Since the late 1980s, four of the five Great Lakes have played host to an increasing number of invasive mussels. First came zebra mussels, followed shortly thereafter by quagga mussels, both members of the Dreissenid family whose native range includes the waters around Ukraine.
Today, the filter-feeders comprise more than 90% of the total animal biomass of the Great Lakes (barring Lake Superior, whose depth and water chemistry make it a less suitable habitat for the two species of mussel).
The extensive presence of these mussels has heavy implications for the chemistry of the Great Lakes.
A new study published in the Proceedings of the Natural Academy of Sciences suggests that these rapacious bivalves play a dominating role in the phosphorus cycle in lakes Huron, Erie, Michigan and Ontario.
And that could have important implications for policymakers attempting to reduce dangerous algae blooms and protect the health of these ecosystems.
“Given current technologies, we are stuck with mussels forever,” said Ted Ozersky, a professor of biology at the University of Minnesota Duluth and one of the study authors. “In the past, we had fairly straightforward models about what happened to water columns as we changed inputs and outputs of phosphorus [from things like agricultural runoff].”
Now, thanks to the mussels, there is far more variability and unpredictability in how much phosphorus is released into the lakes—and that could have cascading impacts on everything from the lakes’ phytoplankton to large fish at the top of the food chain.
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The role of phosphorus in the lakes
Long before human activity had any impact on the chemistry of the Great Lakes, phosphorus was part of the ecosystem. It’s a necessary component for the growth of phytoplankton and other plants, which are in turn consumed by small fish. In the past, the phosphorus cycle changed slowly over long periods of time. Precipitation on land releases small amounts of phosphorus into the lakes, some of it flowing through the water column while the rest is absorbed by sediments at the bottom of the lake.
When the human population around the lakes grew larger, we began inadvertently pumping the lakes full of phosphorus. A combination of agricultural runoff and detergents added far more phosphorus to the water than had ever existed before, leading to massive quantities of algae and other plants.
When the abundance of plants died off, the bacteria that helped them decompose removed oxygen from the water, creating “dead zones”—areas of the lake with very low levels of oxygen where no other creatures could survive. In places like Lake Erie, excess phosphorus has also led to dangerous algal blooms that threaten the health of wildlife and humans alike.
Policy measures implemented beginning in the 1970s aimed at controlling the amount of excess phosphorus getting into the lakes. For a time, it seemed these measures were improving the lakes’ water quality—until the mussels arrived, presenting a new complication.
“If we try to regulate the lakes’ productivity by regulating phosphorus inputs, the lakes may not respond as fast as they used to. It may take decades for us to see the effects,” said Sergei Katsev, a physical and geochemical limnologist and another author of the study. “Whatever the response [to regulation] may be, it might be small compared to random fluctuations that are related to what the mussels are doing, if they’re growing or dying.”
The mussel-phosphorus puzzle
The mussels pull phosphorus from the water column while they’re filtering it, and absorb it into their soft tissues and shells, leaving less phosphorus for all the other organisms in the lake. When a mussel population is growing, there’s less phosphorus in the water.
But if there were to be a mass die-off of mussels, there would be a huge release of phosphorus back into the lake, since the phosphorus would be released from their decaying bodies.
For David Depew, a research scientist for Environment and Climate Change Canada, the study adds to knowledge of the mussel-phosphorus puzzle that scientists have worked on for years.
“A lot of scientists had expected that the mussels play a role, but it’s been really challenging to try and put some sort of brackets on how big that role might be, and that’s where this paper does a good job of fleshing out the range of possibilities we might be dealing with,” Depew said.
Most other studies have looked at mussels in smaller regions and on shorter time frames—say, in the western basin of Lake Erie for five months out of the year. What makes this new study different is that it looks at the four lakes in their entirety over a longer period of around one year. Some nuance is lost in the approach, but Depew says the broader view complements other studies that have previously been done.
Katsev and Ozersky also said the model they built is flexible enough that it could be used to measure mussels’ impact on the phosphorus cycle in other lakes across North America, making it a useful tool in understanding the biogeochemistry of ecosystems being modified by invasive bivalves.
But there’s still plenty of work to be done. Ozersky and Katsev want to measure other nutrients in the ecosystem, like nitrogen and carbon, and how zebra and quagga mussels might be acting on those molecules.
Depew adds that there are still a lot of things we don’t quite understand about the life cycle and population cycles of these mussels.
“In a general sense it’s well accepted that as the population grows, their ability to trap carbon, phosphorus and nitrogen into their tissues and shells will increase, and if the population is waning that capacity decreases,” Depew said. “As for the shorter life cycle components—how much phosphorus might be released through spawning and the subsequent consumption of larvae—I don’t think there’s very good information on that, in part because it’s very difficult to get.”
In other words, the lakes are truly enormous ecosystems with many complicated variables impacting them.
Dreissenid mussels have only been around for about 30 years, and there’s a lot to be learned about their role in the Great Lakes. What is clear is that they’re reshaping the environment and will continue to present a headache for policymakers for the foreseeable future.
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Featured image: Zebra mussel cluster (Photo taken by D. Jude, Univ. of Michigan via NOAA Great Lakes Environmental Research Laboratory Flickr, CC BY-SA 2.0)