NCCOS Investigates Ecosystem Impacts from Great Lakes Sediment Plume
A perennial visitor to the Great Lakes – a vast plume of silt that appears late each winter along the south shores of Lake Michigan, a phenomenon perhaps several thousand years old – posed questions demanding scientific answers.
What is the ecosystem significance of the yearly event? How does it affect nutrient and contaminant transport in the lake? And with what implications for the food web? What is its potential for re-introducing into the water column contaminants from bottom sediments deposited decades earlier? What might the implications be for PCB levels, which, despite having fallen in the environment generally in recent years, continue to be detected at unsafe levels in lake fish?
To answer those and other questions related to the annual plume, the National Oceanic and Atmospheric Administration teamed with the National Science Foundation to study the ecosystem impacts of nutrient and contaminant recycling from Great Lakes sediments.
Important findings of “EEGLE” – the Episodic Events Great Lakes Experiment
- storms are responsible for the annual sediment plume;
- most sediment in the plume is re-suspended off the bottom, not eroded from the shoreline;
- resuspension of particles redistributes contaminants such as PCBs; and
- storm resuspension events re-introduce more phosphorus and approximately as much PCBs into the lake water and its living contents as all external sources combined.
The Great Lakes are the largest system of fresh water on the planet, the source of drinking water for more than 40 million Canadian and U.S. citizens.
The Great Lakes region faces environmental stresses common to all coastal systems, and has a long history of international and interagency collaboration. While water quality has improved overall since the 1970s, hazardous chemicals remain on the bottom of rivers, bays and near- shore waters in the form of contaminated sediments.
20 Percent of Great Lakes Shorelines Contaminated
Approximately 20 percent of Great Lakes shorelines suffer from contaminated sediments. Tissue samples of lake fish continue to show high levels of certain contaminants, leading states to regularly issue advisories warning residents against consuming certain kinds of fish.
In Lake Michigan , for example, fish tissues have repeatedly tested positive for unacceptable levels of chemicals such as PCBs, mercury, and chlordane. A 2002 Environmental Protection Agency assessment of U.S. coastal waters pointed to chemical contamination resulting in fish consumption advisories as one of the greatest environmental problems facing the Great Lakes .
Just as it does along the East, West, and Gulf coasts, NOAA maintains a large multi-disciplinary science presence in the region. In 1997, NOAA’s Coastal Ocean Program, partnering with NOAA’s Great Lakes Environmental Research Laboratory and the National Science Foundation Coastal Ocean Processes Program, launched EEGLE as a comprehensive study of the ecosystem impacts of sediment recycling. The scientists’ five-year effort began with a three-year field study, and was followed by extensive data analysis and modeling. It brought together an impressive team of more than 40 environmental scientists from 17 federal agencies and universities.
The focus of the EEGLE study was the great plume of silt that appears along the south shores of Lake Michigan each year in late winter. Scientists at NOAA’s Great Lakes Environmental Research Laboratory had been the first to capture the full extent of this phenomenon when in 1996 satellite photos taken over a period of unusually cloud free days showed a highly reflective band of water in the lake’s southern basin. The massive turbidity plume extended 10 miles offshore and 200 miles along the southern coastline of Lake Michigan , from Wisconsin, past Chicago, and back up into Michigan.
Massive Recurring Plume Confirmed
Further study of satellite photos and historical data confirmed the massive plume to be a recurring phenomenon. It happens after the ice along the coastline breaks up, when large storms create waves strong enough to stir up particles or silt from the lake bottom. It typically lasts less than a month, eventually veering offshore along the eastern side of the lake, where, coincidentally, sediment accumulations are the highest.
Made aware of the massive extent of this yearly event, scientists began to speculate on its ecosystem significance, raising many of the questions mentioned earlier. Brian Eadie, the project’s lead scientist, was quoted in a January 29, 1998 , Chicago Tribune article, published as the project was just getting under way: “You can still find daily high levels of PCBs in fish, though PCBs were banned 20 years ago. We knew it was happening, we just didn’t know how.”
EEGLE contaminant studies conducted by Eadie and government and academic colleagues have shown that the intense springtime suspended sediment levels in Lake Michigan have important impacts on the cycling of organic contaminants and nutrients.
One effect of this phenomenon is to increase dissolved concentrations of contaminants such as PCBs and PAHs in the water column. These increased concentrations in turn promote the gas-exchange of contaminants to the atmosphere, leading to increased atmospheric deposition of airborne contaminants.
Applying sophisticated research models developed as part of the EEGLE research, modelers lately have been working closely with Great Lakes resource managers to develop models linking sediment transport with ecological factors.
Linking environmental factors with the plume helps managers forecast potential impacts to lakeshore communities. Though important in themselves for understanding the effects of sediment transport on lake ecology, the models also help managers predict impacts on local water quality conditions, such as turbidity and contaminant resuspension.
Using EEGLE to Understand Plume Impacts
The models developed from EEGLE are being used by Great Lakes researchers also to improve understanding of impacts of the annual sediment resuspension plume on the Lake Michigan ecosystem.
For example, recent work by EEGLE researchers has found that the sediment resuspension events alter the short-term nutrient and light climate of the nearshore waters, temporarily reducing phytoplankton reproduction and photosynthesis inside the plume.
Researchers in the EEGLE collaboration have shown that although phytoplankton primary production is reduced in the sediment plumes, secondary bacterial and protozoan production is enhanced by the increased suspension of dissolved carbon and nutrients. This enhancement of microbial production of organisms not dependent on sunlight for energy rivals the production of the light-requiring phytoplankton in clearer waters and has important implications for the spring and summertime growth of zooplankton and fishes.
The net effect of these annual resuspension events, according to John Wickham, the EEGLE project coordinator with NOAA’s National Centers for Coastal Ocean Science, is to promote production of phytoplankton and microbial communities. Because the resuspension event often coincides with the initiation of the annual spring phytoplankton bloom, Wickham says, long-term phytoplankton composition and abundance is enhanced along with microbes such as bacteria and protozoans. As phytoplankton and the microbial community form the base of the food chain for consumers such as zooplankton, larval insects, and larval fishes, these production enhancements cascade up the food chain.
What does the future hold?
In 2003, NOAA and the Cooperative Institute for Limnology and Ecosystem Research co-hosted the NOAA Great Lakes Issues Identification Workshop at the University of Michigan in Ann Arbor. The consensus of workshop participants was that a new program of concerted research is needed to examine the results of recent ecological changes in the Great Lakes on water quality. This research, workshop participants said, should include development of tools to forecast changes in water quality and consequences for the food web.
It’s another example of NCCOS’ research efforts paying dividends long after the actual field work and data analysis has officially ended.
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