**1. Introduction**

Lakes are important land features forming linkages with the hydrological systems at local and transboundary scales, while providing water sources, supporting fisheries, and providing livelihoods to many surrounding communities and different ecosystem services. Small and large lake depositional basins continuously accumulate dissolved or particulate inorganic and organic materials produced within the lake water column (autochthonous) or transported from the catchment area (allochthonous). However, besides lake production, human developments and urbanization around surface waters (lakes and rivers) are also a source of various solid and nutrient-rich materials depositing and accumulating on the lake sediments. When autochthonous matter is dominant, lake sediments may either be either carbonate-rich or silica-rich due to the accumulation of siliceous diatoms, or they may be mostly organic. In the latter case, the organic carbon content may reach 20–25%, but in peat bogs it may be even higher [1, 2]. Particulate matter scavenges chemical species (organic and inorganic pollutants) and, with other geochemical barriers, influences their mobility in the secondary environments. Therefore, understanding the sediment-chemical species associations has given more insights to the chemical element availability for metabolic processes and toxicity under certain environmental conditions. Knowledge of lake bottom dynamics and sedimentological processes is essential in matters related to geographical distribution and potential ecological effects of contaminants and in many practical issues, such as choice of adequate dumping sites for dredging deposits and choice of sampling sites from aquatic pollution control. Sediment accumulation areas prevail where fine materials, medium silt with grain size less than 0.006 mm, can be deposited continuously; sediment transportation zones appear where there is discontinuous deposition of fine particles/aggregates, and sand erosion zones prevail where there is no deposition of fine materials [3]. The sediments within areas of transportation, for natural reasons, are generally very variable (from sand to loose mud); areas of erosion are most frequent in shallow waters and characterized by hard or consolidated deposits (from bare rocks, gravel, and sand to glacial clays), and the deposits from areas of accumulation are always comparatively loose, with high water and organic contents and sometimes with a high content of pollutants [3]. The Lake Victoria (Kenya) gulf waters are eutrophic, and it is continuously fed by several large and small rivers which traverse the fertile agricultural highland areas and transport high levels of eroded materials. Information from sedimentological studies in *L. Victoria* exists but is inadequate due to lack of new studies. The chapter examines the significance of sediment organic matter (SOM) in freshwater aquatic environments, besides other sediment factors, and uses the spatial concentration of total organic carbon determined in surficial sediments of Lake Victoria (Kenya) to explore whether water hyacinth invasion has any significant influence on SOM. It also gives an opportunity to reflect on the potential environmental impacts which can possibly emanate from the new cage aquaculture developments in most of the shallow African inland lakes, such as *L. Victoria* which has a huge potential for cage culture.

#### **1.1. Lake Victoria**

Lake Victoria, the world's second largest freshwater lake (68,800 km<sup>2</sup> in surface area and mean depth of 35 m) supports the world's largest inland fishery and is a transboundary ecosystem with the Nyanza Gulf, on the extreme eastern edge forming the Kenya portion (**Figure 1**). The gulf connects to the open lake via Rusinga Channel. River water inflows account for about 20% of the total water inputs into the main lake. The catchment area and the drainage basin are covered by grassland savanna, wetland vegetation, agricultural crops, and forested highland and mountain areas. It is characterized by high tropical temperatures and heavy rainfall in the highlands, described in a long (April–June) and short (October–December) wet season and a dry season. The lake hydrology, physicochemical environment, and changes in primary production of the lake and gulf areas are well-

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

http://dx.doi.org/10.5772/intechopen.79395

33

Morphometric characteristics from early investigations in L. Victoria and its main gulf areas are provided by Crul [10]. Seismic reflection profiles confirm the origin of the lake as a result of regional tilting and provide an estimated age of 400,000 years for the lake basin [14]. The Nyanza Gulf deepens towards the western direction, as shown by bathymetric map after Kendall [15] and Scholz [16]. Some information on the lake bottom and sediments in the

Nyanza Gulf is provided by Mothersill [17] and Mothersill et al. [18].

described by various authors [4–13].

