North America’s Great Lakes are by far the largest connected freshwater system in the world. In a little over 30 years their biology has been utterly changed with the establishment of invasive populations of zebra and quagga mussels, with an immense decline in the abundance of native plants and animals. A new study shows the quagga mussels have taken over the cycling of the lake’s stores of phosphorus, one of the few elements essential to almost all life. The authors warn many European lakes could be headed for similar declines now the mussels are spreading there, and offer some ideas of how to control the invader.
In the familiar tale of invasive species everywhere, mussels thrived in the Great Lakes after arriving in ballast waters from their native Ukraine, because there were no natural predators to keep them under control. The mussels concentrate pollutants in their tissues, causing problems for the few fish (and humans) that can actually eat them in cleaner conditions.
In the lower lakes there are often more than 10,000 mussels per square meter (1000 per square foot), leaving no room on the lake bed for anything else. They consume so much phytoplankton that away from the shoreline the production of living material from sunlight has fallen by up to 70 percent. "In terms of biomass, quagga mussels are the dominant life form in the Great Lakes," said Professor Sergei Katsev of the University of Minnesota Duluth in a statement.
The mussels arrived when the lakes' ecosystems were recovering from massive algal blooms driven by excess phosphorus runoff that depleted the lakes of oxygen, killing other inhabitants. By restricting the amount of phosphorus entering the lakes all bar Lake Erie, were on the way back to health when the mussels struck.
It turns out the two crises were connected, because the mussels wreck their damage partly by altering the cycling of phosphorus, a Katsev and co-authors reveal in Proceedings of the National Academy of Sciences reveals.
Although zebra and quagga mussels were once performing similar take-overs of the lake bottoms, the quaggas have now beaten the zebra mussels at their own game and dominate. They voraciously consume organisms from the water column, including their phosphorus, and transfer most of it to the sediment through their excretions and when their bodies rot.
“The tissues and shells of quagga mussels now contain nearly as much phosphorus as the entire water columns of the impacted Great Lakes,” the paper reports. The mussels remove phosphorus from the waters many times faster than occurred before their arrival but also re-release it with much greater speed than uncolonized sediments.
"The mussels have short-circuited the normal pathways of the phosphorus cycling in the lakes," lead author Dr Jiying Li, of the Hong Kong University of Science and Technology said. "And the productivity in the lakes is now tied to what the mussel populations are doing."
The consequence is that the availability of phosphorus – and therefore the amount of life that can flourish in the water column – is controlled by mussel numbers. As mussels flourish and die wild swings in availability disrupt the delicate ecosystem of the lakes making survival hard even for those species not directly competing with them.
Although the situation appears to be similar across the middle three lakes, the paper observes important differences in mussel abundance, such that the authors think it might be possible to restore Lake Ontario eventually by further reducing phosphorus input, but something different is needed for the other lakes.