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

Abstract

Abstract. Isotopic composition of nitrate (δ 15 N and δ 18 O) and particulate organic matter (POM; δ 15 N and δ 13 C) 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 δ 15 N and δ 18 O values of nitrate were low (δ 15 N = 2-10 ‰, δ 18 O = 5-8 ‰) during normoxic conditions but increased gradually (the highest at δ 15 N = 27 ‰, δ 18 O = 29 ‰) when anoxic conditions facilitated denitrification in the hypolimnion during spring-early summer. Once nitrate was fully utilized and sulfidic conditions set in, NH + 4 became the dominant inorganic N species, with δ 15 N ranging from 1.3 to 2.6 ‰. Low δ 15 N (∼ −5 ‰) and δ 13 C (−37 to −32 ‰) of POM cooccurring with high NH + 4 and CH 4 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 (δ 15 N = 2-6 ‰). Episodic low δ 15 N 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. δ 13 C POM in the photic zone ranged between −29 and −27 ‰ for most parts of the year. The periods of mixing were characterized by uniform δ 15 N-NO

Nutrient retention behind a tropical mega-dam: a case study of the Sardar Sarovar Dam, India

“…We found strong seasonality in fortnight surface turbidity data at Rajghat, with high turbid (up to 400 NTU) water mostly occur between June and October. Our observations are confirmed by Bardhan et al [5], who reported significantly lower productivity during the monsoon period due to light-limited conditions in the Tillari reservoir of the Western Ghats region. Despite more significant nutrient loading, higher suspended sediment concentrations during monsoon season reduce light penetration.…”

“…Therefore, any large-scale turbulent mixing of reservoir waters and/or air-water interface is unlikely, restricting the loading of fresh oxygen to the anoxic water column. Bardhan et al [5] observed that the water column of 50 m deep Tillari reservoir remains thermally stratified during spring and summer months until the onset of monsoon. Greater reservoir depths, rapid settling of particles, and morphometry may not allow any intense wind-induced mixing and restricts the recycling of P. With changing physiochemical conditions in the reservoir, an additional amount of P may come from dissolution and desorption of P attached with sediment particles and onto the clay minerals.…”

“…We further estimate that about 166 × 10 9 mol of DIN may have lost during 11 years of the study period through denitrification and burial, which may otherwise be used by phytoplankton to fix about 82 × 10 9 mol of atmospheric CO 2 . Studies lead by SWA Naqvi and his research group [5,52,53], have documented the production of CH 4 and N 2 O in the Indian reservoirs. However, the volume of greenhouse gas produced during organic matter decomposition (CO 2 and CH 4 ) and through denitrification (N 2 O) remains undocumented for the SS reservoir.…”

“…A vast number of global studies suggest that damming alters the riverine export of organic carbon and nutrients to the coastal seas the riverine export of organic carbon and nutrients to the coastal seas and influences the primary productivity in the downstream freshwater and saline waters. It is essential as recent studies [5,52,53], have reported low nitrate concentration and absence of measurable oxygen from the hypolimnion in the Indian reservoirs and provided evidence of the formation of CH 4 and N 2 O. It motivated us to measure the changes in nutrient fluxes in Indian rivers and to evaluate its potential impact on downstream ecosystems.…”

δ15N patterns in three subtropical estuaries show switch from nitrogen “reactors” to “pipes” with increasing degradation

“…At the other end of the spectrum, the positive offsets between δ 15 N‐DIN and δ 15 N‐ON in MRE and BRE suggest that N availability exceeded assimilatory demand. This contrasts findings from degraded temperate estuaries with small DON (< 50% TDN) pools (Ye et al ; Bardhan et al ). In BRE, these divergent ON vs. DIN patterns indicate that any NO 3 − or NH 4 + produced during the nitrification and mineralization promoted in other high‐turbidity estuaries (Damashek et al ) is not reassimilated into the biomass, creating the strong apparent fractionation between the PN and DIN pools.…”