Retreating Glaciers May Be Starving Coastal Seas of Key Nutrients — Threatening Phytoplankton, Fisheries and the Ocean’s Carbon Sink

Glacier retreat linked to lower nutrient delivery to coastal waters

New research from the Scripps Institution of Oceanography (University of California, San Diego), published in Nature Communications, shows that meltwater from a rapidly retreating Alaskan tidewater glacier contains lower concentrations of essential micronutrients such as iron and manganese. Scientists analyzed sediment and water samples from two fjords in Southcentral Alaska and found clear geochemical differences tied to glacier movement and retreat.

"The longer you have water in contact with rock or sediments, the more chemical breakdown or weathering takes place," said Sarah Aarons, a geoscience researcher at Scripps and co-author of the study. "A retreating glacier might be sending more sediment to the ocean but with lower concentrations of bioavailable nutrients like iron because more weathering is occurring."

The researchers propose that as glaciers retreat, meltwater spends more time interacting with bedrock and sediment. This extended contact increases weathering and can transform or leach minerals, lowering the concentrations of bioavailable trace elements that are essential for marine life.

Why this matters

Micronutrients such as iron and manganese are critical for phytoplankton growth. Phytoplankton form the base of the marine food web and support commercial fisheries; they also perform roughly half of the world's photosynthetic carbon fixation and produce a substantial fraction of global oxygen. Reduced delivery of bioavailable micronutrients could therefore:

• Limit phytoplankton productivity and shift community composition;
• Undermine fisheries and broader marine food webs;
• Weaken the ocean’s capacity as a carbon sink, potentially accelerating atmospheric CO2 rise and climate warming.

"If we can duplicate these findings elsewhere, the impacts go beyond our scientific understanding of glaciers," said Sarah Aarons. "This could impact the productivity of really significant marine ecosystems, which could have long-term implications for the health of major fisheries."

Lead author Kiefer Forsch emphasized the study's limits: "We see very clear geochemical differences between these two glacier systems that we link to their state of retreat. However, this is a snapshot of two glaciers in one region. Understanding whether these patterns hold across glaciers elsewhere in the world with different bedrock types and stages of retreat will require more research."

Context and next steps

Glacier retreat is largely driven by rising global temperatures from human-caused greenhouse gas emissions. While governments pursue emissions reductions and a transition to renewable energy, this study highlights another way climate-driven glacier change could affect ocean health. The authors call for more comparative studies across regions and bedrock types to determine how widespread the phenomenon is and to quantify ecological consequences.

Bottom line: Early evidence suggests retreating glaciers may deliver more sediment but fewer bioavailable micronutrients to coastal waters — a subtle but potentially far-reaching change with implications for phytoplankton, fisheries and the ocean’s role in moderating climate.