Monsoon decline versus summertime intensification of carbon and nitrogen fixation in a shallow tropical lake

In recent years, changes in lake water levels have emerged as a significant environmental concern, largely attributable to the impacts of climate change and anthropogenic activities. Reductions in lake water levels alter light availability, mixing patterns, nutrient concentrations, biomass, and nutrient assimilation rates. Despite being notable contributors to inland water areas, shallow lakes have received comparatively less attention globally and play a significant role in carbon and nitrogen biogeochemical cycles. This study explores the seasonal influence of water temperature, salinity, and nutrient concentration on phytoplankton primary production, nitrogen assimilation, and dinitrogen fixation rates in a shallow tropical freshwater lake as the water level increases from summer to monsoon. Our results reveal that when water level was reduced during summer, concentrations of particulate and dissolved forms of carbon and nitrogen increased, as did phytoplankton primary production, nitrogen assimilation, and N<inf>2</inf> fixation rates. Higher rates during summer were attributed to increased light availability, temperature, biomass, and nitrogenous nutrient concentrations with reduction in water level. Lower rates during monsoon were due to the dilution of lake water caused by precipitation and runoff resulting in limited biomass and supply of nitrogenous nutrients. This seasonal variation can cascade through trophic levels, affecting primary producers and potentially leading to shifts in community structure and biodiversity. Overall, this study provides valuable insights into the interactions among water level changes, environmental parameters, phytoplankton primary production, nitrogen assimilation, and N<inf>2</inf> fixation rates in tropical semi-arid freshwater shallow lakes facing the threat of lake desiccation.