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Effects of Meltwater on the Arctic Coastal Microbial Food Web: An Experiment With Isotopic Labeling From Young Sound, NE Greenland.

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Title: Effects of Meltwater on the Arctic Coastal Microbial Food Web: An Experiment With Isotopic Labeling From Young Sound, NE Greenland.
Authors: Puts, IC1,2,3,4 (AUTHOR) isolde.puts@umu.se, Mostovaya, A1,2 (AUTHOR), Henson, HC1,2 (AUTHOR), Allais, L1 (AUTHOR), de Kluijver, A5 (AUTHOR), Thyrring, J1,2 (AUTHOR), Middelboe, M6,7 (AUTHOR), Holding, JM1,2,8 (AUTHOR)
Source: Journal of Geophysical Research. Biogeosciences. Mar2026, Vol. 131 Issue 3, p1-15. 15p.
Subject Terms: *Meltwater, *Bacterial growth, *Food chains, *Fjords, *Runoff, *Glacial melting, *Ocean
Geographic Terms: Greenland, Arctic regions
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. Plain Language Summary: Rivers transport increasing amounts of freshwater from melting glaciers and thawing permafrost into the Arctic Ocean. This meltwater often carries sediments, micronutrients, and minerals that coastal microscopic life uses to grow. Currently, it is not understood how this runoff changes coastal water chemistry or how it affects coastal microscopic life, which is important for the rest of the ecosystem. In this experiment, we test how river water affects the growth of various sea‐living microbes (algae and bacteria). We do this by adding either pure freshwater or sediment‐rich glacial meltwater to labeled seawater containing natural microbes, and monitoring their growth, biomass, and fatty acids over 14 days. The experiment shows that meltwater rivers bring silicon, and potentially micronutrients and low amounts of bioavailable carbon to the coast, which stimulates the growth of bacteria, but not algae growth. This is an important observation because it shows that freshwater inputs impact coastal environments and could potentially affect the entire coastal ecosystem. Key Points: We test how sediment‐rich river‐runoff carrying glacial meltwater affects the microbial food‐web in a Greenlandic fjordMeltwater enriches the marine environment with silicon, and low amounts of nitrogen and likely highly bio‐available organic carbonA 16% dilution with river water stimulates bacterial biomass [ABSTRACT FROM AUTHOR]
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Description
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. Plain Language Summary: Rivers transport increasing amounts of freshwater from melting glaciers and thawing permafrost into the Arctic Ocean. This meltwater often carries sediments, micronutrients, and minerals that coastal microscopic life uses to grow. Currently, it is not understood how this runoff changes coastal water chemistry or how it affects coastal microscopic life, which is important for the rest of the ecosystem. In this experiment, we test how river water affects the growth of various sea‐living microbes (algae and bacteria). We do this by adding either pure freshwater or sediment‐rich glacial meltwater to labeled seawater containing natural microbes, and monitoring their growth, biomass, and fatty acids over 14 days. The experiment shows that meltwater rivers bring silicon, and potentially micronutrients and low amounts of bioavailable carbon to the coast, which stimulates the growth of bacteria, but not algae growth. This is an important observation because it shows that freshwater inputs impact coastal environments and could potentially affect the entire coastal ecosystem. Key Points: We test how sediment‐rich river‐runoff carrying glacial meltwater affects the microbial food‐web in a Greenlandic fjordMeltwater enriches the marine environment with silicon, and low amounts of nitrogen and likely highly bio‐available organic carbonA 16% dilution with river water stimulates bacterial biomass [ABSTRACT FROM AUTHOR]
ISSN:21698953
DOI:10.1029/2025JG008921