**2. Sources of organic matter sources in lake sediments**

Lake basins are receptors of different types of dissolved and suspended materials transported by inflowing water, above lake precipitation, and blowing wind. In-lake productivity processes also contribute a significant amount of organic matter and mineral elements, which enter the lake biogeochemical cycles.

Organic matter in aquatic ecosystems ranges from dissolved organic compounds to large aggregates of particulate organic matter and from living to dead materials [20]. Nearly all of the organic carbon in natural waters is in the form of dissolved organic carbon (DOC), colloidal organic carbon (COC), and dead particulate organic carbon (POC). Metabolism of the organic matter and interactions of this material chemically and biologically are, to a significant extent, governed by the size and chemical composition of the organic matter [20]. The productivity of aquatic macrophytes of the wetland and littoral land-water interface zones constitute a major source of organic matter input for a majority of the lakes of the world [20]. A central role in the underwater light climate is played by dissolved organic matter which is present in all aquatic ecosystems. Chromophoric dissolved organic matter (CDOM) originates from the degradation process of organic materials, usually macrophytes and planktons. Recently, using fluorometer data, Alexander and Imberger [21] characterized phytoplankton patchness in the Nyanza Gulf as indicators of phytoplankton diversity and CDOM, while Loiselle et al. [22] determined the spatial dynamics of CDOM of three major bays on the Kenyan and Ugandan shores of Lake Victoria and developed a model to estimate its flow from these bays to the lake.

Lake sediments are comprised mainly of clastic material (sediment of clay, silt, and sand sizes), organic debris, chemical precipitates, or combinations of these. The relative abundance of each depends upon the nature of the local drainage basin, the climate, and the relative age of a lake. Sediments are deposited at different rates in different aquatic environments. Deposition rates vary from site to site, but typical values for the rate at which sediments accumulate in freshwater lake sediments are 1 mm/year (or 1 m per 1000 years). When a watershed drains into a lake, accumulated sediments have the potential to yield information about historic rates of sedimentation that can be used to evaluate the effectiveness of previous erosion control measures [23].

The active lake sediment layer (top layer) forms the exchange zone between sediments and overlying water column. It plays an important role in the biological system; however, it can become anoxic in deeper areas, hence influencing the chemical equilibria and transformations.

#### Degradation of decaying organic matter and organic rich waste discharges can lead to decreases in DO concentrations as a result of the increased microbial activity (respiration). In severe cases of reduced oxygen concentrations (whether natural or man-made), anaerobic conditions can occur (i.e. 0 mg l −1 of oxygen), particularly close to the sediment–water interface as a result of decaying, sedimenting material. Dissolved oxygen levels influence the redox potentials (Eh) and different chemical processes in the aquatic habitats and the distribution of fish species. This is explained by Njiru et al., [24] who explored the impacts of hypoxic conditions to the fishery in Lake Victoria, where, from acoustic data, Nile perch catches increased with increase in DO concentrations and no fish were caught when DO was below 2.5 mg/L−<sup>1</sup> .

Organic matter constitutes a minor but important fraction of lake sediments. It is made-up of a complex mixture of lipids, carbohydrates, proteins, and other biochemicals contained in the tissues of living benthic micro-organisms and contributed from the detritus of organisms formerly living in the lake and its watershed. Humic substances are diagenetically formed from these biochemical starting materials and constitute the major part of the complex mixture of organic materials [25]. Some natural organic compounds significantly affect the quality of water for certain uses, especially those which depend on organoleptic properties (taste and smell). During chlorination for drinking water disinfection, humic and fulvic acids act as precursor substances in the formation of trihalomethanes such as chloroform. In addition, substances included in aquatic humus determine the speciation of heavy metals and some other pollutants because of their high complexing ability. As a result, humic substances affect the toxicity and mobility of metal complexes [2]. Therefore, measurement of the concentrations of these substances can be important for determining anthropogenic impacts on water bodies. Humus is formed by the chemical and biochemical decomposition of vegetative residues and from the synthetic activity of micro-organisms. It is operationally separated into fulvic and humic acid fractions, each being an aggregate of many organic compounds of different masses. Fulvic acid has molecular masses mostly in the range 300–5000, whereas the dominant masses in humic acid exceed 5000. The relative content of fulvic acid in the dissolved humic substances present in freshwaters is between 60 and 90%. Humic and fulvic acids are fairly stable (i.e. their BOD is low). However, these substances are chemically oxidizable and, therefore, can readily affect the results of COD determinations [2].

