**3. Studies on sedimentary environments in Nyanza Gulf of Lake Victoria (Kenya)**

More detailed observations on the sedimentary environments on diatom records and sediment bottom structures in Lake Victoria were made by Stager et al. [55–59] and Scholz et al. [16], with more insights on sediment chronology, accumulation rates, and stratigraphic patterns of biogenic silica accumulation in deepwater cores in the main lake basin determined by Verschuren et al. [60]. Mfundisi [61] analyzed the impact of wetland drainage on soil and plant carbon pools in Yala Swamp. In the developing countries, wetlands are a source of great support to the rural communities, which derive a significant proportion of their livelihood from these areas [62]. However, land cover changes and degradation are often associated with increasing demand for agriculturally fertile soils in the drainage basins. P washed down from the catchments has been identified in the basin and finds its way into the lake especially in areas where macrophytes have been cleared. Upon decomposition of the vegetation matter, the nutrients are released to the bottom mat and sludge [63]. Several studies in lake sediments have tried to explore their potential as sources of labile phosphorus nutrient [64–66] and eutrophication. Other authors have tried to utilize stable isotopes of C and N to understand different sources of OM and processes operating in the C and N cycles [67–69].

#### **3.1. Why study sedimentary organic matter**

**2.3. Methane production**

38 Persistent Organic Pollutants

the final electron acceptor according to the net reaction:

higher probability of continued degradation of the lake environment.

**3. Studies on sedimentary environments in Nyanza Gulf of Lake** 

More detailed observations on the sedimentary environments on diatom records and sediment bottom structures in Lake Victoria were made by Stager et al. [55–59] and Scholz et al. [16], with more insights on sediment chronology, accumulation rates, and stratigraphic patterns of biogenic silica accumulation in deepwater cores in the main lake basin determined by Verschuren et al. [60]. Mfundisi [61] analyzed the impact of wetland drainage on soil and plant carbon

CH<sup>4</sup> + SO4

**Victoria (Kenya)**

Besides methane emissions from wetland areas [46], freshwater anoxic areas are notably important sources. Freshwater bacteria are at the hub of biogeochemical cycles and control water quality in lakes [47]. Methane is oxidized in lakes by a group of bacteria that convert methane and oxygen to cellular material and carbon dioxide [48]. In deeper lakes, methane oxidation occurred mainly within a narrow zone at the boundary of the seasonally mixed layer and the permanently anoxic monimolimnion [49]. Reduction of carbon dioxide and the decomposition of organic sediment material contribute to methanogenesis. Methanogenic bacteria directly reduce the dissolved carbon dioxide near and in the sediments. Simultaneously, methane is formed from organic matter by acetate fermentation. Only three key functional groups of micro-organisms of limited diversity regulate the fluxes of methane on earth, namely, the aerobic methanotrophic bacteria, the methanogenic archaea, and their close relatives, the anaerobic methanotrophic archaea [50]. The anaerobic methanotrophic archaea appear to gain energy exclusively from the anaerobic oxidation of methane, with sulphate as

<sup>2</sup><sup>−</sup> → HCO3

A great deal of biogeochemical research has focused on the causes and effects of the variation in global fluxes of methane throughout earth's history, but the underlying microbial processes and their key agents remain poorly understood [50]. This is a disturbing knowledge gap because 85% of the annual global methane production and about 60% of its consumption are based on microbial processes [50]. Furthermore, wetland ecosystems are vulnerable due to increasing demand for agricultural lands, yet they act as important filters and C sinks. Nutrient concentration gradients were observed between the deep and seasonally stratifying main lake basin and the large, shallow river-influenced Nyanza Gulf, which are connected by the relatively deep and narrow Rusinga Channel [51]. Since the gulf as a whole is P limited, continued P input to this semi-closed part of the lake will result in increased algal blooms and increased eutrophication and therefore negatively affecting the water quality [52, 53]. Increased availability of nutrients in the water helps sustain the healthy water hyacinth, the persistent alien-invasive floating macrophyte species in the Nyanza Gulf. Increased eutrophication [11, 12, 54] process and changes in productivity have been recorded in the lake from several studies and coupled with increased allochthonous material, there is a potentially

<sup>−</sup> + HS<sup>−</sup> + H2 O (1)

Sedimentary records are valuable indicators of the short- and long-term in-lake ecosystems. Long-term effects of anthropogenic activities are better understood when linked to the slower sedimentary processes, especially in large deeper basins. However, such efforts are hampered by the lack of expensive C analysers in most laboratories. The use of the inexpensive and rapid LOI method allows generation of useful information on sediment organic matter contents and is a widely accepted method. Several sediment studies apply this approach [70] in trying to find relationships between organic matter and organic carbon determined from dry combustion. Apart from comprehensive palaeolimnological research efforts made possible by new sediment cores collected in 1995 and 1996 and (eutrophication related changes in the pelagic phytoplankton community of *L. Victoria* as archived in the offshore (in the deepest parts of the lake basin) sedimentary record of biogenic silica, no other detailed information on the spatial distribution of the surficial sediment organic matter on the extreme eastern gulf (*L. Victoria*, Kenya) has been provided. Information gathered in two separate surveys is used to show the spatial distribution of some of the surficial sediment characteristics.

#### **3.2. The physicochemical environment of Nyanza Gulf of Lake Victoria (Kenya)**

In the gulf, most of the sampling sites are nearshore zones of less than 10 m deep (sites of greater 10 m are LS\_37 of 11.0 m and LS\_14 of 13.8 m), when compared to the deeper sites sampled in the main lake (except for the shallow river-mouth areas in the northern area). Surficial sediments in the gulf were observed to consist of dark/gray to brown fine mud and slightly sandy to sandy muds. Observed sediment characteristics are shown in **Table 1** for each site.

**Table 2** shows the mean and range values of the sampled water depth, surface water turbidity, transparency, and chlorophyll-a concentrations. Surface water conditions during the survey and previously collected data are shown in **Figures 2–4**. Although all the three parameters were not determined during all the surveys, available data show a similar trend in water transparency and surface water turbidity. The lake water turbidity showed a high spatial variation during all the sampling surveys. The range values of surface water turbidity were 6.08 NTU to 561 NTU (a maximum value omitted in the graph, at site LS\_ 21 during May 2000). In February 2001 and January–February 2004, surface water turbidity values ranged from 6.45 NTU to 74.8 NTU and 5.08 NTU to 56.6 NTU, respectively.


**Table 1.** Visual descriptions of collected surficial sediments during June–July 2012 survey.


Water transparency values also followed a similar trend as the turbidity in each sampling site. In May 2000, September 2000, December 2000, February 2001, August 2001, July 2003 January–February 2004, and June–July 2012, the values ranged from 0.32 m to 1.5 m, 0.2 to 1.86 m, 0.45 m to 1.6 m, 0.3 m to 2.6 m, 0.35 m to 2.85 m, 0.4 m to 2.2 m, and 0.2 m to 3.2 m). In most of the sites, the values were below 1.5 m (the maximum values were recorded at deeper

**Table 2.** Changes in recorded mean and range values of the sampled water depth, surface water turbidity, transparency,

) 7.70 (3.21) 19 1.26 13.08

Water depth (m) 6.7 (3.5) 21 2.4 14.8 Transparency (m) 0.92 (0.43) 21 0.2 1.86

Water depth (m) 8.0 (6.1) 19 2.5 26.0 Transparency (m) 0.99 (0.43) 19 0.4 2.0

Water depth (m) 5.6 (3.6) 20 2 14 Transparency (m) 0.83 (0.35) 19 0.32 1.5 Turbidity (NTU) 49.9 (122.2) 20 4.86 561

**Figure 2.** Water turbidity and transparency variations during January–February 2004 survey.

**Mean (±SD) n Min. value Max. value**

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sites in the open lake).

and chlorophyll-a concentrations in *L. Victoria* (Kenya).

**December 2000**

**September 2000**

Chlorophyll-a (μgl−<sup>1</sup>

**May 2000**

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**Table 2.** Changes in recorded mean and range values of the sampled water depth, surface water turbidity, transparency, and chlorophyll-a concentrations in *L. Victoria* (Kenya).

**Mean (±SD) n Min. value Max. value**

LS\_34, LS\_13, LS\_37, LS\_30, LS\_31, LS\_15, LS\_9, LS\_32, LS\_33, LS\_24, LS\_14**,** (dark/gray fine muddy sediment, with large and

dark brown sandy muddy sediment); LS\_21 (fine brown muddy sediment with small to slightly larger bivalve shells); LS\_20 (muddy sediment with discrete fine sandy particles with small

LS\_36 and LS\_19 (fine greenish watery muddy sediment signs of

Water depth (m) 9.1 (11.3) 30 0.87 54 Transparency (m) 0.88 (0.85) 24 0.2 3.2

**Table 1.** Visual descriptions of collected surficial sediments during June–July 2012 survey.

Gulf sites Nearshore/offshore zones LS\_1 (brownish muddy sediment); LS\_2, LS\_5, LS\_6, LS\_8,

Open lake sites Off river-mouth zones LS\_23 (slightly sandy brown muddy sediment); LS\_22 (fine

algal deposits)

small empty shell remains) Off river-mouth zones LS\_4, LS\_7, LS\_10, LS\_3, LS\_28 (sandy brown/gray muddy sediment with empty shell remains)

to slightly larger bivalve shells) Nearshore/offshore zones LS\_18, LS\_16, LS\_17, LS\_25 (dark/gray fine muddy sediment);

Water depth (m) 8.2 (7.4) 25 2.5 40 Transparency (m) 1.1 (0.5) 25 0.4 2.2 Turbidity (NTU) 17.98 (15.32) 26 3.0 56.6

Water depth (m) 8.6 (7.5) 25 2.5 40 Transparency (m) 0.99 (0.58) 25 0.35 2.85

Water depth (m) 6.4 (4.0) 24 1.9 18 Transparency (m) 0.96 (0.49) 24 0.3 2.60

Water depth (m) 7.2 (3.4) 19 2.3 12.6 Transparency (m) 0.89 (0.35) 19 0.45 1.6 Turbidity (NTU) 19.16 (16.5) 19 6.45 74.8

) 10.68 (3.73) 25 6.01 22.73

) 20.62 (10.69) 23 8.0 48.02

) 16.2 (8.9) 24 5.5 38.5

**June–July 2012**

**January–February 2004**

40 Persistent Organic Pollutants

Chlorophyll-a (μgl−<sup>1</sup>

Chlorophyll-a (μgl−<sup>1</sup>

Chlorophyll-a (μgl−<sup>1</sup>

**February 2001**

**August 2001**

**July 2003**

Water transparency values also followed a similar trend as the turbidity in each sampling site. In May 2000, September 2000, December 2000, February 2001, August 2001, July 2003 January–February 2004, and June–July 2012, the values ranged from 0.32 m to 1.5 m, 0.2 to 1.86 m, 0.45 m to 1.6 m, 0.3 m to 2.6 m, 0.35 m to 2.85 m, 0.4 m to 2.2 m, and 0.2 m to 3.2 m). In most of the sites, the values were below 1.5 m (the maximum values were recorded at deeper sites in the open lake).

