Seasonal and tidal influence for water quality changes in coastal Bolgoda Lake system, Sri Lanka

The variation in physicochemical properties is a notable feature to identify hydrodynamic and biogeochemical processes in coastal aquatic systems. In this study, the spatial and temporal changes of water quality parameters were measured with respect to spring and neap tides, covering both single and different monsoon periods. Bulk organic geochemical proxies such as total organic carbon to total nitrogen (C/N ratio) and total organic carbon to total sulfur (C/S ratio) were also considered to evaluate organic sources and depositional environments, respectively. The conductivity was decreased with distance from the sea-mouth to stream during the neap tide under the influence of weak precipitation. No significant variation in conductivity can be observed during the spring tide under the influence of strong precipitation. In addition, the conductivity values in North Bolgoda Lake show homogeneous distribution during spring and neap tides within single monsoon period. Therefore, the reduction of freshwater supply (precipitation) had played a significant role in developing salinity stratification along this coastal water body. The seasonal stratification can control organic matter preservation, as oxygen dissolution is decreased with increasing salinity. In contrast, tidal variation and precipitation had no direct connection to change salinity in surrounding groundwater aquifers. Organic geochemical proxies indicate possible oligotrophic to mesotrophic levels of productivity in Bolgoda Lake system. The prominent fluvial process during rainy periods and seawater invasion during dry periods can diminish the accumulation of nutrients and anthropogenic deteriorations in Bolgoda Lake system under sea-mouth open nature.