Organic matter also forms part of the palaeolimnological record preserved in lake sediments. The different types of biota populating a lake and its watershed produce organic matter having distinctive biochemical compositions. Changes in the community structure of these biota create variations in the amounts and types of organic matter deposited at different times in the history of a lake. Organic matter is consequently a dynamic biogeochemical component of sediments [25].

#### **2.1. Roles of lake sediment organic matter**

Nyanza Gulf, a relatively shallow gulf compared to the main lake basin, is composed of numerous inner bays and receives inflowing water from several major rivers. This makes it a distinctly important habitat for several fish species, which are known to breed in the various vegetated bays and river-mouth areas. The gulf is greatly influenced by catchment activities through river inflows and nearby urban human activities. Besides this, an invasive water weed, the water hyacinth, is commonly found in the whole lakeshores and bays. The plant was first reported from Lake Victoria in 1989, and it began to increase in the early 1990s, until

triggered by nutrient enrichment, and the plant floating mats have persisted up to today.

Lake basins are receptors of different types of dissolved and suspended materials transported by inflowing water, above lake precipitation, and blowing wind. In-lake productivity processes also contribute a significant amount of organic matter and mineral elements, which enter the

Organic matter in aquatic ecosystems ranges from dissolved organic compounds to large aggregates of particulate organic matter and from living to dead materials [20]. Nearly all of the organic carbon in natural waters is in the form of dissolved organic carbon (DOC), colloidal organic carbon (COC), and dead particulate organic carbon (POC). Metabolism of the organic matter and interactions of this material chemically and biologically are, to a significant extent, governed by the size and chemical composition of the organic matter [20]. The productivity of aquatic macrophytes of the wetland and littoral land-water interface zones constitute a major source of organic matter input for a majority of the lakes of the world [20]. A central role in the underwater light climate is played by dissolved organic matter which is present in all aquatic ecosystems. Chromophoric dissolved organic matter (CDOM) originates from the degradation process of organic materials, usually macrophytes and planktons. Recently, using fluorometer data, Alexander and Imberger [21] characterized phytoplankton patchness in the Nyanza Gulf as indicators of phytoplankton diversity and CDOM, while Loiselle et al. [22] determined the spatial dynamics of CDOM of three major bays on the Kenyan and Ugandan shores of Lake

Victoria and developed a model to estimate its flow from these bays to the lake.

Lake sediments are comprised mainly of clastic material (sediment of clay, silt, and sand sizes), organic debris, chemical precipitates, or combinations of these. The relative abundance of each depends upon the nature of the local drainage basin, the climate, and the relative age of a lake. Sediments are deposited at different rates in different aquatic environments. Deposition rates vary from site to site, but typical values for the rate at which sediments accumulate in freshwater lake sediments are 1 mm/year (or 1 m per 1000 years). When a watershed drains into a lake, accumulated sediments have the potential to yield information about historic rates of sedimentation that can be used to evaluate the effectiveness of previous erosion control measures [23]. The active lake sediment layer (top layer) forms the exchange zone between sediments and overlying water column. It plays an important role in the biological system; however, it can become anoxic in deeper areas, hence influencing the chemical equilibria and transformations.

**2. Sources of organic matter sources in lake sediments**

by 1998 [19]. Its early spread and nourishment were thought to be

it infested some 200 km2

34 Persistent Organic Pollutants

lake biogeochemical cycles.

The most important determinants of chemical-sediment association are composition or grainsize distribution, pH, salinity, DO<sup>2</sup> , mineralogical content (clay mineral composition, hydrous iron-manganese oxides, carbonate contents), organic matter, and redox potential.

