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
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