**Figure 1.** The map of Lake Victoria (Kenya) showing sampling sites.

Lake Sedimentary Environments and Roles of Accumulating Organic Matter in Biogeochemical… http://dx.doi.org/10.5772/intechopen.79395 33

**Figure 1.** The map of Lake Victoria (Kenya) showing sampling sites.

**1. Introduction**

32 Persistent Organic Pollutants

**1.1. Lake Victoria**

Lakes are important land features forming linkages with the hydrological systems at local and transboundary scales, while providing water sources, supporting fisheries, and providing livelihoods to many surrounding communities and different ecosystem services. Small and large lake depositional basins continuously accumulate dissolved or particulate inorganic and organic materials produced within the lake water column (autochthonous) or transported from the catchment area (allochthonous). However, besides lake production, human developments and urbanization around surface waters (lakes and rivers) are also a source of various solid and nutrient-rich materials depositing and accumulating on the lake sediments. When autochthonous matter is dominant, lake sediments may either be either carbonate-rich or silica-rich due to the accumulation of siliceous diatoms, or they may be mostly organic. In the latter case, the organic carbon content may reach 20–25%, but in peat bogs it may be even higher [1, 2]. Particulate matter scavenges chemical species (organic and inorganic pollutants) and, with other geochemical barriers, influences their mobility in the secondary environments. Therefore, understanding the sediment-chemical species associations has given more insights to the chemical element availability for metabolic processes and toxicity under certain environmental conditions. Knowledge of lake bottom dynamics and sedimentological processes is essential in matters related to geographical distribution and potential ecological effects of contaminants and in many practical issues, such as choice of adequate dumping sites for dredging deposits and choice of sampling sites from aquatic pollution control. Sediment accumulation areas prevail where fine materials, medium silt with grain size less than 0.006 mm, can be deposited continuously; sediment transportation zones appear where there is discontinuous deposition of fine particles/aggregates, and sand erosion zones prevail where there is no deposition of fine materials [3]. The sediments within areas of transportation, for natural reasons, are generally very variable (from sand to loose mud); areas of erosion are most frequent in shallow waters and characterized by hard or consolidated deposits (from bare rocks, gravel, and sand to glacial clays), and the deposits from areas of accumulation are always comparatively loose, with high water and organic contents and sometimes with a high content of pollutants [3]. The Lake Victoria (Kenya) gulf waters are eutrophic, and it is continuously fed by several large and small rivers which traverse the fertile agricultural highland areas and transport high levels of eroded materials. Information from sedimentological studies in *L. Victoria* exists but is inadequate due to lack of new studies. The chapter examines the significance of sediment organic matter (SOM) in freshwater aquatic environments, besides other sediment factors, and uses the spatial concentration of total organic carbon determined in surficial sediments of Lake Victoria (Kenya) to explore whether water hyacinth invasion has any significant influence on SOM. It also gives an opportunity to reflect on the potential environmental impacts which can possibly emanate from the new cage aquaculture developments in most of the shallow African

inland lakes, such as *L. Victoria* which has a huge potential for cage culture.

Lake Victoria, the world's second largest freshwater lake (68,800 km<sup>2</sup>

mean depth of 35 m) supports the world's largest inland fishery and is a transboundary ecosystem with the Nyanza Gulf, on the extreme eastern edge forming the Kenya portion (**Figure 1**). The gulf connects to the open lake via Rusinga Channel. River water inflows

in surface area and

account for about 20% of the total water inputs into the main lake. The catchment area and the drainage basin are covered by grassland savanna, wetland vegetation, agricultural crops, and forested highland and mountain areas. It is characterized by high tropical temperatures and heavy rainfall in the highlands, described in a long (April–June) and short (October–December) wet season and a dry season. The lake hydrology, physicochemical environment, and changes in primary production of the lake and gulf areas are welldescribed by various authors [4–13].

Morphometric characteristics from early investigations in L. Victoria and its main gulf areas are provided by Crul [10]. Seismic reflection profiles confirm the origin of the lake as a result of regional tilting and provide an estimated age of 400,000 years for the lake basin [14]. The Nyanza Gulf deepens towards the western direction, as shown by bathymetric map after Kendall [15] and Scholz [16]. Some information on the lake bottom and sediments in the Nyanza Gulf is provided by Mothersill [17] and Mothersill et al. [18].

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 it infested some 200 km2 by 1998 [19]. Its early spread and nourishment were thought to be triggered by nutrient enrichment, and the plant floating mats have persisted up to today.

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

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

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,

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

The most important determinants of chemical-sediment association are composition or grain-

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

iron-manganese oxides, carbonate contents), organic matter, and redox potential.

, mineralogical content (clay mineral composition, hydrous

in DO concentrations and no fish were caught when DO was below 2.5 mg/L−<sup>1</sup>

therefore, can readily affect the results of COD determinations [2].

of oxygen), particularly close to the sediment–water interface as a result of

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

.

http://dx.doi.org/10.5772/intechopen.79395

35

occur (i.e. 0 mg l

sediments [25].

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

size distribution, pH, salinity, DO<sup>2</sup>

−1
