Nitrogen uptake potential under different temperature-salinity conditions: implications for nitrogen cycling under climate change scenarios

As projected by climate change models, increase in sea surface temperature and precipitation in the future may alter nutrient cycling in the coastal regions due to potential changes in phytoplankton community structure and their ability to assimilate nitrogen (N) and carbon (C). An experiment simulating different temperature and salinity conditions (28^°C-35 ambient conditions, 28^ºC-31, 31^ºC-35 and 31^ºC-31) in mesocosms containing 1000 L of coastal water from the Arabian Sea was performed and N uptake rates were measured using ¹⁵N tracer technique on 2^nd, 5^th, 7^th and 10^th day of the experiment. The results show that, under all conditions, the total N (NO₃^- + NH₄⁺) uptake rates were lower in the beginning and on the final day of the tracer experiment, while it peaked during middle, consistent with chlorophyll a concentrations. Total N uptake rate was significantly lower (p = 0.003) under ambient temperature-lower salinity condition (28^ºC-31) than the others. This indicates that lowering of salinity in coastal regions due to excessive rainfall in the future may affect the N uptake potential of the phytoplankton, which may change the regional C and N budget.