Sediment characteristics were examined in a brackish water lake (Kokilamedu Lake) to find out the nutrient dynamics and its relation to particle size distribution. The lake was inundated by December-2004 tsunami changing it from almost fresh to a brackish water lake. The lake is relatively shallow (1.20 m) at the northern part as compared to the southern region (2.80 m). Analysis of surface sediment revealed that it is basically silty-sand in nature with sand, silt and clay fractions ranging from 63.87-94.62% , 1.60-32.39% and 0.63-14.17% respectively. Sediment of northern portion of the lake is sand dominated in contrast to relatively high silt and clay containing sediment of the southern section. The organic matter (0.62-3.93%) and organic carbon (0.36-2.28%) concentrations were relatively high in silt and clay sediment. Calcium Carbonate (CaCO3) ranged from 0.23-6.25% and it showed relatively higher values towards northern part. All the nutrients, such as NO3-N (1.42-3.40 mg/kg), NH4-N (10.96-68.53 mg/kg), total nitrogen (55.49-238.06 mg/kg), inorganic phosphorus (144.37-280.26 mg/kg) and total phosphorus (140.13-402.70 mg/kg), showed a similar spatial distribution trend with concentrations gradually decreasing from southern to northern section of the lake. Correlation analysis showed that all the nutrients were positively correlated with silt and clay as well as with organic carbon. The high organic carbon rich clayey southern bottom of the lake indicates a sink of terrestrial organic source which significantly contributes nutrients to the eutrophic water column through different biochemical processes. Lack of connectivity and exchange of water with other water bodies further enhances the nutrient accumulation in the bottom of the lake.

Aquatic organisms and plants require certain amount of trace elements and nutrients for their growth. These nutrients are used until they become limiting and further growth is inhibited. The most important micronutrients are nitrogen and phosphorus (Millero, 1996). In the coastal brackish water lake, these nutrients are buried in the sediments and recycled to the nearer water increasing their availability and producing their different forms, which are used by the aquatic plants and organisms for their growth (Arenas and Lanza, 1990; Van-Raaphorst and Kloosterhuis 1994; and Justics et al., 1995). On the other hand, nutrients in fresh with aquatic sediments are mainly related to the assessment of the effect of their increasing concentrations originating from different sources. High value of the nutrients in the sediment and water cause eutrophication of the lake (Vollenweider, 1968; and Sutcliffe and Jones, 1992). Bonanni et al. (1992) showed that sediment plays an important role in the accumulation and regeneration of nutrients. The availability of these micronutrients also depends upon the different physical characteristics of the sediments of the water column. The objective of the present study is to elaborate the relation between geochemical as well as sediment characteristics with organic and inorganic forms of nutrients of the sediment in a brackish water lake.

Soil And Water Sciences Journal, Soil Health Management, Tree Plantations, Soil Organic Carbon, Rainfed Ecosystem, Biological Productivity, Soil Salinity Development, Low Productivity System, Traditional Farming System, Water Management, Agroforestry System, Agriculture Diversification, Decision Support System, Pest Management.