Isotopic composition of nitrate and particulate organic matter in a pristine dam reservoir of western India: implications for biogeochemical processes

Abstract. Isotopic composition of nitrate (δ¹⁵N and δ¹⁸O) and particulate organic matter (POM; δ¹⁵N and δ¹³C) were measured in the Tillari Reservoir, located at the foothills of the Western Ghats, Maharashtra, western India. The reservoir, which is stratified during spring–summer and autumn seasons but gets vertically mixed during the southwest monsoon (SWM) and winter, is characterized by diverse redox nitrogen transformations in space and time. The δ¹⁵N and δ¹⁸O values of nitrate were low (δ¹⁵N  =  2–10 ‰, δ¹⁸O  =  5–8 ‰) during normoxic conditions but increased gradually (the highest at δ¹⁵N  =  27 ‰, δ¹⁸O  =  29 ‰) when anoxic conditions facilitated denitrification in the hypolimnion during spring–early summer. Once nitrate was fully utilized and sulfidic conditions set in, NH₄⁺ became the dominant inorganic N species, with δ¹⁵N ranging from 1.3 to 2.6 ‰. Low δ¹⁵N (∼ −5 ‰) and δ¹³C (−37 to −32 ‰) of POM co-occurring with high NH₄⁺ and CH₄ in sulfidic bottom waters were probably the consequence of microbial chemosynthesis. Assimilation of nitrate in the epilimnion was the major controlling process on the N isotopic composition of POM (δ¹⁵N  =  2–6 ‰). Episodic low δ¹⁵N values of POM (−2 to 0 ‰) during early summer, coinciding with the absence of nitrate, might arise from N fixation, although further work is required to confirm the hypothesis. δ¹³C POM in the photic zone ranged between −29 and −27 ‰ for most parts of the year. The periods of mixing were characterized by uniform δ¹⁵N–NO₃⁻ and δ¹⁸O–NO₃⁻ at all depths. Higher POM (particulate organic carbon, POC, as well as particulate organic nitrogen, PON) contents and C ∕ N values with lower δ¹³C POM during the SWM point to allochthonous inputs. Overall, this study, the first of its kind in the Indian subcontinent, provides an insight into biogeochemistry of Indian reservoirs, using stable carbon and nitrogen isotopes as a tool, where the monsoons play an important role in controlling vertical mixing and dynamics of carbon and nutrients.