Dynamic changes, cycling and downward fate of dissolved carbon and nitrogen photosynthetically-derived from glaciers...

Glaciers play key roles in capturing, storing, and transforming global carbon and nitrogen, thereby contributing markedly to their cycles. However, an integrated mechanistic approach is still lacking regarding glacier's primary producers (PP), in terms of stable dissolved inorganic carbon isotope (δ¹³C-DIC) and its relationship with dissolved carbon and nitrogen transformation d ynamic changes/cycling. Here, we sampled waters from glaciers, streams, tributaries, and the Indus River (IR) mainstream in the Upper IR Basin, Western Himalaya. Dissolved organic matter (DOM) appears to increase, on average, by ∼2.5-23.4% with fluctuations when passing from glaciers to streams-tributaries-IR mainstream (the upper and lower parts, respectively) continuum, implying that DOM originates from glaciers PP and is subsequently degraded. The corresponding fluctuations are observed for fluorescent DOM (FDOM), dissolved organic nitrogen (8.0-106.8%), NO₃^--N (-13.5/+16.6%), NH₄⁺-N (-8.8/+13.0%), and NO₂^--N (70.7-217.5%). These variations are associated with overall DOM/FDOM transformations, with the production of ending byproducts (e.g. CO₂/DIC). The δ¹³C-DIC values fluctuated from glaciers (-5.3 ± 2.5‰) to streams (-4.4 ± 2.1‰), tributaries (-4.3 ± 1.6‰), and IR mainstream (-4.2 ± 1.3‰). The δ¹³C-DIC data are consistent with C transformations that involve lighter CO₂ emission into the atmosphere, whereas highly depleted DIC/CO₂ is the signature of DOM degradation after its fresh production from glaciers PP which originated by photosynthetic activities (e.g. uptake/sink of atmospheric CO₂: -8.4‰). Finally, glacier-fed meltwaters would simultaneously contribute to the biogeochemical characteristics of downward margins and specific ecosystems (lake/pond/groundwater/hot springs) via transformation dynamics/cycling of dissolved C and N with high photo/microbial lability. Our results highlight the substantial contribution of western Himalayan glaciers-derived DOM to the global C and N cycles.