In situ experiments were conducted to determine the Net Ecosystem Production (NEP) in cryoconite holes from the surface of two glaciers (Patsio glacier and Chhota Shigri glacier) in the Western Himalaya during the melt season from August to September 2019. The study aimed to gain an insight into the factors controlling microbial activity on glacier surfaces in this region. A wide range of parameters, including sediment thickness, TOC %, TN %, chlorophyll-a concentration, altitudinal position, and grain size of the cryoconite mineral particles were considered as potential controlling factors. From redundancy analysis, the rate of Respiration observed in cryoconite at Chhota Shigri glacier was predominantly explained by sediment thickness in cryoconite holes (37.1 % of the total variance, p < 0.05) with Photosynthesis largely explained by the chlorophyll-a content of the sediment (39.6 %, p < 0.05). NEP was explained primarily by the TOC content and sediment thickness in cryoconite holes (35.8 % and 22.1 % respectively, p < 0.05). The altitudinal position of the cryoconite is strongly correlated with biological activity, suggesting that the stability of cryoconite holes was an important factor driving primary productivity and respiration rate on the surface of Chhota Shigri glacier. We calculated that the number of melt seasons required to accumulate organic carbon in thin sediment layers (<0.3 cm), based on our measured NEP rates, ranged from 11 to 70 years, indicating that the organic carbon in cryoconite holes largely derives from allochthonous inputs, such as elsewhere on the glacier surface. Phototrophic biomass in the same thin sediment layer of cryoconite was estimated to take atleast 4 months to be produced in situ (with mean estimated time upto 1.7 ± 1.5 years). Organic matter accumulated inside the cryoconite holes both through allochthonous deposition 2 and via biological activity on the glacier surface in these areas may have the potential to export dissolved organic matter and associated nutrients to downstream ecosystems. Given the importance of Himalayan glaciers as a vital water source for millions of people downstream, this study highlights the need for further investigation in aspects of the quantification of in situ produced organic matter and its impact on supraglacial melting in the Himalaya.
<p>Nutrients deposited and cycled on the glacier surfaces are important not only because of their role in the global biogeochemical cycling in the downstream environments, but also because of their importance as a primary food source for microbes inhabiting glacial surfaces, their ice surface darkening properties, and the consequent potential for enhancing glacier melt. The present study focuses on the Chhota Shigri (CS) Glacier in the North-Western (NW) Indian Himalayan region. The dissolved organic carbon (DOC) concentration in the bare ice is relatively higher than the other aspects studied in the glacial environment of CS indicating that much of the active microbial activity occurring in the bare ice. Total inorganic nitrogen (TIN) is typically concentrated in the snow, which is the major contributor of NO<sub>3</sub><sup>-</sup>. The rapid declining of TIN in the bare ice as compare to other aspects and its enrichment in DOC suggests for a more active microbial activity occurrence in the exposed ice rather than in isolated cryoconites holes in the Chhota Shigri Glacier. The net biological productivity indicates the dominance of net gross photosynthesis over respiration, suggesting a net autotrophic production in CS.</p>
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