**Figure 2.** Water turbidity and transparency variations during January–February 2004 survey.

**Figure 3.** Water turbidity and transparency variations during February 2001 survey.

The months of May 2000, December 2000, and June–July 2012 fall within the wet season, whereas the rest of the sampling surveys were conducted during the drier season experienced around the lake basin. Calculated correlations showed strong positive correlations between water transparency and water depth with a significant Pearson r coefficient (p < 0.01 level) of 0.817 (2012 survey, n = 24); 0.717 (2004 survey, n = 26); 0.828 (2003 survey, n = 25); 0.792 (August 2001, n = 24); 0.478 (p < 0.05; February 2001, n = 19); 0.496 (p < 0.05, December 2000,

**Figure 5.** Chlorophyll-a concentration variations during August 2001\*\*, July 2003, and January–February 2004 surveys.

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The surficial sediments were characterized by a relatively high water content, with over 89% of the samples containing water contents of greater than 75%. The values ranged from a minimum of 27.34% to a maximum value of 91.55%. Sediment organic matter contents (1 hour drying values) were more variable (**Figure 6**) and ranged from 1.90% to 33.47%. Lake sites with notably low sediment-water contents were LS\_20, LS\_21, and LS\_23, whereas lower contents (less than 20%) of organic matter were found in sites LS\_20; LS\_23; LS\_34, LS\_13, LS\_1, and

The overall mean (± standard deviation) carbonate content (**Figure 7**) was 2.26 ± 1.48% (n = 29). Surficial sediment mean carbonate contents ranged from 0.21 ± 0.01% to 8.09 ± 0.36% for all the stations in 2012. The distribution of calcium carbonate contents in surficial sediments (**Figure 10**) shows a relatively low range when compared to values obtained in 2003/2004 which ranged from 0.5% to 21.8% (n = 29), with a mean (standard deviation) of 8.9 ± 6.6%. The differences are mainly attributed to the method of determination used. Current sediment carbonate concentrations are based on the gravimetric method as opposed to the wet digestion method used for the data of 2003/2004. Shell remains in surficial sediments seem to contribute to the variable concentrations found, although most of the samples contained relatively low amounts. These values are lower when compared to bottom sediments from more calcareous formations, although the contributions of other possible biochemical sources have also not

n = 21); 0.631 (September 2000, n = 19), and 0.541 (p < 0.05, May 2000, n = 20).

LS\_ 32; LS\_11; LS\_17; LS\_14; LS\_22; and LS\_9 and LS\_10.

been assessed.

**Figure 4.** Surface water turbidity and transparency variations during may 2000 and December 2000 surveys.

Changes in spatial variations in chlorophyll-a concentrations have been reported previously, but only a few measurements were available from the various survey data discussed (**Figure 5**). In September 2000, August 2001, July 2003, and January–February 2012, the values ranged from 3.48 μgl−<sup>1</sup> to 13.08 μgl−<sup>1</sup> , 5.4 μgl−<sup>1</sup> to 38.5 μgl−<sup>1</sup> , 8.95 μgl−<sup>1</sup> to 48.02 μgl−<sup>1</sup> , and 6.01 μgl−<sup>1</sup> to 22.73 μgl−<sup>1</sup> , respectively. Although the values showed high spatial variability, relatively higher concentrations were evident in the gulf waters during May 2000 and July 2003. However, most of the chlorophyll-a concentrations showed insignificant and negatively correlated with water depth and water transparency. Significant correlations of the chlorophyll-a concentration with water transparency were observed during February 2001 (r = −0.679, p < 0.01, n = 19). Similarly, in January–February 2004, the concentrations of chlorophyll-a were negatively and significantly correlated with surface water turbidity (r = −0.770, p < 0.01, n = 26).

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**Figure 5.** Chlorophyll-a concentration variations during August 2001\*\*, July 2003, and January–February 2004 surveys.

The months of May 2000, December 2000, and June–July 2012 fall within the wet season, whereas the rest of the sampling surveys were conducted during the drier season experienced around the lake basin. Calculated correlations showed strong positive correlations between water transparency and water depth with a significant Pearson r coefficient (p < 0.01 level) of 0.817 (2012 survey, n = 24); 0.717 (2004 survey, n = 26); 0.828 (2003 survey, n = 25); 0.792 (August 2001, n = 24); 0.478 (p < 0.05; February 2001, n = 19); 0.496 (p < 0.05, December 2000, n = 21); 0.631 (September 2000, n = 19), and 0.541 (p < 0.05, May 2000, n = 20).

The surficial sediments were characterized by a relatively high water content, with over 89% of the samples containing water contents of greater than 75%. The values ranged from a minimum of 27.34% to a maximum value of 91.55%. Sediment organic matter contents (1 hour drying values) were more variable (**Figure 6**) and ranged from 1.90% to 33.47%. Lake sites with notably low sediment-water contents were LS\_20, LS\_21, and LS\_23, whereas lower contents (less than 20%) of organic matter were found in sites LS\_20; LS\_23; LS\_34, LS\_13, LS\_1, and LS\_ 32; LS\_11; LS\_17; LS\_14; LS\_22; and LS\_9 and LS\_10.

**Figure 4.** Surface water turbidity and transparency variations during may 2000 and December 2000 surveys.

from 3.48 μgl−<sup>1</sup>

42 Persistent Organic Pollutants

22.73 μgl−<sup>1</sup>

to 13.08 μgl−<sup>1</sup>

, 5.4 μgl−<sup>1</sup>

**Figure 3.** Water turbidity and transparency variations during February 2001 survey.

Changes in spatial variations in chlorophyll-a concentrations have been reported previously, but only a few measurements were available from the various survey data discussed (**Figure 5**). In September 2000, August 2001, July 2003, and January–February 2012, the values ranged

concentrations were evident in the gulf waters during May 2000 and July 2003. However, most of the chlorophyll-a concentrations showed insignificant and negatively correlated with water depth and water transparency. Significant correlations of the chlorophyll-a concentration with water transparency were observed during February 2001 (r = −0.679, p < 0.01, n = 19). Similarly, in January–February 2004, the concentrations of chlorophyll-a were negatively and

, 8.95 μgl−<sup>1</sup>

, respectively. Although the values showed high spatial variability, relatively higher

to 48.02 μgl−<sup>1</sup>

, and 6.01 μgl−<sup>1</sup>

to

to 38.5 μgl−<sup>1</sup>

significantly correlated with surface water turbidity (r = −0.770, p < 0.01, n = 26).

The overall mean (± standard deviation) carbonate content (**Figure 7**) was 2.26 ± 1.48% (n = 29). Surficial sediment mean carbonate contents ranged from 0.21 ± 0.01% to 8.09 ± 0.36% for all the stations in 2012. The distribution of calcium carbonate contents in surficial sediments (**Figure 10**) shows a relatively low range when compared to values obtained in 2003/2004 which ranged from 0.5% to 21.8% (n = 29), with a mean (standard deviation) of 8.9 ± 6.6%. The differences are mainly attributed to the method of determination used. Current sediment carbonate concentrations are based on the gravimetric method as opposed to the wet digestion method used for the data of 2003/2004. Shell remains in surficial sediments seem to contribute to the variable concentrations found, although most of the samples contained relatively low amounts. These values are lower when compared to bottom sediments from more calcareous formations, although the contributions of other possible biochemical sources have also not been assessed.

**Figure 6.** Mean (±SD) sediment organic matter variations at different drying durations and temperature using 2012 survey sediment samples.

Relationships between some of the sediment characteristics determined are shown in **Figures 8** and **9**. The organic matter content shows a strong positive correlation with water content and water depth at 0.01 level of significance (two-tailed test). Calculated bulk density values (**Figure 9a** and **b**) were within a narrow range from 0.0205 gcm−<sup>3</sup> to 0.0875 gcm−<sup>3</sup> , with higher values in sediments of relatively low percentage of OM and water contents. The sediment bulk density relationship with the sediment OM and water contents was best described by an exponential relationship (r2 = 0.900 and r2 = 0.630), with a strong negative correlation coefficient. The water content of surface sediments varies from about 30–50% in minerogenic deposits from areas of erosion to approximately 95–99% in highly organic sediments [3]. Part of the sediment

**Figure 8.** (a) The relationship between surficial sediment organic matter and water depth (r = 0.533, n = 27). (b) the

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**Figure 9.** (a) Relationship between the sediment-water content and bulk density. (b) Relationship between the sediment

organic matter content and bulk density.

relationship between surficial sediment organic matter and water content (r = 0.661, r<sup>2</sup> = 0.437; n = 27).

**Figure 7.** Mean (±SD) sediment carbonate content variations during June 2012 survey in *L. Victoria* (Kenya).

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**Figure 8.** (a) The relationship between surficial sediment organic matter and water depth (r = 0.533, n = 27). (b) the relationship between surficial sediment organic matter and water content (r = 0.661, r<sup>2</sup> = 0.437; n = 27).

**Figure 9.** (a) Relationship between the sediment-water content and bulk density. (b) Relationship between the sediment organic matter content and bulk density.

**Figure 7.** Mean (±SD) sediment carbonate content variations during June 2012 survey in *L. Victoria* (Kenya).

Relationships between some of the sediment characteristics determined are shown in **Figures 8** and **9**. The organic matter content shows a strong positive correlation with water content and water depth at 0.01 level of significance (two-tailed test). Calculated bulk density values

**Figure 6.** Mean (±SD) sediment organic matter variations at different drying durations and temperature using 2012

values in sediments of relatively low percentage of OM and water contents. The sediment bulk density relationship with the sediment OM and water contents was best described by an exponential relationship (r2 = 0.900 and r2 = 0.630), with a strong negative correlation coefficient. The water content of surface sediments varies from about 30–50% in minerogenic deposits from areas of erosion to approximately 95–99% in highly organic sediments [3]. Part of the sediment

to 0.0875 gcm−<sup>3</sup>

, with higher

(**Figure 9a** and **b**) were within a narrow range from 0.0205 gcm−<sup>3</sup>

survey sediment samples.

