Biogeochemical responses to mixing of glacial meltwater and hot spring discharge in the Mount St. Helens crater

Environments where geothermal waters and glacier meltwater mix are common on Earth, yet little is known about the biogeochemical processes that occur when hot, reduced geothermal water mixes with cold, oxidized glacial meltwater in natural systems. Mount St. Helens provides an ideal location to study the interaction between geothermal and glacier waters since the water sources, and their mixing environment in Step Creek, are exposed in the volcanic crater. We find that the two water sources contain distinct major ion, trace element, dissolved organic matter (DOM) and biological signatures. The hot spring contains high concentrations of biogeochemically-reactive components (e.g. siderophile and chalcophile trace elements and DOM) compared to the glacier discharge, but a large fraction of these solutes do not remain in solution after the waters mix. In contrast, glacier discharge contains fewer solutes, but most of these solutes remain in solution after the waters mix. The mixing of glacier and hot spring water in Step Creek supports seston and benthic ecosystems that have higher phototrophic and microbial biomass than those in the source waters, suggesting that the mixing environment in this high-gradient stream provide a more comprehensive suite of soluble and essential nutrients that promote primary production and DOM cycling.