Inter-seasonal variation in nitrogen uptake rates of the eutrophic Cochin estuary and adjacent coastal Arabian Sea.

The study assessed the Cochin estuary and adjacent coastal Arabian Sea for their seasonal variation in nitrate (NO 3 ⁻) and ammonium (NH 4 ⁺) uptake rates by total and nano + picoplankton using the ¹⁵N tracer technique. The results suggested that the NO 3 ⁻ and NH 4 ⁺ uptake rates in the Cochin estuary are higher than those in the adjacent coastal Arabian Sea. NO 3 ⁻ and NH 4 ⁺ uptake rates in the nearshore stations in the off Cochin station were high, indicating the influence of the eutrophic estuary. NO 3 ⁻ and NH 4 ⁺ uptake rates conducted in off Mangalore transect were significantly lower than those of the off Cochin as it does not have an exchange with eutrophic systems. The nano + picoplankton's contribution to the total DIN uptake rates in the Cochin estuary was 77–98 %, indicating the relevance of nano + pico phytoplankton in the N cycling of the region. • Nano + pico phytoplankton contributes up to 77-98 % of the total DIN uptake rate in the Cochin estuary. • Evidence of the influence of eutrophication in the estuary of DIN uptake rates in the coastal Arabian Sea. • DIN uptake potential of phytoplankton depends on TN:TP ratio rather than the individual nutrient concentrations. [ABSTRACT FROM AUTHOR]