The fate of the enormous amount of reactive nitrogen released to the environment by human activities in India is unknown. Here we show occurrence of seasonal stratification and generally low concentrations of dissolved inorganic combined nitrogen, and high molecular nitrogen (N2) to argon ratio, thus suggesting seasonal loss to N2 in anoxic hypolimnia of several dam-reservoirs. However, 15N-experiments yielded low rates of denitrification, anaerobic ammonium oxidation and dissimilatory nitrate reduction to ammonium—except in the presence of methane (CH4) that caused ~12-fold increase in denitrification. While nitrite-dependent anaerobic methanotrophs belonging to the NC10 phylum were present, previously considered aerobic methanotrophs were far more abundant (up to 13.9%) in anoxic hypolimnion. Methane accumulation in anoxic freshwater systems seems to facilitate rapid loss of reactive nitrogen, with generally low production of nitrous oxide (N2O), through widespread coupling between methanotrophy and denitrification, potentially mitigating eutrophication and emissions of CH4 and N2O to the atmosphere.

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Abstract. The western Indian continental shelf is one of the most productive coastal systems of the world ocean. This system experiences extreme changes in its oxygen regime, being normoxic from November to May and suboxic (denitrifying)/anoxic from June to October, owing to the biogeochemical response to cyclical monsoonal influence. In order to understand the impact of the seasonally varying oxygen regime on benthic mineralization, nutrient exchange and, in turn, on the shelf ecosystem, we carried out the first ever intact-core incubations during two contrasting seasons -spring intermonsoon and fall intermonsoon (late southwest monsoon) at a 28 m-deep fixed site on the inner shelf off Goa, dominated by fine-grained cohesive sediments. The results showed that incomplete sediment oxygen consumption (SOC) occurred during April as opposed to the complete SOC and subsequent sulfide flux observed in the fall intermonsoon incubations. The sediments acted as a perennial net source of DIN (dissolved inorganic nitrogen i.e. NO Slow oxidation of organic carbon (C org ) under anoxia, lower temperature and reduced benthic faunal activity appeared to decrease benthic mineralization by 25 % as suggested by the drop in the C org oxidation rate from 63.8 mmol C m −2 d −1 in April to 47.8 mmol C m −2 d −1 in October. This indicated a higher preservation of C org during the late southwest monsoon. Sediment porosity, C org content and nutrients did not show significant variations from April to October. Porewaters were found to be enriched with NH

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Emission of methane (CH4), a potent greenhouse gas, from tropical reservoirs is of interest because such reservoirs experience conducive conditions for CH4 production through anaerobic microbial activities. It has been suggested that Indian reservoirs have the potential to emit as much as 33.5 MT of CH4 per annum to the atmosphere. However, this estimate is based on assumptions rather than actual measurements. We present here the first data on dissolved CH4 concentrations from eight freshwater reservoirs in India, most of which experience seasonal anaerobic conditions and CH4 buildup in the hypolimnia. However, strong stratification prevents the CH4-rich subsurface layers to ventilate CH4 directly to the atmosphere, and surface water CH4 concentrations in these reservoirs are generally quite low (0.0028-0.305 μM). Moreover, only in two small reservoirs substantial CH4 accumulation occurred at depths shallower than the level where water is used for power generation and irrigation, and in the only case where measurements were made in the outflowing water, CH4 concentrations were quite low. In conjunction with short periods of CH4 accumulation and generally lower concentrations than previously assumed, our study implies that CH4 emission from Indian reservoirs has been greatly overestimated.

The western Indian continental shelf is one of most productive coastal systems of the world ocean. This system undergoes extreme change in oxygen regime being normoxic from November to May and suboxic/anoxic from June to October owing to the biogeochemical response to cyclical monsoonal influence. In order to understand its impact on benthic mineralization, nutrient exchange and in turn on the shelf ecosystem, we carried out first ever intact core incubation experiments by covering two contrasting seasons i.e. Spring intermonsoon and fall intermonsoon (late southwest monsoon). The results show that the shelf sediments act as a perennial net source of DIN, PO₄^3– and SiO₄^4– to the overlying water column. DIN efflux increased from 1.4 to 3.21 mmol m^–2 d^–1 from April to October of which NH₄⁺ comprises 59–100%. During oxic regime about 75% of diffusing NH₄⁺ appears to be nitrified (2.55 mmol m^–2 d^–1) of which about 77% remains coupled to benthic denitrification. Consequently 58% of NH₄⁺ flux gets lost in active coupled nitrification-denitrification process causing substantial N loss (1.98 mmol m^–2 d^–1) in the sediment. The continental shelf sediment switches over from being a NO₃^– source during oxic regime to a NO₃^– sink during low oxygen regime. During suboxia benthic denitrification being fed by NO₃^– from overlying water causes N loss at a rate of 1.04 mmol m^–2 d^–1. N loss continues even in sulfidic condition during October possibly through chemolithoautotrophic denitrification at a potential rate of 3.21 mmol m^–2 d^–1. PO₄^3– flux increased more than 4 fold during October as compared to April due to reductive dissolution of Fe and Mn oxides. The SiO₄^4– flux increases during anoxia due to higher availability of siliceous ooze as a result of diatom blooms during the monsoon season.

