Effects of glacial meltwater on the coastal microbial food web: an experiment

The Arctic Ocean is predicted to freshen by 30-50% by 2100, with coastal areas receiving significant freshwater from melting glaciers and permafrost. This influx of freshwater, along with increased sediment and carbon inputs, is altering the water chemistry along Greenland’s coasts, potentially impacting food webs. In this study, we examine how sediment-rich, glacial meltwater affects the microbial food web in a high Arctic fjord. We differentiate the effects of freshening alone (using Milli-Q water) from those combined with land-derived compounds (glacial meltwater). Seawater was enriched with ¹³C-HCO₃^- and diluted with 16% freshwater using either glacial meltwater or Milli-Q water. We monitored the response of natural microbial producers to both treatments over 14 days by measuring bacterial and algal biomass and production, as well as phospholipid-derived fatty acids (PLFAs) and their δ¹³C signature. Our results indicate that glacial runoff enriches the marine environment with silicon, and likely nitrogen and carbon, that significantly influence microbial production. Specifically, glacial runoff and a modest 16% freshening boost bacterial production and biomass, but not algal (primary) production. Overall, our study demonstrates that glacial runoff increases bacterial coastal production, potentially impacting the entire ecosystem and highlighting the significant influence of terrigenous freshwater inputs on coastal environments.