Spatial variation of nitrogen uptake rates in the largest brackish water lagoon of Asia (Chilika, India)

Abstract

Inland and coastal water bodies around the world are susceptible to eutrophication due to inputs of anthropogenic nutrients, such as nitrogen (N) and phosphorus (P). Studies related to quantification of uptake rates of these nutrients and the factors governing these rates are limited in tropical aquatic systems, leading to inadequate understanding of N and carbon (C) cycling in these environments. Here, we report the rates of dissolved inorganic N (DIN) uptake and N2 fixation along with N isotopic composition of particulate organic matter (δ15NPOM) in Asia's largest brackish water lagoon (Chilika, India). The experiments were carried out at sixteen different locations in the lagoon where NO3− and NH4+ uptake rates varied from 0.004 to 1.42 μmol N L−1h−1 (average ∼ 0.51 ± 0.51 μmol N L−1h−1) and 0.19–1.76 μmol N L−1h−1 (average ∼ 1.11 ± 0.43 μmol N L−1h−1), respectively. In general, NH4+ was preferred substrate in the lagoon as depicted from relative preference index (RPINH4+ >1). Despite higher DIN concentrations, the lagoon showed diazotrophic activity as evidenced by considerable N2 fixation rates at majority of stations. Preliminary calculations of the lagoon-wide N budgeting indicate N2 fixation and atmospheric deposition to be significant sources of new N to the lagoon. δ15NPOM in the lagoon ranged from 1.34 to 8.31‰ (average ∼ 3.99 ± 1.68‰) indicating contributions from marine and terrestrial sources along with N2 fixation to the organic matter pool.