Micromelt sampling of the glacier algal nutrient environment

Zygnematophycean "glacier algae" form extensive blooms on ablating glacier surfaces despite the ultra-oligotrophic conditions apparent. Previous work has postulated that this oligotrophic bloom paradox is due to (i) lower nutrient requirements of glacier algae, (ii) efficient uptake and storage of the nutrients available, and/or (iii) ineffective characterisation of the actual nutrient environment that glacier algae experience. We investigate the latter here by directly sampling the thin (∼2 mm) melt water film in which glacier algal cells reside across three glaciers in Svalbard during the 2023 melt season, comparing to outcomes from more typical bulk ice sampling techniques. Micromelt samples generally contained increased concentrations of ammonium (NH4+), nitrate (NO3-), nitrite (NO2-), and phosphate (PO43-), though trends were not uniform, and concentrations remained well within oligotrophic levels. Several major ion species were significantly increased in micromelt fractions as compared to bulk samples, indicating aeolian deposition and marine aerosol influences on the glacier algal environment. In turn, enhanced micromelt dissolved organic carbon concentrations (DOC) indicated likely DOC delivery by glacier algae to the microbial food web from the onset of bloom formation. Taken together, datasets reveal new fine-scale heterogeneity in the glacier algal meltwater environment.