A comprehensive modeling study shows how subglacial meltwater triggers an additional summer bloom of phytoplankton in Disko Bay. At the same time, the climate-relevant carbon sequestration increases only slightly.
In Qeqertarsuup Tunua, Greenland’s Disko Bay, a study led by the Moss Landing Marine Laboratories shows how subglacial discharge transports nutrients from the fjord floor into the sunlit surface layer, thereby strengthening a second, summer bloom of phytoplankton.
The meltwater doesn’t supply the nutrients itself; as it rises—fresh water is lighter than seawater—it pulls them up from depth to the surface. In a region where, in summer, nitrate—essential for phytoplankton growth—is largely used up, this process makes the crucial difference: productivity lasts longer and cascades through the food web until the nitrate is depleted again.
The study, published on August 5 in Nature Communications Earth & Environment, focuses on Jakobshavn Glacier (Greenlandic: Sermeq Kujalleq), Greenland’s most active outlet glacier. At the height of the summer melt, about 1,200 cubic meters of freshwater per second flow from its subglacial drainage system into the sea. At roughly 850 meters depth, where the glacier rests on bedrock, this freshwater meets the salty fjord water and, being less dense, rises as a turbulent buoyant plume—effectively acting like an “escalator” for nutrients.
To quantify how subglacial meltwater affects primary production in Disko Bay, the research team ingested nearly all available high-resolution ocean observations from the past three decades into the complex ECCO–Darwin model (Estimating the Circulation and Climate of the Ocean–Darwin).
The results show that this meltwater-driven upwelling pump increases summer phytoplankton production in the study area by about 15 to 40 percent. Satellite-based chlorophyll observations confirm this pattern, particularly downstream of the glacier fronts.
Those observations also indicate that Arctic phytoplankton growth rates have been rising since the late 1990s—and this new Disko Bay study offers a clear mechanism behind that trend.
The nutrient boost increases phytoplankton primary production—more CO₂ is fixed into biomass. At the same time, CO₂ is less soluble in summer’s warmer, fresher surface waters than in spring, so despite the markedly higher phytoplankton production, the ocean as a whole takes up and stores only about 3 percent more CO₂.
“We reconstructed what’s happening in one key system, but there’s more than 250 such glaciers around Greenland,” said Dustin Carroll, an oceanographer at San José State University and co-author of the study, in a press release from NASA’s Jet Propulsion Laboratory. He added that the simulations will be extended to the entire Greenland coast and beyond.
Climate models for Greenland project a 100–300% increase in ice-sheet runoff, routed through englacial and subglacial drainage, by the end of the century. Where tidewater glaciers like Sermeq Kujalleq retain deep grounding lines, summertime primary production is likely to continue rising, driven by ongoing glacier fertilization from meltwater plumes.
Ecologically, the impact is tangible. With phytoplankton at the base of the food web, changes in its productivity ripple through all higher levels—from zooplankton to fish and marine mammals. The simulations indicate not only more biomass but also a shift in community composition: diatoms as well as tiny unicellular eukaryotes and cyanobacteria increase. In Greenlandic fjords, where stocks such as the economically important Greenland halibut are closely tied to productivity, this can affect yields. And last but not least, ringed seals, narwhals, and polar bears that use the fjords seasonally may indirectly benefit from the higher nutrient availability.
“We didn’t build these tools for one specific application. Our approach is applicable to any region, from the Texas Gulf to Alaska,” Michael Wood, an oceanographer at San José State University’s Moss Landing Marine Laboratories and lead author of the study, said in the press release. “Like a Swiss Army knife, we can apply it to lots of different scenarios.”