Columbus, OH New Ohio Sea Grant research by Dr. Virginie Bouchard has found that "outwelling" occurs in freshwater ecosystems – seiche events transport large quantities of nutrients from a wetland to its connecting Great Lake.
For more than 40 years, scientists have recognized how important tides are to the transfer of organic matter from the salt marshes in Europe to their adjoining marine waters. Known as "outwelling," tides can force the transport of organic matter produced in the marshes to the connecting water body.
Yet, for freshwater ecosystems, there is very little information about whether outwelling exists between the Great Lakes and their adjacent wetlands. "Great Lakes seiche events could act as key vehicles to transport massive amounts of organic matter to the adjacent lake," states Bouchard. Once in the lake, the nutrients can be replenished back into the aquatic food webs.
To determine whether the outwelling concept can be applied to such ecosystems, Bouchard used Metzger Marsh, a 906-acre wetland located along Lake Erie, as her test site. "By examining its controlled opening to the Lake, we can estimate how much or if at all organic matter travels between the two ecosystems."
Before organic matter could be traced, she first needed to establish if there was a water exchange between the wetland and Lake Erie. To do this, Bouchard designated 18 sites in and outside of Metzger Marsh and took water samples from each site. Each sample was examined to determine the amount of oxygen and hydrogen each had. "Every site has a unique amount of hydrogen and oxygen or what is called an isotopic signature that we can use to track where water from each site goes. If we find that signature in the Lake Erie water, we know that water traveled from a specific spot in the wetland into Lake Erie," explains Bouchard.
What she found was 75 percent of the wetland had a hydrologic connection to Lake Erie on a daily basis. "Water that originally was deep in the wetland traveled almost two and a half kilometers across the wetland into the waters of Lake Erie." When it was a storm event, that number rose to almost 100 percent of the wetland.
By discovering a definite water exchange between the two ecosystems, Bouchard could then determine if the seiche events carried anything besides water. Carbon, a sign that organic matter is present, was used as the isotopic signature for each of the 18 sites.
"Our research found that in general, water going out of Metzger had more organic matter than the water coming into the wetland," explains Bouchard. Many of her sampling found total organic carbon levels 50 to 70 percent higher in the water going out of the wetland. "This data unequivocally confirms that outwelling occurs between freshwater wetlands and their connected lake," states Bouchard. "Metzger Marsh with its highly productive vegetation gets the needed organic matter out to Lake Erie."
Bouchard’s next step will be to see which aquatic food webs the organic matter travels into once it is in Lake Erie. To do this, Bouchard will again use carbon and nitrogen as her isotopic signatures and track the integration of the isotopes into the food web.
"We hope to show that not only is the export of organic matter between a wetland and a lake substantial, but outwelling serves an overall beneficial role to the lake food web," concludes Bouchard. "We may need to have a mosaic of wetlands in the Great Lakes – connected and diked wetlands – in order to benefit all types of needs."