With its bitter cold snaps and blasts of lake-effect snow, winter 2024-2025 made its presence known in the Great Lakes region. Ice cover on the lakes spiked above average in February. And while levels mostly hovered below average for the rest of the season, they were much closer to normal than the meager freezes of the previous two winters.
On Lake Erie, ice coverage reached its seasonal peak of 95 percent in February 2025, a typical number for the shallowest of the Great Lakes. As spring neared, Erie’s icy lid gradually dissipated to reveal signs of life in its surface waters.
The OLI (Operational Land Imager) on Landsat 8 acquired this image of the central part of the lake on March 10. Ice cover stood at about 33 percent that day, and the ice-free water displayed swirling bands containing a mix of sediment and phytoplankton.
Resuspended sediment often produces colorful patterns in Lake Erie’s surface waters, especially in spring. Strong winds and the currents they generate can stir up material from the shallow lakebed. The lake bottom is rich in quartz sand and silt, as well as calcium carbonate from limestone. A lot of resuspension was occurring at the time of this image, said Michael McKay, director of the Great Lakes Institute for Environmental Research at the University of Windsor, accounting for some of the color.
At the same time, McKay added, elevated levels of a type of phytoplankton called diatoms were present. Communities of these microscopic organisms, which have silica shells and plenty of chlorophyll, may have contributed to the color of surface waters. Although Lake Erie, specifically its western basin, is a hotspot for harmful algal blooms in the summer, different phytoplankton groups can thrive in the lake other times of year.
Samples collected by a Canadian Coast Guard icebreaker on March 10 contained elevated chlorophyll concentrations indicative of a spring bloom and showed that diatoms made up 80 to 90 percent of the phytoplankton present, said McKay. The samples came from locations near the ice edge and extending west approximately 80 kilometers (50 miles), and the measurements corroborated those from samples collected by a U.S. Coast Guard icebreaker in Lake Erie’s central basin three days earlier.
Scientists discovered in the early 2000s that diatoms could live within and just below Lake Erie’s ice. As the main components of winter-spring blooms, these microbes play important roles in the lake ecosystem, including cleaning pollutants from water and forming the base of the aquatic food web.
Subsequent work has revealed their clever strategy for surviving harsh winter conditions. The diatoms have a symbiotic relationship with a type of bacteria that can form ice crystals. That relationship enables diatoms to attach to the underside of lake ice and access the light needed to perform photosynthesis. Sparser ice cover during some winters has resulted in lower abundance of some diatom species, researchers found, with implications for the lake ecosystem that are not yet fully understood.
“We spent decades ignoring this time of year,” said Steven Wilhelm, director of the Aquatic Microbial Ecology Research Group at the University of Tennessee. “[McKay and I] were lucky to be part of a team that had a chance to start reinvestigating Lake Erie in the winter about 18 years ago.” Many questions around relationships between the seasons—how diatom blooms might shape the onset and severity of summertime blooms, for example—remain ripe for more investigation, he said. “Clearly the lake is active and alive year-round.”
NASA Earth Observatory image by Michala Garrison, using Landsat data from the U.S. Geological Survey. Story by Lindsey Doermann.
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