Nitrogen Uptake Dynamics in a Tropical Eutrophic Estuary (Cochin, India) and Adjacent Coastal Waters

Quantification of nitrogen (N) transformation rates in tropical estuarine-coastal water coupled systems undergoing anthropogenic disturbances is scant. A thorough understanding of these metabolic rates is required to evolve a mitigation strategy to save such systems from further degradation. Here, we report the first measurements of ammonium (NH₄ ⁺) and nitrate (NO₃ ⁻) uptake along with N₂ fixation rates in the Cochin estuary, a tropical eutrophic ecosystem along the west coast of India, and two transects (off Cochin and off Mangalore) in the coastal Arabian Sea. In general, the Cochin estuary sustained higher uptake rates of NH₄ ⁺ (0.32–0.91 μmol N l⁻¹ h⁻¹) and NO₃ ⁻ (0.01–0.38 μmol N l⁻¹ h⁻¹) compared to coastal waters. The N uptake in the nearshore waters of Cochin transect (NH₄ ⁺ : 0.34 μmol N l⁻¹ h⁻¹ and NO₃ ⁻ : 0.18 μmol N l⁻¹ h⁻¹) was influenced more by estuarine discharge than was the Mangalore transect (NH₄ ⁺ : 0.02 μmol N l⁻¹ h⁻¹ and NO₃ ⁻ : 0.03 μmol N l⁻¹ h⁻¹). Despite high dissolved inorganic nitrogen (DIN) concentrations, the Cochin estuary also showed higher N₂ fixation rates (0.59–1.31 nmol N l⁻¹ h⁻¹) than the coastal waters (0.33–0.55 nmol N l⁻¹ h⁻¹). NH₄ ⁺ was the preferred substrate for phytoplankton growth, both in the Cochin estuary and coastal waters, indicating the significance of regenerative processes in primary production. A significant negative correlation between total nitrogen (TN): total phosphorus (TP), and NH₄ ⁺ uptake (as well as N₂ fixation) rates in the estuary suggests that nutrient stoichiometry plays a major role in modulating N transformation rates in the Cochin estuary.