Abstract:

The Arctic Ocean freshwater content is predicted to increase by 30%–50% during the 21st century, and especially coasts receive substantial amounts of freshwater from melting glaciers and thawing permafrost. The resulting increase in sediment and carbon inputs from rivers is changing seawater chemistry
along Arctic coasts, potentially disrupting food web processes. Here, we test how sediment‐rich river runoff carrying glacial meltwater affects the microbial food web in a Greenlandic fjord, by separating the effects of freshening alone from those combined with river‐ and land‐derived compounds (river discharge). After enriching seawater with 13C‐HCO3 􀀀 and freshening it by 16% with either Milli‐Q or river water, we monitored the response of natural microbial producers in both treatments. We tracked bacterial and algal biomass and production over 14 days and assessed fatty acids and their δ13C signatures. The results show that meltwater enriches the marine environment with silicon and nitrogen. Furthermore, a 16% dilution with river water stimulates bacterial —but not algal— production and biomass. The observed effects of meltwater are possibly due to impacts of labile organic carbon, inorganic (micro) nutrient inputs, or even the increased silicate concentration supporting biosilicifying bacteria. Still, the specific environmental factors supporting the increased bacterial activity remain speculative. Given the limited research on meltwater effects on microbial processes, we highlight key knowledge gaps and provide guidance for future experimental setups. Overall, we show that increased river discharge carrying meltwater enhances bacterial production, potentially impacting the entire ecosystem and emphasizing the significant influence of terrigenous freshwater inputs on coastal environments.

https://doi.org/10.1029/2025JG008921