44 Persistent Organic Pollutants

water is bound to crystals in chemical structures or forms film tightly adsorbed to sediment particles, and the rest constitutes the mobile medium, which surrounds the sediment particles and takes part in exchange processes between the particulate and dissolved phases, as well as in exchange processes between sediments and lake water [3].

In Lake Victoria, thermal stratification, leading to hypoxia, was observed in the late 1920s [4]. Hypoxia conditions were restricted to the deeper waters (>60 m) and for shorter periods during the rainy season [6]. Lake Victoria has three phases of thermal stratification; moderate stratification occurs between September and December, stable stratification in January to March/April, and deep strong mixing in June to July [6, 73]. Stratification of the water column isolates the bottom water from exchange with oxygen-rich surface water and the atmosphere, while decomposition of organic matter in the isolated bottom water consumes dissolved oxygen [24]. For lakes, factors affecting vertical water mixing such as wind and temperature can lower DO in bottom waters to anoxic levels. Increase in deepwater temperatures, increases thermal stratification stability [74]. Thermal stability makes the lake less able to mix effectively and promotes low oxygen conditions in deepwaters during stratification period

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Low dissolved oxygen or hypoxic conditions can be due to natural causes such as algal respiration, seasonal flooding, stratification, and anthropogenic causes. Low dissolved oxygen environments vary in temporal frequency, seasonality, and persistence [75–77]. Hypoxia occurs naturally in habitats characterized by low mixing or light limited, heavily vegetated swamps and backwaters that circulate poorly, stratify, and have large loads of terrestrial organic matter [78, 79]. Levels of hypoxia are mainly determined by primary productivity, depth, and temperature of the aquatic body [77]. Increasing and widespread deepwater anoxia in Lake Victoria might put at risk the entire fishery [80]. In Lake Victoria, severe hypoxic conditions

bottom area [81]. Water hyacinth is notably a persistent floating macrophyte in Lake Victoria. Shading of the water by the hyacinth curtailed photosynthesis, while microbial breakdown of decaying plant material used the available oxygen. The waters below water hyacinth recorded

the distribution of fish by blocking migratory routes of those escaping low DO and predation [83]. However, studies by Njiru et al. [82] found the hyacinth to have led to recovery of the native species which were more hypoxia tolerant such as catfishes, lungfish, and tilapia. Hypoxia is physiologically stressful for fish, shellfish, and invertebrates with prolonged exposure to anoxia being fatal to most aquatic fauna [76, 84]. Njiru et al. [24] explored the impacts of hypoxia on the fishery of Lake Victoria. Hypoxia exposure can prompt both lethal and sublethal effects in fishes, leading to reduced feeding, reproductive, growth, metabolism, and slower reaction time. These effects vary across fish species [84] but also depend on the frequency, intensity, and duration of the hypoxic events [76]. In shallower lake areas, with permanent cover of macrophytes, especially large extensive floating water hyacinth, deoxygenated waters may influence the distribution of low oxygen-intolerant fish species in the lake.

Sediments accumulated in lake basins consist of various organic and inorganic materials mostly utilized as proxies for climatic changes and as historical records of the lake. The gulf's bottom deposits are mainly a combination of the several externally transported materials, surface runoffs, and shoreline-eroded materials and slowly settling suspended loads. An equally important source is the autochthonous organic matter which is aerially derived materials. The first observations on the nature of the bottom of Lake Victoria were made by Graham and

**3.3. Sources of organic matter in Lake Victoria sediments**

) now persists at depths below 40–50 m which cover about 35% of the lake's total

making it inhabitable to most fish [82]. Additionally, the weed affected

between September and April [73].

(<1.0 mgl−<sup>1</sup>

DO as low as 0.1 mgl−<sup>1</sup>

Surface and bottom lake water temperature show about 2°C range (surface 23.9°C to 25.9°C, bottom 23.0°C to 25.2°C) and a difference of about 0.9°C to 0.7°C. There are no large variations of the gulf water temperature when compared to the whole lake surface and > 50 m depth mean temperatures reported in 2009 ([13], 25.88 ± 0.86°C surface and 24.89 ± 0.23°C > 50 m) and 1927 ([71], 24.69 ± 0.71°C surface and 23.32 ± 0.29°C > 50 m). Studies from main lake [72] show that between 1927 and 2008 the lake's temperature increased by 0.99°C at the surface and by 1.34°C at depths >50 m, with the rate of warming increasing most rapidly between 2000 and 2008 in the whole lake. In February 2000 there were marked thermal discontinuities in the water column at a number of deep stations, with marked oxyclines at depths ranging from 30 to 50 m and with all stations being anoxic from 50 m downwards [72]. In contrast, in February 2007 the lake's temperature had risen, especially at the bottom, and both the thermal discontinuities and oxyclines were much reduced, only one station recording a dissolved oxygen concentration of <2.0 mg l −1 at 50 m [72].

Lake waters are more towards neutral alkaline (pH values of 7.59–8.51) and of high oxygenation levels in the surface layers, and sometimes appreciable oxygen levels occur even in deeper waters (above 3.45 mgl−<sup>1</sup> in 2012). In July 2003, the dissolved oxygen in bottom water stations was above 2 mgl−<sup>1</sup> in all stations. However, in some instances, very low to anoxic conditions have been reported in deep sites. Sitoki et al. [13] reported mean dissolved oxygen of 2.55 mgl−<sup>1</sup> at stations of greater than 40 m depth (**Figure 10**) from lakewide surveys in 2000 and 2001 (February to March) and 5.75 mgl−<sup>1</sup> (August to September). But less deoxygenation of lakewide bottom was reported at the same stations in February–March and August–September of 2006 to 2009 surveys with mean DO concentrations of 6.13 mgl−<sup>1</sup> and 5.80 mgl−<sup>1</sup> , respectively.

**Figure 10.** Mean dissolved oxygen concentration (mgl−<sup>1</sup> ) at greater than 40 m lake water depth during February–march and August–September lakewide surveys in different years (values adapted from Sitoki et al.) [13].

In Lake Victoria, thermal stratification, leading to hypoxia, was observed in the late 1920s [4]. Hypoxia conditions were restricted to the deeper waters (>60 m) and for shorter periods during the rainy season [6]. Lake Victoria has three phases of thermal stratification; moderate stratification occurs between September and December, stable stratification in January to March/April, and deep strong mixing in June to July [6, 73]. Stratification of the water column isolates the bottom water from exchange with oxygen-rich surface water and the atmosphere, while decomposition of organic matter in the isolated bottom water consumes dissolved oxygen [24]. For lakes, factors affecting vertical water mixing such as wind and temperature can lower DO in bottom waters to anoxic levels. Increase in deepwater temperatures, increases thermal stratification stability [74]. Thermal stability makes the lake less able to mix effectively and promotes low oxygen conditions in deepwaters during stratification period between September and April [73].

water is bound to crystals in chemical structures or forms film tightly adsorbed to sediment particles, and the rest constitutes the mobile medium, which surrounds the sediment particles and takes part in exchange processes between the particulate and dissolved phases, as well as

Surface and bottom lake water temperature show about 2°C range (surface 23.9°C to 25.9°C, bottom 23.0°C to 25.2°C) and a difference of about 0.9°C to 0.7°C. There are no large variations of the gulf water temperature when compared to the whole lake surface and > 50 m depth mean temperatures reported in 2009 ([13], 25.88 ± 0.86°C surface and 24.89 ± 0.23°C > 50 m) and 1927 ([71], 24.69 ± 0.71°C surface and 23.32 ± 0.29°C > 50 m). Studies from main lake [72] show that between 1927 and 2008 the lake's temperature increased by 0.99°C at the surface and by 1.34°C at depths >50 m, with the rate of warming increasing most rapidly between 2000 and 2008 in the whole lake. In February 2000 there were marked thermal discontinuities in the water column at a number of deep stations, with marked oxyclines at depths ranging from 30 to 50 m and with all stations being anoxic from 50 m downwards [72]. In contrast, in February 2007 the lake's temperature had risen, especially at the bottom, and both the thermal discontinuities and oxyclines were much reduced, only one station recording a dissolved

in exchange processes between sediments and lake water [3].

−1

at 50 m [72].

Lake waters are more towards neutral alkaline (pH values of 7.59–8.51) and of high oxygenation levels in the surface layers, and sometimes appreciable oxygen levels occur even in deeper

have been reported in deep sites. Sitoki et al. [13] reported mean dissolved oxygen of 2.55 mgl−<sup>1</sup> at stations of greater than 40 m depth (**Figure 10**) from lakewide surveys in 2000 and 2001

bottom was reported at the same stations in February–March and August–September of 2006 to

in 2012). In July 2003, the dissolved oxygen in bottom water stations

(August to September). But less deoxygenation of lakewide

and 5.80 mgl−<sup>1</sup>

) at greater than 40 m lake water depth during February–march

, respectively.

in all stations. However, in some instances, very low to anoxic conditions

oxygen concentration of <2.0 mg l

(February to March) and 5.75 mgl−<sup>1</sup>

**Figure 10.** Mean dissolved oxygen concentration (mgl−<sup>1</sup>

and August–September lakewide surveys in different years (values adapted from Sitoki et al.) [13].

2009 surveys with mean DO concentrations of 6.13 mgl−<sup>1</sup>

waters (above 3.45 mgl−<sup>1</sup>

was above 2 mgl−<sup>1</sup>

46 Persistent Organic Pollutants

Low dissolved oxygen or hypoxic conditions can be due to natural causes such as algal respiration, seasonal flooding, stratification, and anthropogenic causes. Low dissolved oxygen environments vary in temporal frequency, seasonality, and persistence [75–77]. Hypoxia occurs naturally in habitats characterized by low mixing or light limited, heavily vegetated swamps and backwaters that circulate poorly, stratify, and have large loads of terrestrial organic matter [78, 79]. Levels of hypoxia are mainly determined by primary productivity, depth, and temperature of the aquatic body [77]. Increasing and widespread deepwater anoxia in Lake Victoria might put at risk the entire fishery [80]. In Lake Victoria, severe hypoxic conditions (<1.0 mgl−<sup>1</sup> ) now persists at depths below 40–50 m which cover about 35% of the lake's total bottom area [81]. Water hyacinth is notably a persistent floating macrophyte in Lake Victoria. Shading of the water by the hyacinth curtailed photosynthesis, while microbial breakdown of decaying plant material used the available oxygen. The waters below water hyacinth recorded DO as low as 0.1 mgl−<sup>1</sup> making it inhabitable to most fish [82]. Additionally, the weed affected the distribution of fish by blocking migratory routes of those escaping low DO and predation [83]. However, studies by Njiru et al. [82] found the hyacinth to have led to recovery of the native species which were more hypoxia tolerant such as catfishes, lungfish, and tilapia. Hypoxia is physiologically stressful for fish, shellfish, and invertebrates with prolonged exposure to anoxia being fatal to most aquatic fauna [76, 84]. Njiru et al. [24] explored the impacts of hypoxia on the fishery of Lake Victoria. Hypoxia exposure can prompt both lethal and sublethal effects in fishes, leading to reduced feeding, reproductive, growth, metabolism, and slower reaction time. These effects vary across fish species [84] but also depend on the frequency, intensity, and duration of the hypoxic events [76]. In shallower lake areas, with permanent cover of macrophytes, especially large extensive floating water hyacinth, deoxygenated waters may influence the distribution of low oxygen-intolerant fish species in the lake.