Concentrations of some inorganic and organic contaminants (PAH, PCBs, OCs, Hg, and alkylated metal forms) can become increasingly high and important sources of exposure, with transfers along lake food chains in the long term. Contaminated sediments are therefore a concern to lake managers of surface lake waters receiving significant amounts of anthropogenic wastes, hence the development and use of sediment quality guidelines in protecting the aquatic fauna and flora. Managing contaminated sediment in place requires a complete chemical, biological, and physical understanding of the fate and mobility of contaminants. Most of them adsorb onto suspended and deposited particles at the bottom, where they form important secondary sources. Soils and sediments are important "sinks" for such contaminants because of their enormous quantities and their abilities to pick up, or sorb, large amounts of a wide variety of contaminants. It is essential to understand the mechanism by which the contaminant is sorbed to soil and sediment [26]. Compared to other natural environmental substrates, sediments have a greater capacity to bind Hg more than 90% of total Hg in sediment-water system which is sorbed or held in sediments [27].

decreases its exposure and transport to other parts of the ecosystem, such as fish and plankton [26]. The way in which contaminants are sorbed into soil or sediment varies with the nature of the contaminant and the makeup of the soil and sediment [38, 39]. The composition of soil and sediment includes both mineral matter and organic matter as the primary constituents. Under relatively dry conditions, the soil/sediment mineral matter acts as an adsorbent, where the sorbed organic compounds are held on the surface of the mineral grains. The soil/sediment organic matter (SOM) acts as an absorbent or a partition medium, where the sorbed organic compounds dissolve (partition) into the matrix of the entire SOM. The soil or sediment, then, is characterized as a dual-function sorbent, in which the mineral matter sorbs the contaminant by adsorption, while the SOM sorbs the contaminant by a partition process [40–42]. Because so many environmental contaminants are transported by groundwater and surface water, it is important to understand the unique function of the soil organic matter within these aquatic systems and how the partition processes affect the fate of common environmental contaminants.

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The organic carbon partitioning coefficient, Koc, data of both carbon tetrachloride (CT) and 1,2-dichlorobenzene on most normal soils from shallow depths are shown to be quite invariant, suggesting that soil organic matter at such depths from diverse geographic sources maintains a comparable polar-to-nonpolar balance and possibly a comparable composition [26]. However, soil samples are genuinely different from bed-sediment samples in terms of their Koc values. The average Koc values for nonpolar solutes on bed sediments are about twice those on soils, suggesting that sediment organic matter is in general less polar in nature than soil organic matter. This unique geochemical disparity has not been documented until now [26]. The observed difference in Koc between the soil and bed-sediment samples must be taken into account in future studies. Because it has now been shown that Koc values for soils are largely consistent worldwide, just as Koc values for sediments are largely consistent worldwide, Koc values can be used to assess the sorption of nonpolar contaminants to the organic matter of different soils/sediments. This finding should result in considerable savings in cost and time for contamination studies [26].

Although suspended sediment as a whole can be examined as a transport mechanism for trace metals, additional insight can be gained from a knowledge of the particle-size distribution. The most significant factor controlling sediment capacity for retaining trace metals is grain size, according to Jenne et al. [43]. The correlation between decreasing grain size and increasing trace-metal concentrations is significant. This correlation is a result of factors, both physical and chemical, that most notably are related to the large surface area per unit weight of fine particles (such as clays) compared to larger particles. According to Horowitz [44] the increase

A recent view of the role of clays as metal concentrators implies that clays function as substrates for the precipitation and flocculation of organic matter and secondary minerals such as hydrous iron and manganese oxides. Those secondary minerals, in turn, become substrates for the accumulation of trace metals rather than the clays themselves, according to Jenne [45]. There is some debate whether accumulation takes place by organics and secondary minerals or whether there is direct physical and chemical interaction with the clays. However, there is general agreement that the largest concentrations of metals are present with the clays and that the following clay minerals have a decreasing order of association with trace metals: montmo

commonly is more than a hundredfold.

rillonite>vermiculite>illite, chlorite>kaolinite.

Stable isotope signatures (δ<sup>13</sup>C and δ15N) of organic matter in lake sediment cores have been used as indicators of historic lacustrine productivity [28, 29] and past nitrogen sources [28, 30], respectively. Total carbon (TC) to total nitrogen (TN) ratios of OM can be used to assess the relative contributions of higher plants versus phytoplankton to sediments [31–34]. In *L. Victoria* (Tanzania), the potential to discriminate between input sources of organic material into lakes using stable isotope signals in sediment and suspended particulate matter was demonstrated, where stable C and N ratios showed that inshore areas in bays without pronounced fluvial input were dominated by autochthonous planktonic organic matter; conversely, for bays receiving significant fluvial inflows, the composition of organic matter depended on the extent of preservation of shoreline vegetation [35].

