Retreating glaciers may deliver fewer nutrients to the ocean

Glaciers are more than frozen giants. They connect the land with the sea in ways that are often overlooked. Every summer, meltwater is released from glaciers that rushes toward the ocean, cloudy with fine sediment.

The meltwater carries vital nutrients that marine life depends on. But new research shows this nutrient flow is changing as glaciers shrink. To investigate, a team from UC San Diego’s Scripps Institution of Oceanography studied two Alaskan glaciers.

The research shows that retreating glaciers may release water rich in sediment but poor in the nutrients that feed marine ecosystems.

Two glaciers, one difference

The team compared two glaciers on Alaska’s Kenai Peninsula. Aialik still reaches the sea, while Northwestern has retreated inland by more than 15 kilometers since 1950.

The glaciers grind across the same kind of bedrock, but their waters tell different stories.

Northwestern’s meltwater carried fewer forms of iron and manganese that plankton can actually use. Those metals are crucial for phytoplankton, which form the base of ocean food chains.

“If we can duplicate these findings elsewhere, the impacts go beyond our scientific understanding of glaciers,” said Sarah Aarons, a geochemist at Scripps.

“This could impact the productivity of really significant marine ecosystems, which could have long term implications for the health of major fisheries.”

How glaciers change nutrients

The difference comes down to how long meltwater stays trapped beneath the ice. In a retreating glacier, water lingers longer. It weaves through sediment and rock, allowing more chemical reactions to occur.

Those reactions turn nutrients like iron into forms that marine organisms can’t absorb.

By contrast, stable glaciers like Aialik flush meltwater quickly into the sea. The water carries “fresher” minerals – tiny reactive particles that plankton thrive on.

The chemistry changes not because of what glaciers erode, but because of how long the water interacts with rock before escaping.

Testing the meltwater

Researchers collected samples from the cloudy meltwater plumes in each fjord. Aialik’s plume showed higher levels of reactive metals. Northwestern’s had fewer.

“The longer you have water in contact with rock or sediments the more chemical breakdown or weathering takes place,” she said.

“So 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,” Aarons explained.

These differences affect the entire food web. Fewer nutrients mean less phytoplankton, which means less food for fish. Even regions known for rich marine life – like the Gulf of Alaska – could see long-term effects if this pattern continues.

Glaciers losing nutrients

Tidewater glaciers – those that flow directly into the sea – move through a recurring cycle of advance, stability, and retreat. Each stage alters how nutrients move from land to ocean.

Stable glaciers pump nutrient-rich water upward through subglacial plumes that mix with seawater. Once a glacier retreats, those plumes vanish. The new meltwater streams across bare rock instead, carrying weathered, nutrient-poor sediment.

This process doesn’t just affect Alaska. Similar changes are appearing in Greenland, Antarctica, and Svalbard. Glaciers there are losing their role as nutrient suppliers to the ocean.

Beyond the local impacts

Kiefer Forsch, the study’s lead author, summed up the concern. “We see very clear geochemical differences between these two glacier systems that we link to their state of retreat,” he said.

“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.”

These shifts matter globally. Glacial meltwater influences how carbon cycles through the ocean. When phytoplankton growth drops, less carbon dioxide gets pulled from the atmosphere.

The changes beginning in Alaskan fjords could echo across the planet’s climate systems.

The work ahead

Future studies will test whether this trend appears in other glacial regions. Aarons stressed that such research depends on public support.

“Funding from NSF allows us to understand how this landscape is responding to a warming planet, and has a direct impact upon the many people who subsist on these lands and visit these glacial fjords for their abundant and diverse wildlife,” added Aarons.

As glaciers retreat further, the link between ice and ocean weakens, disrupting a system that has quietly sustained marine life for thousands of years.

Once a powerful natural conveyor of nutrients, glacial meltwater now carries less of what ocean ecosystems depend on.

This shift affects everything from tiny phytoplankton to large fish populations. It also signals how climate change can alter connections between distant environments, turning what was once a life-giving flow from mountains to the sea into a warning about the planet’s changing balance.

The study is published in the journal Nature Communications.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–