UMD scientists study phytoplankton, the base of Great Lakes food chain
The EPA-funded project could help protect water quality and steer fisheries management.
DULUTH — The base of the food chain across the Great Lakes is changing in ways affecting the top of the food chain, the fish we love to catch and eat, and scientists at the Natural Resources Research Institute at the University of Minnesota Duluth are trying to find out why.
The problems are different in each lake, but start with tiny plants called phytoplankton, microscopic marine algae.
In a balanced ecosystem, phytoplankton are the food for small creatures called zooplankton and for small fish, which in turn are eaten by bigger fish. But the Great Lakes systems aren’t all in balance.
The NRRI researchers will track how phytoplankton are changing, identify reasons for the changes and make recommendations on what can be done to protect water quality in the lakes and manage the fish in them.
“It’s more of a food web, really, than a food chain. It’s all interconnected. And the people who are trying to manage fish species out there, and protect water quality, need to know what’s happening at the base of the food web,” said Euan Reavie, the NRRI scientist heading the $3 million study funded by the Environmental Protection Agency as part of the Great Lakes Restoration Initiative.
From invasive species like mussels to warming waters and ongoing pollution, each lake faces multiple stressors. In Lake Michigan, billions of invasive quagga mussels have filtered out virtually all of the phytoplankton in the deep parts of the lake. Reavie says that’s created a virtual dead zone for deepwater fish like cisco, lake trout and whitefish.
“The fish in the deepest parts of Lake Michigan are starving,” he added, even as fish near shore, like salmon, are doing OK.
The latest NRRI effort started this spring and will continue for five summer seasons with a dozen scientists fanning-out across the lakes, on boats like the EPA’s Lake Guardian.
UMD scientists have been studying sediments in the Great Lakes for decades and crews have been looking at deepwater phytoplankton in the lakes since 2007, Reavie said.
The latest effort started this spring and will run across five summers.
“The goal is to provide managers with some information that they can use to help guide their management decisions,” Reavie said.
Scientists also can use core samples of sediment from the lake bottom to see how phytoplankton has changed — the type and number of them — from decades, even centuries ago.
“That's how we know the phytoplankton in Lake Superior are entirely different now than they were 100 years ago and 300 years ago,” Reavie said. “The ones here before are gone.” So far it’s not clear how or why that happened, or the impact it’s having on the food chain above it — species like herring, whitefish and lake trout that also have been around for centuries
In Lake Erie, huge algae blooms, some of them toxic, are growing more often and cover more water in recent years. Researchers know part of the problem is phosphorus-heavy runoff from agricultural fields in the lake’s watershed.
But scientists like Reavie discovered the problem also starts upstream, in Lake Huron, where invasive mussels have filtered out most of the small algae. That’s created a massive surplus of silica in Lake Huron (formerly picked up by the diatom algae) that's now flowing down into Lake Erie’s western basin, Reavie said, and that’s helped fuel the explosion of algae in recent years.
“Sometimes it’s a combination of things happening that can trigger the problem. And it may not be very obvious,” Reavie said. “The agricultural issues (for Lake Erie) get all the headlines. But the silica is also part of the equation.”
In Lake Superior, a few recent algal blooms, in a lake where they had never historically occurred before, have caused concern. So far, Reavie noted the problem has been limited to the South Shore area close to shore, essentially between the Twin Ports and Ashland. Scientists believe the problem is in part caused by runoff after big rainfall storms on shore. The runoff is carrying all sorts of material — animal waste, agricultural runoff and more — that are fertilizing the algae in the big lake.
But it’s also likely the Lake Superior algal blooms are being spurred by warmer water in recent years. Algae needs warmer water to grow. Both the increased frequency of rainfall and the increase in water temperature are the result of climate change already occurring in the region.
“We know Lake Superior is warming faster than any other lake in the world,” Reavie said. “That’s going to allow for more algae blooms. And probably a few other problems.”