Sediment metabolism (e.g. measured as sediment oxygen uptake) is highly varied and depends on the individual characteristics of a lake. Profundal sediments may have a high organic carbon content (up to 25%) and are more homogenous compared with littoral sediments, with a resultant high sediment metabolism [36]. Organic matter degradation is influenced by the presence of benthic fauna, which enhance the oxidation of the sediments and organic matter cycling by increasing oxygen supply during respiratory pumping [37]. Benthic animals and microbes depend on organic matter for their nutrition. Because many components of organic matter are relatively easily oxidized forms of reduced carbon, the dissolved oxygen concentration of sediment porewater is controlled largely by the availability of oxidizable organic matter below the sediment–water interface [25]. Organic matter so resuspended is re-exposed to oxidation in the water column. Resuspension is usually greater in large, wellmixed lakes than in small lakes. Bioturbation, or biological mixing, of surface sediments also prolongs the exposure of organic matter to oxidation and adds to this the degradation due to the nutritional needs of benthic fauna [25].

#### **2.2. Chemical adsorption to sediments and soil**

Sorption to soils and sediments is probably the most influential factor on the transport and fate of organic contaminants in the environment. The extent of the sorption to soil and sediment affects not only the contaminant level in an ecosystem but the movement and fate of the contaminant as well. For example, in a hydrogeologic system, the increased sorption of a contaminant to soil and sediment reduces its level in the adjacent water column and thus decreases its exposure and transport to other parts of the ecosystem, such as fish and plankton [26]. The way in which contaminants are sorbed into soil or sediment varies with the nature of the contaminant and the makeup of the soil and sediment [38, 39]. The composition of soil and sediment includes both mineral matter and organic matter as the primary constituents. Under relatively dry conditions, the soil/sediment mineral matter acts as an adsorbent, where the sorbed organic compounds are held on the surface of the mineral grains. The soil/sediment organic matter (SOM) acts as an absorbent or a partition medium, where the sorbed organic compounds dissolve (partition) into the matrix of the entire SOM. The soil or sediment, then, is characterized as a dual-function sorbent, in which the mineral matter sorbs the contaminant by adsorption, while the SOM sorbs the contaminant by a partition process [40–42]. Because so many environmental contaminants are transported by groundwater and surface water, it is important to understand the unique function of the soil organic matter within these aquatic systems and how the partition processes affect the fate of common environmental contaminants.

along lake food chains in the long term. Contaminated sediments are therefore a concern to lake managers of surface lake waters receiving significant amounts of anthropogenic wastes, hence the development and use of sediment quality guidelines in protecting the aquatic fauna and flora. Managing contaminated sediment in place requires a complete chemical, biological, and physical understanding of the fate and mobility of contaminants. Most of them adsorb onto suspended and deposited particles at the bottom, where they form important secondary sources. Soils and sediments are important "sinks" for such contaminants because of their enormous quantities and their abilities to pick up, or sorb, large amounts of a wide variety of contaminants. It is essential to understand the mechanism by which the contaminant is sorbed to soil and sediment [26]. Compared to other natural environmental substrates, sediments have a greater capacity to bind Hg more than 90% of total Hg in sediment-water system which is sorbed or held

Stable isotope signatures (δ<sup>13</sup>C and δ15N) of organic matter in lake sediment cores have been used as indicators of historic lacustrine productivity [28, 29] and past nitrogen sources [28, 30], respectively. Total carbon (TC) to total nitrogen (TN) ratios of OM can be used to assess the relative contributions of higher plants versus phytoplankton to sediments [31–34]. In *L. Victoria* (Tanzania), the potential to discriminate between input sources of organic material into lakes using stable isotope signals in sediment and suspended particulate matter was demonstrated, where stable C and N ratios showed that inshore areas in bays without pronounced fluvial input were dominated by autochthonous planktonic organic matter; conversely, for bays receiving significant fluvial inflows, the composition of organic matter depended on the extent of preser-

