Freshwater Estuaries: Exploring an Unusual Ecosystem

The Saint Louis River Estuary is a rare type of freshwater ecosystem that receives significant amounts of water from both the incoming rivers and Lake Superior.

While estuaries are common along coastlines where rivers empty into the big blue sea, it’s less common to find a river running into a large-enough body of freshwater like Lake Superior. By definition alone, the St. Louis River Estuary is unique, but it is further noteworthy for all that it provides. The estuary is an important migratory bird fly-way and nesting site. It is an important nursery for all sorts of Lake Superior fish, from walleye to channel catfish to lake sturgeon. The estuary was once filled with beds of wild rice, a staple food for the native Ojibwe people. And today it flows between Duluth, Minnesota and Superior, Wisconsin, where it’s home to the largest active harbor in the Great Lakes.

Understanding the physical, chemical and biological processes going on in the estuary is essential in efforts to protect and restore it and manage its multiple resources for multiple interests. Luke Loken, a Center for Limnology grad student in Emily Stanley’s lab is currently exploring seasonal patterns in water chemistry in the estuary. At the end of February he presented at the Saint Louis River Estuary Summit meeting in Superior Wisconsin. The meeting was highly focused on the research ongoing in the estuary and environmental concerns of local citizens. Luke sent along this explanation of estuary dynamics and a note from the field.

Rivers dominate the estuary during high flow periods such as spring snow-melt and large rain events. When the river discharge is low, Lake Superior can more easily flow back into the estuary. As lakes become larger, a phenomenon called “seiche energy” is produced. Seiches have effects similar to the tides of the ocean. They are are created by wind pushing water to one side of the lake, which changes lake levels on both sides of the lake. (For example, if the wind is easterly, areas on the east side of Lake Superior see a rise in water level, while cities in the west, like Superior and Duluth watch water levels drop) When the winds die down, the water returns as a slow moving wave that surges into river mouths and temporarily reversing flow directions.
 

During winter months, the lake seiche effect is still intact because Lake Superior does not fully freeze. River inputs are minimal in the winter, so the lake has the greatest ability to infiltrate the estuary. When river discharge is low, as in winter, this seiche event can push lake water further into the Saint Louis River Estuary and change water levels from between 2 to 12 inches.
 

Because my research focuses on the nutrient and carbon patterns of the estuary, and because winter water chemistry samples are underrepresented in field studies, I figured I would sample under the ice to bring a more comprehensive set of data to the summit. It took some work, but I managed to collect water from six sites under about 2 feet of ice. I’ll combine the winter data with my 2012 and 2013 open-water data to provide a comprehensive seasonal pattern in water chemistry within the estuary.
 
With this data I hope to understand how far into the estuary the lake can penetrate, how that changes through the year, and what it means as far as nutrient inputs and water chemistry for this unique ecosystem.
 

Hopefully what Luke learns can help us better understand estuary dynamics and lead to better management of the resource for multiple interests. It’s truly an economic and ecological treasure shared between Minnesota and Wisconsin.

To learn more about the estuary, visit this amazing site from the St. Louis River Estuary project, featuring stories, science and video vignettes on everything form shipping to fishing to ricing in the estuary. And “like” the Lake Superior National Estuarine Research Reserve page on Facebook to stay up to date on the latest research.