#### **3.3. Sources of organic matter in Lake Victoria sediments**

Sediments accumulated in lake basins consist of various organic and inorganic materials mostly utilized as proxies for climatic changes and as historical records of the lake. The gulf's bottom deposits are mainly a combination of the several externally transported materials, surface runoffs, and shoreline-eroded materials and slowly settling suspended loads. An equally important source is the autochthonous organic matter which is aerially derived materials. The first observations on the nature of the bottom of Lake Victoria were made by Graham and Worthington during their survey in 1927–1928 [71]. They stated that nearly all the bottom of the lake was covered with a fine greenish-black mud which was almost entirely composed of the dead shells of diatoms [10]. First palaeolimnological records of bottom sediments in the northern part of the main Lake Victoria (outside the Nyanza Gulf) were provided by Kendall [15]. Scholz et al. [16] study revealed that fine-grained late-Pleistocene and late-Holocene sediments having a maximum open-basin thickness of about 8 metres overlie older desiccated lake sediments, alluvial materials, Precambrian crystalline, and tertiary volcanic rocks depending on the position in the lake. The distribution of these sediments mimics bathymetry. Thicker sediment layers may be found near bathymetric heights and inshore waters. Analyses of the composition of oven-dried samples of mud revealed 24–64% silica, 4–25% carbon, 1.5–4.8% iron, 0.6–2.2% nitrogen, 0.5–2% sulfur, 0.04% phosphate, 0.3% Ca, 0.07% Mg, 0.05% K, and 0.03% Na [10]. Talbot and Livingstone [85] used geochemical analysis of organic matter in sediment cores from Pilkington bay, Ugandan area of *L. Victoria* in 1971 (depth of 32 m and 66 m) to provide a history of lake level fluctuations. The core sediments were characterized by moderate to low organic contents and very low hydrogen index. Very low HI from sediments rich in algal remains indicates severe degradation of the organic material. At 6.9 m and 9.8 m downcore Ibis core 1, the TOC values were 18.6 and 2.6%, respectively [85]. Similarly, at 7.3 m and 10.3 m downcore Ibis core 3, the TOC values were 7.9 and 2.1%, respectively [85]. TOC and HI both decline as an exposure surface is approached, mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation. The carbon isotopic composition of the organic matter shows significant variations through the core δ<sup>13</sup>Corg values were > −20‰ at and below the discontinuity surfaces and < −20‰ above the upper shell bed. According to Talbot and Livingstone [84], variations in the carbon isotopic composition of the bulk organic matter may reflect mainly changes in the relative contributions from different plant communities, which are also related to changes in the lake. Although all aquatic algae and many vascular plants have carbon isotopic compositions in the range from −22‰ to −30‰, plants using the C4 photosynthetic pathway have range from −9‰ to −16‰ [86, 87]. Variations in the bulk organic matter isotopic composition of organic matter in Lake Victoria are likely principal to reflect varying contributions from plant communities with different proportions of C<sup>3</sup> and C<sup>4</sup> plants. In particular a sediment dominated by inputs of aquatic algae or forest trees is likely to have a δ<sup>13</sup>C between −20‰ and − 30‰, while substantial inputs from C4 graminoid plants from either marshes or terrestrial grassland will tend to produce a δ<sup>13</sup>Corg between −10‰ and − 15‰. Characterization of stable isotope signatures of various organisms from the lake shows varied values, but the information is limited spatially. Studies on sources of carbon in the *L. Victoria* ecosystem reported mean (±SD) values of stable C isotope values for organisms collected at deep site and littoral site in Napoleon Gulf in 1995 [69] for δ13C were gastropods (−19.0 ± 1.7), bivalves (−18.0 ± 0.7), *Rastrineobola argentea* (−16.9 ± 0.8), and *Lates niloticus* (−18.9 ± 0.2). The δ13C of plants ranged from −8.8‰ to −24.6‰, while in fishes the values ranged from −18.6‰ to −24.5‰, suggesting assimilation of mostly C3 sources for the fishes [67]. According to Gichuki et al. [68], aquatic macrophytes from wetland ecosystems in Nyanza Gulf had δ<sup>13</sup>C isotopic ratios ranging from −8.92 to −29.18, and for the dominant macrophytes, they observed most enriched δ<sup>13</sup>C values for *Cyperus papyrus* and most depleted δ<sup>13</sup>C values for *Eichhornia crassipes*. Fractionation of carbon isotopes during photosynthesis is a key parameter for understanding organic carbon isotope signatures in aquatic ecosystems. Photosynthetic fractionation of carbon isotopes can occur at the diffusion, dissolution, and

carboxylation step [88]. During photosynthesis, plants preferentially acquire the lighter carbon isotope, 12C. Consequently, plant organic matter has a lighter isotope ratio than the source inorganic carbon [88]. More recently, on the Tanzanian portion of the lake, Machiwa [35] demonstrated the potential to discriminate between input sources of organic material into lakes using stable isotope signals in sediment and suspended particulate matter. Variations in δ<sup>13</sup>C, δ15N, and C/N ratios in SPM and sediments in inshore areas were due to differences in the proportions and sources of autochthonous or allochthonous matter. Watershed characteristics, such as urbanization, and lake characteristics, such as algal blooms, also immensely influenced the stable isotope signal of the organic matter in sediments [35]. Although this study did not incorporate isotopic studies, it clearly shows that interpretation of such information demands more spatial and temporal information on isotopic patterns to understand within lake variations in allochthonous inputs, considering the high seasonal variability in a tropical

Lake Sedimentary Environments and Roles of Accumulating Organic Matter in Biogeochemical…

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49

Short cores from the open lake document a shift in lake conditions beginning in the 1930s that progressed to the major ecosystem collapse of the early 1980s [14]. The coincidence of the shift in sediment properties in the 1930s with the beginning of rapid expansion of human population and agricultural activity suggests cause and effect. It is conceivable that the lake experienced similar conditions due to natural causes between about 9800 and 7500 years ago [14]. From core studies, deep rift lake basins [89], with anoxic depths below 250 m, contains thick sequence of biologically undisturbed, finely laminated muds and silts. Similar low oxygen conditions to anoxic conditions reported in deep sites of Lake Victoria a relatively shallow basin can possibly promote accumulation of significant amounts of organic matter after deposition, depending on type of sedimenting materials, the extents of bioturbation processes, and redox conditions. There are no experimental studies on downward flux measurements of particulate carbon in the gulf areas. In shallower areas, downward fluxes of particulate organic matter may be highly variable, and the remobilization of deposited sediments through resuspension caused by physical mixing processes may contribute to variations in

Understanding sources, dispersion pathways, and sinks of sedimentary materials can improve our understanding of various geochemical cycles in the lake. In aquatic environments dissolved phosphate is consumed during growth of phytoplankton and is regenerated during bacterial decomposition of organic matter. The regenerated phosphate may be released to the overlying water, reprecipitated within the sediment as authigenic phase or adsorbed by other constituents of sediment [90]. Development of anoxia in sediments will lead to reduction of iron oxides and release of sequestered P. Adsorption on oxidized surface sediment affects the

Besides the sediment physical factors, iron-manganese oxides, clay minerals, organic matter, and reactive Fe are among the sedimentary geochemical factors controlling chemical partitioning and bioavailability. Linnik and Zubenko [92] showed that the release of heavy metals from bottom sediments was promoted, for example, by a deficit in dissolved oxygen, a decrease in pH and redox potential (Eh), and an increase in mineralization and in dissolved organic matter concentration. Compared with other natural environmental substrates, sediments have a greater capacity to bind Hg. More than 90% of the total Hg in sediment-water system is

type of climate experienced in the lake region.

sediment composition and low-sediment organic matter.

flux of phosphate from the sediment to the overlying water [91].

carboxylation step [88]. During photosynthesis, plants preferentially acquire the lighter carbon isotope, 12C. Consequently, plant organic matter has a lighter isotope ratio than the source inorganic carbon [88]. More recently, on the Tanzanian portion of the lake, Machiwa [35] demonstrated the potential to discriminate between input sources of organic material into lakes using stable isotope signals in sediment and suspended particulate matter. Variations in δ<sup>13</sup>C, δ15N, and C/N ratios in SPM and sediments in inshore areas were due to differences in the proportions and sources of autochthonous or allochthonous matter. Watershed characteristics, such as urbanization, and lake characteristics, such as algal blooms, also immensely influenced the stable isotope signal of the organic matter in sediments [35]. Although this study did not incorporate isotopic studies, it clearly shows that interpretation of such information demands more spatial and temporal information on isotopic patterns to understand within lake variations in allochthonous inputs, considering the high seasonal variability in a tropical type of climate experienced in the lake region.

