The spread of quagga mussels across the American Great Lakes diverted supplies of phosphorous – a major biological nutrient – to the ecosystem, according to research published this week in PNAS.
The health of aquatic ecosystems depends on the supply of key nutrients, especially phosphorous. Too much phosphorous results in an unwanted excess, and a lot of effort is gone into preventing the contamination of water bodies with phosphorous. In the world’s largest freshwater ecosystem, the Great Lakes of North America, this control may have recently lost due to invasive species. According to a new study, the quagga mussels, which spread to four of the Five Great Lakes, have accumulated large amounts of phosphorous in their biomass, to the point that their activities now regulate the supply of phosphorous to the ecosystem.
The invaders, belonging to the Ponto-Caspian region of Eurasia, appeared in the Great Lakes region in the late 1980s, and by the late 2000s, they had spread over vast expanses of lower sediment in all lakes except for Lake Superior. Its biological effects on the ecosystem have been well recognized, but the effects on the chemical system have remained uncertain.
Researchers from the Great Lakes Observatory at the University of Minnesota Duluth analyzed the phosphorous cycle in Lakes Michigan, Huron, Erie and Ontario. They used a mass balance model, which they calibrated with the team’s measurements of sediments and mussels at multiple locations in Lakes Michigan and Huron. The results show that phosphorous concentrations in the invaded Great Lakes are now regulated by the clustering dynamics of one benthic species, the quagga mussel.
“Quagga mussels are small, hard-shelled creatures that live on the bottom of the lake and filter water, removing phytoplankton and other small particles,” explains Ted Ozerski, assistant professor of biology who co-led the study. They now occupy the lake floor at densities in excess of 10,000 individuals per square meter (6 mussels per square inch). “In terms of biomass, mussels are the dominant form of life in the Great Lakes,” says Sergey Katsev, a professor at the Large Lakes Observatory who oversaw the geochemical aspects of the research.
By filtering organic particles from lake water and recycling phosphorous along with their excreta and faeces, mussels significantly alter the natural rates at which phosphorous is exchanged between lake water and sediments. According to the study, mussels in Lake Michigan not only remove phosphorous from the water at a rate ten times faster than it did two decades ago, but also resupply the water column with eight times the amount of phosphorous reaching the lake from the entire watershed. This kind of “internal loading” effectively separates phosphorous dynamics from the input to the watershed, leaving the system open to poorly anticipated fluctuations when mussel populations increase or decrease.
“Mussels have shortened the natural pathway of the phosphorous cycle in lakes,” explains lead author Jing Li, a former UMD postdoctoral researcher and assistant professor at the Hong Kong University of Science and Technology. Productivity in the lakes is now linked to what the mussel population is doing.
Growing populations of mussels are able to absorb large amounts of phosphorous from the water column, which is partly responsible for increasing the purity of water in the Great Lakes in recent years. In contrast, death events are able to release large amounts of phosphorous back into the ecosystem. As a result, phosphorous is regulated by mussel population dynamics and may respond only slowly to our efforts to control phosphorous runoff from watersheds.
The results of the study appearing this week PNAS, It turns out that one invasive species can have disastrous consequences for geochemical cycles in even the largest aquatic ecosystems in the world. According to the authors, this warns of similar environmental changes in lakes invaded by mussels across Europe and North America and calls for a new model for managing aquatic ecosystems.
The study was funded by the US National Science Foundation with an OCE-1737368 grant.