Porewater was found to be enriched with NH₄⁺, PO₄^3– and SiO₄^4– while depleted in NO₃^– and NO₂^– in these organic rich sediments. Sedimentary oxygen consumption decreased by ~28% under anoxia presumably due to the decrease of temperature as well as lower abundance of benthic fauna. Anoxia also appears to reduce benthic mineralization by 25% as C_org mineralization rate decreased from 63.83 mmol C m^–2 d^–1 in April to 47.83 mmol C m^–2 d^–1 in October. This is explained as due to slow oxidation of refractory C_org under anoxia apart from effect of lower te...

Abstract. Phytoplankton and bacterial pigment compositions were determined by high performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS) in two freshwater reservoirs (Tillari Dam and Selaulim Dam), which are located at the foothills of the Western Ghats in India. These reservoirs experience anoxia in the hypolimnion during summer. Water samples were collected from both reservoirs during anoxic periods while one of them (Tillari Reservoir) was also sampled in winter, when convective mixing results in well-oxygenated conditions throughout the water column. During the period of anoxia (summer), bacteriochlorophyll (BChl) e isomers and isorenieratene, characteristic of brown sulfur bacteria, were dominant in the anoxic (sulfidic) layer of the Tillari Reservoir under low light intensities. The winter observations showed the dominance of small cells of Chlorophyll b-containing green algae and cyanobacteria, with minor presence of fucoxanthin-containing diatoms and peridinin-containing dinoflagellates. Using total BChl e concentration observed in June, the standing stock of brown sulfur bacteria carbon in the anoxic compartment of Tillari Reservoir was estimated to be 2.27 gC m−2, which is much higher than the similar estimate for carbon derived from oxygenic photosynthesis (0.82 gC m−2. The Selaulim Reservoir also displayed similar characteristics with the presence of BChl e isomers and isorenieratene in the anoxic hypolimnion during summer. Although sulfidic conditions prevailed in the water column below the thermocline, the occurrence of photo-autotrophic bacteria was restricted only to mid-depths (maximal concentration of BChl e isomers was detected at 0.2% of the surface incident light). This shows that the vertical distribution of photo-autotrophic sulfur bacteria is primarily controlled by light penetration in the water column where the presence of H2S provides a suitable biogeochemical environment for them to flourish.

Phytoplankton and bacterial pigment compositions were determined by high performance liquid chromatography (HPLC) and liquid chromatography- mass spectrometry (LCMS) in two freshwater reservoirs (Tillari Dam and Selaulim Dam), which are located at the foothills of the Western Ghats in India. These reservoirs experience anoxia in the hypolimnion during summer. Water samples were collected from both reservoirs during anoxic periods while one of them (Tillari Reservoir) was also sampled in winter, when convective mixing results in well-oxygenated conditions throughout the water column. During the periods of anoxia (summer), bacteriochlorophyll (BChl) <i>e</i> isomers and isoreneiratene, characteristic of brown sulfur bacteria, were dominant in the anoxic (sulfidic) layer of the Tillari Reservoir under low light intensities. The winter observations showed the dominance of small cells of Chlorophyll-<i>b</i> containing green algae and cyanobacteria, with minor presence of fucoxanthin-containing diatoms and peridinin-containing dinoflagellates. Using total BChl-<i>e</i> concentration observed in June, the standing stock of brown sulfur bacteria carbon in the Tillari Reservoir was computed to be 2.4 gC m⁻², which is much higher than the similar estimate for carbon derived from oxygenic photosynthesis (0.82 gC m⁻²). These results highlight the importance of anoxygenic photosynthetic biomass in tropical freshwater systems. The Selaulim Reservoir also displayed similar characteristics with the presence of BChl-<i>e</i> isomers and isorenieratene in the anoxic hypolimnion during summer. Although sulfidic conditions prevailed in the water column below the thermocline, the occurrence of photoautotrophic bacteria was restricted only to mid-depths (maximal concentration of BChl-<i>e</i> isomers was noted at 0.2 % of the surface incident light). This shows that the vertical distribution of photoautotrophic sulfur bacteria is primarily controlled by light penetration in the water column where the presence of H₂S provides a suitable biogeochemical environment for them to flourish