Worthington during their survey in 1927–1928 [71]. They stated that nearly all the bottom of the lake was covered with a fine greenish-black mud which was almost entirely composed of the dead shells of diatoms [10]. First palaeolimnological records of bottom sediments in the northern part of the main Lake Victoria (outside the Nyanza Gulf) were provided by Kendall [15]. Scholz et al. [16] study revealed that fine-grained late-Pleistocene and late-Holocene sediments having a maximum open-basin thickness of about 8 metres overlie older desiccated lake sediments, alluvial materials, Precambrian crystalline, and tertiary volcanic rocks depending on the position in the lake. The distribution of these sediments mimics bathymetry. Thicker sediment layers may be found near bathymetric heights and inshore waters. Analyses of the composition of oven-dried samples of mud revealed 24–64% silica, 4–25% carbon, 1.5–4.8% iron, 0.6–2.2% nitrogen, 0.5–2% sulfur, 0.04% phosphate, 0.3% Ca, 0.07% Mg, 0.05% K, and 0.03% Na [10]. Talbot and Livingstone [85] used geochemical analysis of organic matter in sediment cores from Pilkington bay, Ugandan area of *L. Victoria* in 1971 (depth of 32 m and 66 m) to provide a history of lake level fluctuations. The core sediments were characterized by moderate to low organic contents and very low hydrogen index. Very low HI from sediments rich in algal remains indicates severe degradation of the organic material. At 6.9 m and 9.8 m downcore Ibis core 1, the TOC values were 18.6 and 2.6%, respectively [85]. Similarly, at 7.3 m and 10.3 m downcore Ibis core 3, the TOC values were 7.9 and 2.1%, respectively [85]. TOC and HI both decline as an exposure surface is approached, mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation. The carbon isotopic composition of the organic matter shows significant variations through the core δ<sup>13</sup>Corg values were > −20‰ at and below the discontinuity surfaces and < −20‰ above the upper shell bed. According to Talbot and Livingstone [84], variations in the carbon isotopic composition of the bulk organic matter may reflect mainly changes in the relative contributions from different plant communities, which are also related to changes in the lake. Although all aquatic algae and many vascular plants have carbon isotopic compositions in the range from −22‰

photosynthetic pathway have range from −9‰ to −16‰ [86, 87].

sources for

plants. In particular a sediment dominated by inputs of aquatic algae

Variations in the bulk organic matter isotopic composition of organic matter in Lake Victoria are likely principal to reflect varying contributions from plant communities with different

or forest trees is likely to have a δ<sup>13</sup>C between −20‰ and − 30‰, while substantial inputs from

the fishes [67]. According to Gichuki et al. [68], aquatic macrophytes from wetland ecosystems in Nyanza Gulf had δ<sup>13</sup>C isotopic ratios ranging from −8.92 to −29.18, and for the dominant macrophytes, they observed most enriched δ<sup>13</sup>C values for *Cyperus papyrus* and most depleted δ<sup>13</sup>C values for *Eichhornia crassipes*. Fractionation of carbon isotopes during photosynthesis is a key parameter for understanding organic carbon isotope signatures in aquatic ecosystems. Photosynthetic fractionation of carbon isotopes can occur at the diffusion, dissolution, and

the values ranged from −18.6‰ to −24.5‰, suggesting assimilation of mostly C3

 graminoid plants from either marshes or terrestrial grassland will tend to produce a δ<sup>13</sup>Corg between −10‰ and − 15‰. Characterization of stable isotope signatures of various organisms from the lake shows varied values, but the information is limited spatially. Studies on sources of carbon in the *L. Victoria* ecosystem reported mean (±SD) values of stable C isotope values for organisms collected at deep site and littoral site in Napoleon Gulf in 1995 [69] for δ13C were gastropods (−19.0 ± 1.7), bivalves (−18.0 ± 0.7), *Rastrineobola argentea* (−16.9 ± 0.8), and *Lates niloticus* (−18.9 ± 0.2). The δ13C of plants ranged from −8.8‰ to −24.6‰, while in fishes

to −30‰, plants using the C4

and C<sup>4</sup>

proportions of C<sup>3</sup>

48 Persistent Organic Pollutants

C4

Short cores from the open lake document a shift in lake conditions beginning in the 1930s that progressed to the major ecosystem collapse of the early 1980s [14]. The coincidence of the shift in sediment properties in the 1930s with the beginning of rapid expansion of human population and agricultural activity suggests cause and effect. It is conceivable that the lake experienced similar conditions due to natural causes between about 9800 and 7500 years ago [14]. From core studies, deep rift lake basins [89], with anoxic depths below 250 m, contains thick sequence of biologically undisturbed, finely laminated muds and silts. Similar low oxygen conditions to anoxic conditions reported in deep sites of Lake Victoria a relatively shallow basin can possibly promote accumulation of significant amounts of organic matter after deposition, depending on type of sedimenting materials, the extents of bioturbation processes, and redox conditions. There are no experimental studies on downward flux measurements of particulate carbon in the gulf areas. In shallower areas, downward fluxes of particulate organic matter may be highly variable, and the remobilization of deposited sediments through resuspension caused by physical mixing processes may contribute to variations in sediment composition and low-sediment organic matter.

Understanding sources, dispersion pathways, and sinks of sedimentary materials can improve our understanding of various geochemical cycles in the lake. In aquatic environments dissolved phosphate is consumed during growth of phytoplankton and is regenerated during bacterial decomposition of organic matter. The regenerated phosphate may be released to the overlying water, reprecipitated within the sediment as authigenic phase or adsorbed by other constituents of sediment [90]. Development of anoxia in sediments will lead to reduction of iron oxides and release of sequestered P. Adsorption on oxidized surface sediment affects the flux of phosphate from the sediment to the overlying water [91].

Besides the sediment physical factors, iron-manganese oxides, clay minerals, organic matter, and reactive Fe are among the sedimentary geochemical factors controlling chemical partitioning and bioavailability. Linnik and Zubenko [92] showed that the release of heavy metals from bottom sediments was promoted, for example, by a deficit in dissolved oxygen, a decrease in pH and redox potential (Eh), and an increase in mineralization and in dissolved organic matter concentration. Compared with other natural environmental substrates, sediments have a greater capacity to bind Hg. More than 90% of the total Hg in sediment-water system is sorbed or held in sediments [27]. Sediments therefore serve are useful long-term records of the material influx and provide historical records for evaluating past and other ecological changes occurring in the lake. However, very few studies have been concentrated in the gulf, as the best depositional basins for provision of long-term records are in the main lake. Therefore, continuous provision of data on the sedimentary environment is valuable in understanding processes within these areas and their influences on the above water quality, considering the fact that the area receives significant inputs from the inflowing rivers and nearby human activities.

catchment of *L. Victoria* found out that less loading will occur from riverine systems that have lower discharges, consist of nonagricultural land, and have low basin slope. Soil erosion and leaching in this tropical region increase dramatically as slope incline increases. A sediment core was taken 55 m water depth off western Kenya (core at station 103) and the diatom and chemical stratigraphy analyzed [9]. The dry weight sedimentation rate increased after 1960 to 90 gm−<sup>2</sup> yr.−<sup>1</sup>

production of *Cyclostephanos* concurrent with an increase in C and N deposition early in the century [9]. These trends continued until the decade of the 1960s when P and biogenic Si deposition began a rapid increase. By the late 1960s, *Melosira* spp. was nearly gone largely replaced in the diatom community by *Nitzschia*. By the late 1970s, modern rates of C, N, P, and Si deposition were established as was the modern diatom community [9]. The increased deposition rates of N beginning in the 1920s and P beginning in the 1950s were likely results of watershed and airshed disturbance [9]. Primary production was extremely high as lake level rose in its first 500 years,

Concentrations of P in the gulf lake water (2000–2002) were found to be different from those in the main lake with phosphorus fractions, soluble reactive P, and total P being significantly higher in the main lake than in the gulf [53]. Well-oxygenated conditions in the gulf keep the

between the gulf and the main lake. In the gulf and the Rusinga Channel, the less bioavailable apatite phosphorus dominated, whereas in the deeper main lake, organic P was the major fraction illustrating the importance of anaerobic release of P from sediments and acceleration

Chemical processes within the sediments play an important role in the P fluxes within and from sediments. In aquatic environment dissolved phosphate is consumed during growth of phytoplankton and is regenerated during bacterial decomposition of organic matter [91]. Much of the regeneration takes place in the water, but in relatively shallow environments such as lakes, estuaries, and continental shelves, sediments may play an important role in the regeneration of phosphate [97]. The regenerated phosphate may be released to the overlying water, reprecipitated within the sediment as authigenic phase or adsorbed by other constituents of sediment. Adsorption on metal oxides in the sediment has been identified as one of the principal reactions involving phosphate [90]. Mortimer [98] has described the oxidized surface layer as a trap for phosphate. The reactions that release phosphate to the porewater are desorption of phosphate from surface sites on sediment particles, mineralization of organic

matter, and reduction of iron oxides in the anoxic zone of the sediments [99].


dry weight and was highest in the Rusinga Channel, the exchange zone

[66]. Total P in the sediment ranged from

nourished by the high input of nutrients from the flooded landscape [14].

end of the gulf is exceptionally high, >1500 mgkg−<sup>1</sup>

of internal P loading in the main lake [65].

before then. There is an interpretable sequence which begins with increasing

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51

from 57 gm−<sup>2</sup> yr.−<sup>1</sup>

PO4

812.7 to 1738 mgkg−<sup>1</sup>

The six coring stations, representative of the main lake depositional basin, were located at a water depth range between 48 and 68 m [60], but none was located in the Nyanza Gulf, where the depths are below 60 m. Accumulation of fine-grained Holocene sediments in offshore regions of Lake Victoria is restricted to the deep east-central basin floor [16, 59]. However, surficial sediment deposits the deep gulf, and open stations contain similar organic rich muds. Verschuren et al. [60] noted that with increasing distance from the depositional centre, progressively thinner sheets of Holocene sediments occur, for example, about 5.4 m at station V95-1G and 3.6 m at V96-1MC (eastern central basin), reflecting bottom dynamics that become less and less favorable for undisturbed sediment accumulation. According to Verschuren et al. [60], in the transect of stations examined, greater physical biological sediment mixing at shallower depths is evident in the lack of flocculent surface muds in cores V96-1MC and V96-8MC (western central basin). In these shallow sites, organic matter decreases from 32% at the top of the short sediment core surface, to 23% at bottom of core compared to a change of 14% (top of core) to 10% (end of core). The deepest site short cores at 58 m and 68 m water depth had a sub-oxic sediment–water interface (0.0 mgl−<sup>1</sup> to 0.7 mgl−<sup>1</sup> ), with high and near constant organic matter contents along the sediment core (35–38% near top surface of the core and 33–35% end of core). The core closer to the extreme western side of the open lake portion of the Nyanza Gulf contained 27% (top) and 25% (end of core).

#### **3.4. Sedimentation and nutrients in Lake Victoria**

Rivers contribute a significant load of suspended materials annually into the lake. Although it is not possible to provide current rates of resuspended loads, the inputs of organic matter into the depositional areas seem significant from the productive water column (autochthonous sources). Johnson [92] summarized published sedimentation rates for several lakes. There is surprisingly little difference between the sedimentation rates observed in tectonic versus glacial lakes, even though the relief, and hence, the sediment supply rate per unit area of drainage basin should be higher around tectonic lakes. Verschuren et al. [60] established average recent sedimentation accumulation rates of 0.032 ± 0.001 to 0.028 ± 0.001 gcm−<sup>2</sup> yr.−<sup>1</sup> from 210Pb dating of sediment cores in deep depositional sites in the main lake. The rates are lower but comparable with other variable rates reported for other temperate lakes [93]. Variable sedimentation rates are reported in different types of lakes, but the mean values reported by Verschuren et al. [60] for the lake appear to fall within the low end values for other lakes. In L. Erie, sedimentation rates [94, 95] estimated from 35 stations varied from less than 20 mgcm−<sup>2</sup> yr.−<sup>1</sup> to greater than 1000 mgcm−<sup>2</sup> yr.−<sup>1</sup> with an average of 230 mgcm−<sup>2</sup> yr.−<sup>1</sup> .

