Contributions of biological domains to nitrogen biogeochemical cycling in a High Arctic glacial ecosystem during summer melt

In this study coupled investigations of solute chemistry and isotopic tracers (δ15N, δ18O-NO3 and δ18O-H2O) of snow and streams are used to reveal the competitive and diverse nature of biological process within a High Arctic glacial ecosystem (Midtre Lovénbreen, Svalbard). This includes potential mineralisation and nitrification of dissolved organic nitrogen (DON) and ammonia within subglacial flow paths and denitrification in hyporheic zone of proglacial streams. During the initial melt phase (up to mid July), most of nitrate in the proglacial streams was directly derived from snow. But as the hyporheic zone gradually became more active after mid July and its relative contribution of nitrate to the proglacial streams water increased downstream. This was evident with a considerable downstream loss of nitrate and an increase in δ15N-NO3 (-7.82 to 14.91‰) values in the proglacial meltwater streams. At the same time non-snowpack nitrate was detected in subglacial runoff indicating as significant presence of metabolically active heterotrophic microbial consortia beneath the glacier. The observed snow packs δ18O-NO3 values (76 to 80‰) were strongly diagnostic of an atmospheric origin. However with the initiation of melt, this provided subglacial and proglacial aquatic ecosystems with a major primary source of nitrogen. Therefore nitrogen leaving this glacial watershed in runoff (δ18O-NO3 values 64.44 to 3.78‰) had a markedly different composition.