Sediment metabolism (e.g. measured as sediment oxygen uptake) is highly varied and depends on the individual characteristics of a lake. Profundal sediments may have a high organic carbon content (up to 25%) and are more homogenous compared with littoral sediments, with a resultant high sediment metabolism [36]. Organic matter degradation is influenced by the presence of benthic fauna, which enhance the oxidation of the sediments and organic matter cycling by increasing oxygen supply during respiratory pumping [37]. Benthic animals and microbes depend on organic matter for their nutrition. Because many components of organic matter are relatively easily oxidized forms of reduced carbon, the dissolved oxygen concentration of sediment porewater is controlled largely by the availability of oxidizable organic matter below the sediment–water interface [25]. Organic matter so resuspended is re-exposed to oxidation in the water column. Resuspension is usually greater in large, wellmixed lakes than in small lakes. Bioturbation, or biological mixing, of surface sediments also prolongs the exposure of organic matter to oxidation and adds to this the degradation due to

Sorption to soils and sediments is probably the most influential factor on the transport and fate of organic contaminants in the environment. The extent of the sorption to soil and sediment affects not only the contaminant level in an ecosystem but the movement and fate of the contaminant as well. For example, in a hydrogeologic system, the increased sorption of a contaminant to soil and sediment reduces its level in the adjacent water column and thus

in sediments [27].

36 Persistent Organic Pollutants

vation of shoreline vegetation [35].

the nutritional needs of benthic fauna [25].

**2.2. Chemical adsorption to sediments and soil**

The organic carbon partitioning coefficient, Koc, data of both carbon tetrachloride (CT) and 1,2-dichlorobenzene on most normal soils from shallow depths are shown to be quite invariant, suggesting that soil organic matter at such depths from diverse geographic sources maintains a comparable polar-to-nonpolar balance and possibly a comparable composition [26]. However, soil samples are genuinely different from bed-sediment samples in terms of their Koc values. The average Koc values for nonpolar solutes on bed sediments are about twice those on soils, suggesting that sediment organic matter is in general less polar in nature than soil organic matter. This unique geochemical disparity has not been documented until now [26]. The observed difference in Koc between the soil and bed-sediment samples must be taken into account in future studies. Because it has now been shown that Koc values for soils are largely consistent worldwide, just as Koc values for sediments are largely consistent worldwide, Koc values can be used to assess the sorption of nonpolar contaminants to the organic matter of different soils/sediments. This finding should result in considerable savings in cost and time for contamination studies [26].

Although suspended sediment as a whole can be examined as a transport mechanism for trace metals, additional insight can be gained from a knowledge of the particle-size distribution. The most significant factor controlling sediment capacity for retaining trace metals is grain size, according to Jenne et al. [43]. The correlation between decreasing grain size and increasing trace-metal concentrations is significant. This correlation is a result of factors, both physical and chemical, that most notably are related to the large surface area per unit weight of fine particles (such as clays) compared to larger particles. According to Horowitz [44] the increase commonly is more than a hundredfold.

A recent view of the role of clays as metal concentrators implies that clays function as substrates for the precipitation and flocculation of organic matter and secondary minerals such as hydrous iron and manganese oxides. Those secondary minerals, in turn, become substrates for the accumulation of trace metals rather than the clays themselves, according to Jenne [45]. There is some debate whether accumulation takes place by organics and secondary minerals or whether there is direct physical and chemical interaction with the clays. However, there is general agreement that the largest concentrations of metals are present with the clays and that the following clay minerals have a decreasing order of association with trace metals: montmo rillonite>vermiculite>illite, chlorite>kaolinite.

#### **2.3. Methane production**

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 the final electron acceptor according to the net reaction:

$$\rm{CH}\_4\rm{+SO}\_4^{2-} \rightarrow \rm{HCO}\_3^- + \rm{HS}^- + \rm{H}\_2\rm{O} \tag{1}$$

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

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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

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

from 6.45 NTU to 74.8 NTU and 5.08 NTU to 56.6 NTU, respectively.

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

of OM and processes operating in the C and N cycles [67–69].

**3.1. Why study sedimentary organic matter**

sediment characteristics.

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 higher probability of continued degradation of the lake environment.