Although soil types and land uses vary in different drainage basins, the surrounding gulf areas are under the same typical tropical wet climate. Lindenschmidt et al. [96] in a neighbouring rural catchment of *L. Victoria* found out that less loading will occur from riverine systems that have lower discharges, consist of nonagricultural land, and have low basin slope. Soil erosion and leaching in this tropical region increase dramatically as slope incline increases. A sediment core was taken 55 m water depth off western Kenya (core at station 103) and the diatom and chemical stratigraphy analyzed [9]. The dry weight sedimentation rate increased after 1960 to 90 gm−<sup>2</sup> yr.−<sup>1</sup> from 57 gm−<sup>2</sup> yr.−<sup>1</sup> before then. There is an interpretable sequence which begins with increasing production of *Cyclostephanos* concurrent with an increase in C and N deposition early in the century [9]. These trends continued until the decade of the 1960s when P and biogenic Si deposition began a rapid increase. By the late 1960s, *Melosira* spp. was nearly gone largely replaced in the diatom community by *Nitzschia*. By the late 1970s, modern rates of C, N, P, and Si deposition were established as was the modern diatom community [9]. The increased deposition rates of N beginning in the 1920s and P beginning in the 1950s were likely results of watershed and airshed disturbance [9]. Primary production was extremely high as lake level rose in its first 500 years, nourished by the high input of nutrients from the flooded landscape [14].

sorbed or held in sediments [27]. Sediments therefore serve are useful long-term records of the material influx and provide historical records for evaluating past and other ecological changes occurring in the lake. However, very few studies have been concentrated in the gulf, as the best depositional basins for provision of long-term records are in the main lake. Therefore, continuous provision of data on the sedimentary environment is valuable in understanding processes within these areas and their influences on the above water quality, considering the fact that the

The six coring stations, representative of the main lake depositional basin, were located at a water depth range between 48 and 68 m [60], but none was located in the Nyanza Gulf, where the depths are below 60 m. Accumulation of fine-grained Holocene sediments in offshore regions of Lake Victoria is restricted to the deep east-central basin floor [16, 59]. However, surficial sediment deposits the deep gulf, and open stations contain similar organic rich muds. Verschuren et al. [60] noted that with increasing distance from the depositional centre, progressively thinner sheets of Holocene sediments occur, for example, about 5.4 m at station V95-1G and 3.6 m at V96-1MC (eastern central basin), reflecting bottom dynamics that become less and less favorable for undisturbed sediment accumulation. According to Verschuren et al. [60], in the transect of stations examined, greater physical biological sediment mixing at shallower depths is evident in the lack of flocculent surface muds in cores V96-1MC and V96-8MC (western central basin). In these shallow sites, organic matter decreases from 32% at the top of the short sediment core surface, to 23% at bottom of core compared to a change of 14% (top of core) to 10% (end of core). The deepest site short cores at 58 m and 68 m water

constant organic matter contents along the sediment core (35–38% near top surface of the core and 33–35% end of core). The core closer to the extreme western side of the open lake portion

Rivers contribute a significant load of suspended materials annually into the lake. Although it is not possible to provide current rates of resuspended loads, the inputs of organic matter into the depositional areas seem significant from the productive water column (autochthonous sources). Johnson [92] summarized published sedimentation rates for several lakes. There is surprisingly little difference between the sedimentation rates observed in tectonic versus glacial lakes, even though the relief, and hence, the sediment supply rate per unit area of drainage basin should be higher around tectonic lakes. Verschuren et al. [60] established average recent sedimentation accumulation rates of 0.032 ± 0.001 to 0.028 ± 0.001 gcm−<sup>2</sup> yr.−<sup>1</sup>

210Pb dating of sediment cores in deep depositional sites in the main lake. The rates are lower but comparable with other variable rates reported for other temperate lakes [93]. Variable sedimentation rates are reported in different types of lakes, but the mean values reported by Verschuren et al. [60] for the lake appear to fall within the low end values for other lakes. In L. Erie, sedimentation rates [94, 95] estimated from 35 stations varied from less than 20

Although soil types and land uses vary in different drainage basins, the surrounding gulf areas are under the same typical tropical wet climate. Lindenschmidt et al. [96] in a neighbouring rural

to 0.7 mgl−<sup>1</sup>

with an average of 230 mgcm−<sup>2</sup> yr.−<sup>1</sup>

), with high and near

from

.

area receives significant inputs from the inflowing rivers and nearby human activities.

depth had a sub-oxic sediment–water interface (0.0 mgl−<sup>1</sup>

**3.4. Sedimentation and nutrients in Lake Victoria**

mgcm−<sup>2</sup> yr.−<sup>1</sup>

50 Persistent Organic Pollutants

of the Nyanza Gulf contained 27% (top) and 25% (end of core).

to greater than 1000 mgcm−<sup>2</sup> yr.−<sup>1</sup>

Concentrations of P in the gulf lake water (2000–2002) were found to be different from those in the main lake with phosphorus fractions, soluble reactive P, and total P being significantly higher in the main lake than in the gulf [53]. Well-oxygenated conditions in the gulf keep the PO4 -P strongly bound to mineral particles, whereas in the main lake, where deeper depths allows for development of anoxia, it is released into solution [53]. In 2005 and 2006, an assessment of the potential for sediments to contribute to the water column P concentrations in Lake Victoria showed that sediment total TP, apatite phosphorus, inorganic phosphorus, and organic phosphorus increased in sediments along the gulf towards the main lake, while the non-apatite inorganic phosphorus (NAIP) increases were less defined [65]. The longitudinal gradient of sediment TP and its fractions in Nyanza Gulf is a result of high rates of terrigenous input and resuspension and transport of the light, phosphorus-rich inorganic and organic matter towards the main lake [64]. The non-apatite inorganic P concentration on the western end of the gulf is exceptionally high, >1500 mgkg−<sup>1</sup> [66]. Total P in the sediment ranged from 812.7 to 1738 mgkg−<sup>1</sup> dry weight and was highest in the Rusinga Channel, the exchange zone between the gulf and the main lake. In the gulf and the Rusinga Channel, the less bioavailable apatite phosphorus dominated, whereas in the deeper main lake, organic P was the major fraction illustrating the importance of anaerobic release of P from sediments and acceleration of internal P loading in the main lake [65].

Chemical processes within the sediments play an important role in the P fluxes within and from sediments. In aquatic environment dissolved phosphate is consumed during growth of phytoplankton and is regenerated during bacterial decomposition of organic matter [91]. Much of the regeneration takes place in the water, but in relatively shallow environments such as lakes, estuaries, and continental shelves, sediments may play an important role in the regeneration of phosphate [97]. The regenerated phosphate may be released to the overlying water, reprecipitated within the sediment as authigenic phase or adsorbed by other constituents of sediment. Adsorption on metal oxides in the sediment has been identified as one of the principal reactions involving phosphate [90]. Mortimer [98] has described the oxidized surface layer as a trap for phosphate. The reactions that release phosphate to the porewater are desorption of phosphate from surface sites on sediment particles, mineralization of organic matter, and reduction of iron oxides in the anoxic zone of the sediments [99].

#### **3.5. Inferences from stable isotope signatures in lake sediments and their applications**

Organic rich sediments have been reported in sediments from depositional basins of the main Lake Victoria [100, 101]. Previous studies show a slight variability in SOM contents (**Figures 11** and **12**). Mean sedimentation rates, and organic carbon (total core length mean) of sediment core 103 (core recovered at 56 m water depth in 1990 by Hecky) [9], 96–5 MC (core recovered at 68 m water depth in 1996 by Verschuren et al. [102], and Itome (core recovered at 25 m water depth in 1995 by Campbell et al. [103] were 100, 320, and 276 gm−<sup>2</sup> yr.−<sup>1</sup> and 168, 204, and 184 mgg−<sup>1</sup> dry weight, respectively. A similar trend in sediment organic matter emerged in surficial sediments in the relatively shallower Nyanza Gulf, with significant spatial differences especially in the zones with significant influence of externally derived riverine inputs. According to Hecky et al. [100], the isotopic analysis of all the three sediment cores recovered by Hecky [9], Verschuren et al. [102], and Campbell et al. [103] recorded an increase in the isotopic signature of the sedimentary organic matter (from a baseline −22‰ to −21‰ in the deepest core and no clear trends for inshore Itome core, which was significantly δ<sup>13</sup>C enriched). According to Hecky et al. [100], a Mwanza-Port Bell (southern shore-northern shore) surface water transect in October 1995, of main *L. Victoria*, show highly correlated POC and PN (r = 0.96) with a mean molar C/N ratio of 7.2 with higher ratios nearshore (up to 8.4). These low, near Redfield ratios, values for C:N are characteristic of Lake Victoria [104] and are indicative that the particulate matter sampled was of algal origin. Isotopically depleted PC occurred offshore, and δ<sup>13</sup>C was increasingly enriched nearer the coasts as depth shoaled. The δ15N of the PN showed an inverse pattern with highest δ15N values observed offshore.

Consequently, there was a strong inverse relationship between δ<sup>15</sup>N and δ<sup>13</sup>C, with the former decreasing at high algal abundances and the latter increasing as PC increased. The scatter of the isotopic signatures with increasing algal abundances indicates that these relationships

Lake Sedimentary Environments and Roles of Accumulating Organic Matter in Biogeochemical…

Nyanza Gulf is more active in terms of exchanges between the land and the lake, when compared to the offshore depositional basins. Again, exports of organic rich waters from urbanized areas act as important modifiers of sedimentary organic matter derived from in-lake

A major portion of the organic matter in freshwater systems originates from the terrestrial environment. The bulk of allochthonous organic matter consists of structurally complex polysaccharides, lignocelluloses, and other complex organic compounds [105]. Autochthonous production is also an important source of organic matter in lakes and is generally less refractory than allochthonous carbon [106, 107]. Autochthonous organic matter is primarily derived from phytoplankton and macrophytes, but, in shallow clear lakes of Nyanza Gulf from April 1994 to March 1995 from different surveys conducted in Nyanza Gulf between April 1994 to 2012 where light penetrates to

Littoral sediments are often heterogeneous because of the presence of rooted macrophytes and the resuspension of particles in shallow water during windy periods, and the net sedimentation of organic matter is small because of alternating sedimentation and resuspension episodes [36]. In addition there can be a significant downward transportation of material along the lake bottom resulting in deposition of organic matter in the profundal sediments [36]. Submerged aquatic macrophytes are generally soft in nature, moderately rich in protein, and are preferred by different herbivorous fish. The production of littoral and pelagic phytoplankton and of the dominant biomass of submerged macrophytes and epiphytes in littoral

Much of the organic matter produced by these larger aquatic plants remains in the wetlands and littoral zones of lakes and undergo decomposition [20]. During senescence and after

and DIN [100]. The

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53

are dynamic in nature responding to the availability and demand for CO<sup>2</sup>

**Figure 12.** Mean sediment organic matter contents (±SD) in surficial sediments.

**3.6. Significance of floating water hyacinth mats in Lake Victoria on sediment** 

the bottom, benthic micro-algae can also be an important source of detritus [106].

zone contributes to the total sediment organic matter accumulated over time.

productivity and higher plant production.

