Nitrogen content and composition in two contrasting estuaries in West Bengal: indicators of natural vs anthropogenic impact?.

EINSPORN, M.; FISCHER, P.; BATABYAL, P. ; PALIT, A.; LARA, R.J.

Bremen

Congreso; GDCh Wissenschaftsforum Chemie; 2011

Gesellschaft Deutscher Chemiker

Human activity has more than doubled the input of fixed nitrogen (N) into terrestrial ecosystems [1] and its delivery to coastal waters leads to eutrophication and oxygen deficiencies in estuaries worldwide [2]. The Ganges basin is densely populated and intensively used. Although models rank the Ganges the second largest riverine N source to the sea (3.78 * 1012 g N year-1 [3]), empirical data on the nitrogen load is scarce. We sampled the River Hooghly close to Kolkata and the mangrove dominated Matla Estuary during dry season in January 2011 in order to characterize the N load and to delineate the importance of human impact. Dissolved inorganic nitrogen (DIN) concentrations at Hooghly sites were low compared to other large human-impacted rivers (e.g., Huanghe: 260 μM; Changjiang: 80 μM DIN [4]). Planktonic DIN uptake was probably light limited due to high total suspended matter (TSM) instead, adsorption of ammonium to clays could contribute to low DIN and the absence of ammonium. In contrast, the low DIN in the Matla related to the reduced input from the agriculture hinterland during dry season. Contrary to DIN, particulate nitrogen (PN) was enriched at the urban Howrah Bridge site. Elevated amino acid (AA) concentrations and AA-N % pointed at the anthropogenic input of sewage material. At the Matla estuary, amino acids indicated the presence of degraded organic matter (OM), consistent with reduced primary production. Low δ15 N in the Matla estuary pointed at biological N fixation. δ15 N in both areas suggested little influence of wastewater on TSM which would result in elevated δ15 N [5]. Our study reveals only little influence of human impact during dry season while we expect a larger contribution from the agricultural hinterland and the urban areas in the rainy season.