Abstract
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.
Highlights
Nitrogen is an essential macronutrient wherein the availability often limits primary production in aquatic ecosystems
Based on the vertical temperature distribution it appears that the reservoir gets vertically mixed through convective overturning in winter (December to February, with the exact duration of mixing depending upon meteorological conditions prevailing in a given year)
The reservoir gets vertically mixed during the southwest monsoon (SWM) season as well as in winter; the water column remained stratified during other parts of the year
Summary
Nitrogen is an essential macronutrient wherein the availability often limits primary production in aquatic ecosystems. It is a polyvalent element that undergoes redox transformation between the terminal oxidation states of +5 and −3. These transformations involve isotopic fractionation to varying degrees, and natural abundance of stable isotopes (15N and 14N) in various N species provides useful insight into nitrogen cycling besides its sources/sinks in the oceanic (Altabet, 1988; Sigman et al, 2005), coastal (Thunell et al, 2004; Hu et al, 2016) and estuarine (Cifuentes et al, 1988; Savoye et al, 2012) water bodies and sediments. Some of the best studied freshwater ecosystems in this regard are Lake Lugano at the Swiss–Italian border, Lake Kinneret in Israel and Lake Superior in the USA. In the eutrophic Lake Lugano, the highly depleted δ13C and δ15N of the near-bottom particulate organic matter (POM) established the active presence of methanotrophic bacteria during suboxic conditions (Lehmann et al, 2004)
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