**organic matter accumulation**

**Figure 11.** Monthly sediment organic matter contents in surficial sediments.

Lake Sedimentary Environments and Roles of Accumulating Organic Matter in Biogeochemical… http://dx.doi.org/10.5772/intechopen.79395 53

**Figure 12.** Mean sediment organic matter contents (±SD) in surficial sediments.

Consequently, there was a strong inverse relationship between δ<sup>15</sup>N and δ<sup>13</sup>C, with the former decreasing at high algal abundances and the latter increasing as PC increased. The scatter of the isotopic signatures with increasing algal abundances indicates that these relationships are dynamic in nature responding to the availability and demand for CO<sup>2</sup> and DIN [100]. The Nyanza Gulf is more active in terms of exchanges between the land and the lake, when compared to the offshore depositional basins. Again, exports of organic rich waters from urbanized areas act as important modifiers of sedimentary organic matter derived from in-lake productivity and higher plant production.

#### **3.6. Significance of floating water hyacinth mats in Lake Victoria on sediment organic matter accumulation**

A major portion of the organic matter in freshwater systems originates from the terrestrial environment. The bulk of allochthonous organic matter consists of structurally complex polysaccharides, lignocelluloses, and other complex organic compounds [105]. Autochthonous production is also an important source of organic matter in lakes and is generally less refractory than allochthonous carbon [106, 107]. Autochthonous organic matter is primarily derived from phytoplankton and macrophytes, but, in shallow clear lakes of Nyanza Gulf from April 1994 to March 1995 from different surveys conducted in Nyanza Gulf between April 1994 to 2012 where light penetrates to the bottom, benthic micro-algae can also be an important source of detritus [106].

Littoral sediments are often heterogeneous because of the presence of rooted macrophytes and the resuspension of particles in shallow water during windy periods, and the net sedimentation of organic matter is small because of alternating sedimentation and resuspension episodes [36]. In addition there can be a significant downward transportation of material along the lake bottom resulting in deposition of organic matter in the profundal sediments [36]. Submerged aquatic macrophytes are generally soft in nature, moderately rich in protein, and are preferred by different herbivorous fish. The production of littoral and pelagic phytoplankton and of the dominant biomass of submerged macrophytes and epiphytes in littoral zone contributes to the total sediment organic matter accumulated over time.

Much of the organic matter produced by these larger aquatic plants remains in the wetlands and littoral zones of lakes and undergo decomposition [20]. During senescence and after

**Figure 11.** Monthly sediment organic matter contents in surficial sediments.

**3.5. Inferences from stable isotope signatures in lake sediments and their** 

Organic rich sediments have been reported in sediments from depositional basins of the main Lake Victoria [100, 101]. Previous studies show a slight variability in SOM contents (**Figures 11** and **12**). Mean sedimentation rates, and organic carbon (total core length mean) of sediment core 103 (core recovered at 56 m water depth in 1990 by Hecky) [9], 96–5 MC (core recovered at 68 m water depth in 1996 by Verschuren et al. [102], and Itome (core recovered at 25 m water depth in 1995 by Campbell et al. [103] were 100, 320, and 276 gm−<sup>2</sup> yr.−<sup>1</sup>

ter emerged in surficial sediments in the relatively shallower Nyanza Gulf, with significant spatial differences especially in the zones with significant influence of externally derived riverine inputs. According to Hecky et al. [100], the isotopic analysis of all the three sediment cores recovered by Hecky [9], Verschuren et al. [102], and Campbell et al. [103] recorded an increase in the isotopic signature of the sedimentary organic matter (from a baseline −22‰ to −21‰ in the deepest core and no clear trends for inshore Itome core, which was significantly δ<sup>13</sup>C enriched). According to Hecky et al. [100], a Mwanza-Port Bell (southern shore-northern shore) surface water transect in October 1995, of main *L. Victoria*, show highly correlated POC and PN (r = 0.96) with a mean molar C/N ratio of 7.2 with higher ratios nearshore (up to 8.4). These low, near Redfield ratios, values for C:N are characteristic of Lake Victoria [104] and are indicative that the particulate matter sampled was of algal origin. Isotopically depleted PC occurred offshore, and δ<sup>13</sup>C was increasingly enriched nearer the coasts as depth shoaled. The δ15N of the PN showed an inverse pattern with highest δ15N values observed offshore.

dry weight, respectively. A similar trend in sediment organic mat-

and

**applications**

52 Persistent Organic Pollutants

168, 204, and 184 mgg−<sup>1</sup>

death of organisms, much of the organic matter is released as soluble compounds. The particulate components decompose at various rates depending on their location, composition, and environmental conditions, particularly those of temperature and oxygen availability. When decomposing tissue falls to the sediments in detrital masses, the environmental of the aggregates rapidly becomes anaerobic. Under these reducing conditions, rates of decomposition are decreased greatly [20].

Water hyacinth has contributed to several to various socio-economic impacts in the lake fishery, such as lake transportation, artisanal fishing. Also, Ofulla et al. [108] study on associations between aquatic macrophytes and vector snails for schistosomiasis illustrated that *B. sudanica* and *B. africanus*, the two most common snail hosts of schistosomiasis in the Nyanza Gulf, were found associated with the aquatic macrophytes in the lake waters. Besides this, a form of ecological succession (the progressive displacement of one or more species of plants by other species) has been observed in Lake Victoria, in which stationary mats of water hyacinth along the shores and banks of rivers were replaced by other aquatic plants such as hippo grass (*Vossia cuspidata*) and other aquatic sedges such as *Cyperus papyrus* and climbing plants such as *Ipomoea aquatica* [109–113], creating concern among stakeholders in the region. The dispersal and continuous presence of water hyacinth is therefore still a concern in *L. Victoria* since the 1990s. Kisumu Bay, Homa Bay, Asembo Bay, Luangwa Gembe, off Sondu-Miriu river-mouth, off Kibos river-mouth, and Dunga are in-lake areas commonly found associated with macrophytes, including water hyacinth as observed by Ofulla et al. [109]. Besides this, large *Cyperus papyrus* swamps are found around major river-mouths of Sondu-Miriu, Nyando, Kisumu Bay, Kibos river-mouth, and Dunga areas. Persistence of extensive floating mats of water hyacinth and other macrophytes in major bays of Nyanza Gulf (**Plates 1a, 1b,** and **2a**) has the potential to introduce a lot of plant debris and decaying particulate organic materials into the lake sediments (**Plates 2b** and **2c**).

#### **3.7. Are future potential impacts from cage culture on lake sediments of concerns?**

In recent years, aquaculture production has increased worldwide, mainly due to the increasing demand from aquaculture produce and the need for improved food security. These developments are supposed to improve income and livelihoods but can generate negative

impacts such as pollution, landscape modification, or biodiversity change, if best aquaculture practices are not followed. However, it should be recognized that to date the majority of aquaculture practices have had few adverse effects on ecosystem [114]. First studies on impact of fish cage operations on surrounding water environment in L. Malawi [115, 116] were minimal despite substantial discharges from the cages, due to dispersion by water currents and aggregation of wild fish species feeding on the wastes. Experimental studies on cage culture from Uganda and Tanzanian side of *L. Victoria* found no consistent environmental changes using water quality parameters, phytoplankton and macro-invertebrates [117, 118]. Effect of nutrient discharge on DO was not pronounced. Nevertheless, some cases of environmental degradation in coastal areas have occurred due to, for example, intensive cage culture operations in Europe and shrimp farming practices in Southeast Asia and Latin America [114, 119]. Fecal contamination implies a high risk of contracting waterborne diseases if the water is used for drinking purposes without pretreatment. *Escherichia coli* (*E. coli*) is used as an indicator for human and animal fecal pollution of water. Urban sewage and industrial effluent [120]

**Plate 2.** Undegraded water hyacinth and other macrophytes (a), retrieved from a trawl net and freshly retrieved surficial sediments from shallow riverine zones (b) and from deep open lake areas (c) using a Ponar grab sampler, in Nyanza gulf

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of Lake Victoria in June–July 2017.

**Plate 1.** Water hyacinth and other macrophytes in Nyanza gulf of Lake Victoria in July 2012 (a) and June–July 2017 (b).

Lake Sedimentary Environments and Roles of Accumulating Organic Matter in Biogeochemical… http://dx.doi.org/10.5772/intechopen.79395 55

death of organisms, much of the organic matter is released as soluble compounds. The particulate components decompose at various rates depending on their location, composition, and environmental conditions, particularly those of temperature and oxygen availability. When decomposing tissue falls to the sediments in detrital masses, the environmental of the aggregates rapidly becomes anaerobic. Under these reducing conditions, rates of decomposi-

Water hyacinth has contributed to several to various socio-economic impacts in the lake fishery, such as lake transportation, artisanal fishing. Also, Ofulla et al. [108] study on associations between aquatic macrophytes and vector snails for schistosomiasis illustrated that *B. sudanica* and *B. africanus*, the two most common snail hosts of schistosomiasis in the Nyanza Gulf, were found associated with the aquatic macrophytes in the lake waters. Besides this, a form of ecological succession (the progressive displacement of one or more species of plants by other species) has been observed in Lake Victoria, in which stationary mats of water hyacinth along the shores and banks of rivers were replaced by other aquatic plants such as hippo grass (*Vossia cuspidata*) and other aquatic sedges such as *Cyperus papyrus* and climbing plants such as *Ipomoea aquatica* [109–113], creating concern among stakeholders in the region. The dispersal and continuous presence of water hyacinth is therefore still a concern in *L. Victoria* since the 1990s. Kisumu Bay, Homa Bay, Asembo Bay, Luangwa Gembe, off Sondu-Miriu river-mouth, off Kibos river-mouth, and Dunga are in-lake areas commonly found associated with macrophytes, including water hyacinth as observed by Ofulla et al. [109]. Besides this, large *Cyperus papyrus* swamps are found around major river-mouths of Sondu-Miriu, Nyando, Kisumu Bay, Kibos river-mouth, and Dunga areas. Persistence of extensive floating mats of water hyacinth and other macrophytes in major bays of Nyanza Gulf (**Plates 1a, 1b,** and **2a**) has the potential to introduce a lot of plant debris and decaying particulate organic

**3.7. Are future potential impacts from cage culture on lake sediments of concerns?**

In recent years, aquaculture production has increased worldwide, mainly due to the increasing demand from aquaculture produce and the need for improved food security. These developments are supposed to improve income and livelihoods but can generate negative

**Plate 1.** Water hyacinth and other macrophytes in Nyanza gulf of Lake Victoria in July 2012 (a) and June–July 2017 (b).

tion are decreased greatly [20].

54 Persistent Organic Pollutants

materials into the lake sediments (**Plates 2b** and **2c**).

**Plate 2.** Undegraded water hyacinth and other macrophytes (a), retrieved from a trawl net and freshly retrieved surficial sediments from shallow riverine zones (b) and from deep open lake areas (c) using a Ponar grab sampler, in Nyanza gulf of Lake Victoria in June–July 2017.

impacts such as pollution, landscape modification, or biodiversity change, if best aquaculture practices are not followed. However, it should be recognized that to date the majority of aquaculture practices have had few adverse effects on ecosystem [114]. First studies on impact of fish cage operations on surrounding water environment in L. Malawi [115, 116] were minimal despite substantial discharges from the cages, due to dispersion by water currents and aggregation of wild fish species feeding on the wastes. Experimental studies on cage culture from Uganda and Tanzanian side of *L. Victoria* found no consistent environmental changes using water quality parameters, phytoplankton and macro-invertebrates [117, 118]. Effect of nutrient discharge on DO was not pronounced. Nevertheless, some cases of environmental degradation in coastal areas have occurred due to, for example, intensive cage culture operations in Europe and shrimp farming practices in Southeast Asia and Latin America [114, 119].

Fecal contamination implies a high risk of contracting waterborne diseases if the water is used for drinking purposes without pretreatment. *Escherichia coli* (*E. coli*) is used as an indicator for human and animal fecal pollution of water. Urban sewage and industrial effluent [120] around Kisumu and Homa Bay areas are considered more vulnerable to fecal contamination, than open lake sites. Safe water quality implies low risk from bacteriological pollution and acceptable properties in terms of chemical, oduor, and taste [121]. Under the World Health Organization (WHO) guidelines, the bacterium *E. coli* should not be detected in a given 100 ml sample of drinking water [122]. Increased nutrients from atmosphere, land and fecal contamination, fuel the growth, and proliferation of algal blooms in surface water. Cyanobacteria are potentially toxic to humans and animals and can also degrade the ecological and esthetic values of water. From previous phytoplankton surveys in Kisumu Bay of the Nyanza Gulf of *L. Victoria* (Kenya), cyanobacteria were the most dominant, contributing 50% to the total phytoplankton biovolume. The highest MC concentrations were recorded between November and March coinciding with the wet season when rainfall and nutrient enrichment from the catchment increased [13]. Algal blooms were thicker and more frequent in Murchison Bay than Napoleon Gulf. Highly toxic blue-green algal scums (*Microcystis* and *Anabaena* spp.) and water hyacinth (*Eichhornia crassipes*) congregated along the shores of the city of Gaba (Uganda) water intake sites. Microcystin levels were between <0.5 μgL−<sup>1</sup> and 3 μgL−<sup>1</sup> [123]. In bays of *L. Victoria* (Tanzania), the level of water exchange from individual bays to the main basin is an important factor influencing eutrophication and microcystin production in nearshore habitats [124]. Microcystins were found in closed bay sites, and concentrations ranged from 0.4 to 13 μg l −1 microcystin-LR equivalent and coincided with high abundance of *Microcystis* spp.

is readily available and is commonly used at farms in response to outbreaks. However, the amount of antibiotics released depends upon the fish species, amount of feeding activity, and absorption in the fish digestive tract [125]. The most obvious detrimental effect of extensive use of antimicrobials in aquaculture is selection of fish and shellfish pathogens resistant to multiple antimicrobials. Antimicrobial resistance determinants in piscine pathogens could also be acquired from environmental antimicrobial-resistant bacteria that have been selected by residual antimicrobials in water and sediments [130, 131]. Considerations suggest that excessive aquacultural use of antimicrobials may potentially have major effects on animal and human health as well as on the environment. There are no detailed assessments to provide

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information on amounts of antimicrobials in use aquaculture and potential effects.

surface waters.

lake environment.

**4. Conclusion**

storage in dams and shallow lakes.

Biofouling adds weight to nets and equipment, and it changes hydrodynamics of fish cage systems. Chemical antifoulants are used to control or eliminate the growth of marine organisms which attach to aquaculture cages, ropes, and structures [125], and hence their toxic effects on other nontarget organisms around fish farms are of concern. Heavy and persistent biofouling impedes water flow through cages, increases BOD in cages, causes net drag, and can shorten the useful life of nets and ropes [132, 126]. Finally, as documented in a recent study by Biginagwa et al. [133], concerns are emerging on the possible effects of microplastics on fish and as sources of organic contaminants in the lake, although there are increasing public awareness campaigns of uncontrolled disposal of all types of plastics in

The cage culture industry is at its infancy in Kenya but with a huge interest in the technology. Currently in *L. Victoria*, cage farms are operational. It is expected that increasing investment in cages (in Kenya, Uganda, and Tanzanian sections of *L. Victoria*) will also create a high demand on feeds and seed, with extensive areas under farms in this transboundary ecosystem. In marine aquaculture, there are many lessons learnt with regards to negative environmental impacts from cage farms. Although there are no incidents of contamination from such activities, it is therefore prudent to ensure full implementation of guidelines on best management practices and awareness creation to ensure cage operations and other farm activities promote a sustainable fishery as a whole. This calls for frequent monitoring of the

Sediments eroded from watersheds are important determinant factors in water quality and integrated water resource management. Increased concentrations in surface water causes increased turbidity, reduced under water light transmission and siltation, and reduced water

Maintaining good sediment and water quality today is prioritized due to human health related concerns from faecal contamination from untreated and partially treated domestic wastewaters; discharge of industrial chemical effluents; increased use of pesticides, fertilizers, and herbicides for crop production and livestock rearing; and persistent organic pollutants

and pharmaceutical residues which are endocrine disruptors and carcinogens.

Particulate matter fluxes into the bottom sediments remove and incorporate particulate-bound metals from the water column, where they can become permanently buried or reintroduced back through sediment resuspension, especially in shallower areas. Similarly types of feed and sources are important factors which can contribute to accumulation of non-essential metals and other organics in aquatic food chains and caged fish. Supplementary fish diets improve fish nutrition. However, understanding how dietary MeHg affects its bioaccumulation in caged fish fed from marine based feeds is unknown but of importance. Variations in sediment metal contents have been reported in the lake from different surveys in 2010, 2012, and 2013. Total mercury concentration in sediments fell within the range of values recorded elsewhere but with low ng/l concentrations in the water. There is paucity of data on Hg contents in biota from the gulf, apart from initial results by Campbell et al. [103]. Recent research indicates that heavy metals do tend to accumulate in the sediments below fish farms. However, most studies have found that concentrations are within acceptable environmental guidelines even at farms that have been in production for many years [125]. Toxic metals that bioaccumulate and magnify along food chains are a concern to human health worldwide. Improvement in feed formulations is expected to decrease Zn loading to the marine environment, as many manufacturers are adding lower amounts of a more available form, zinc methionine [126]. Recently moderate to high levels of sum of seven PCB concentration were provided in sediments and fish, with median values of 2.2 to 96.3 μgKg−<sup>1</sup> dry weight and 300 to 3000 μgKg−<sup>1</sup> lipid weight, respectively. Fish cestode endoparasites biomagnified PCB levels than levels in fish hosts. These levels were found to be comparable with ranges of the PCB values found in other places of the world [127]. In a recent study, Abong'o et al. [128] indicates occurrences of some OCs around island waters of *L. Victoria*, supporting previous reports of use of the same pesticides in the wider catchment by Musa et al. [129]. Antibiotics and therapeutants are administered to recover sick fish, but this has declined in marine aquaculture. Commercially medicated feed is readily available and is commonly used at farms in response to outbreaks. However, the amount of antibiotics released depends upon the fish species, amount of feeding activity, and absorption in the fish digestive tract [125]. The most obvious detrimental effect of extensive use of antimicrobials in aquaculture is selection of fish and shellfish pathogens resistant to multiple antimicrobials. Antimicrobial resistance determinants in piscine pathogens could also be acquired from environmental antimicrobial-resistant bacteria that have been selected by residual antimicrobials in water and sediments [130, 131]. Considerations suggest that excessive aquacultural use of antimicrobials may potentially have major effects on animal and human health as well as on the environment. There are no detailed assessments to provide information on amounts of antimicrobials in use aquaculture and potential effects.

Biofouling adds weight to nets and equipment, and it changes hydrodynamics of fish cage systems. Chemical antifoulants are used to control or eliminate the growth of marine organisms which attach to aquaculture cages, ropes, and structures [125], and hence their toxic effects on other nontarget organisms around fish farms are of concern. Heavy and persistent biofouling impedes water flow through cages, increases BOD in cages, causes net drag, and can shorten the useful life of nets and ropes [132, 126]. Finally, as documented in a recent study by Biginagwa et al. [133], concerns are emerging on the possible effects of microplastics on fish and as sources of organic contaminants in the lake, although there are increasing public awareness campaigns of uncontrolled disposal of all types of plastics in surface waters.

The cage culture industry is at its infancy in Kenya but with a huge interest in the technology. Currently in *L. Victoria*, cage farms are operational. It is expected that increasing investment in cages (in Kenya, Uganda, and Tanzanian sections of *L. Victoria*) will also create a high demand on feeds and seed, with extensive areas under farms in this transboundary ecosystem. In marine aquaculture, there are many lessons learnt with regards to negative environmental impacts from cage farms. Although there are no incidents of contamination from such activities, it is therefore prudent to ensure full implementation of guidelines on best management practices and awareness creation to ensure cage operations and other farm activities promote a sustainable fishery as a whole. This calls for frequent monitoring of the lake environment.
