Flood Response: Lake Superior's Dark Surprise
Water sample data from 3km beyond the Aerial Lift Bridge. The flood occurred on June 19-20, 2012. Within a month Total Phosphorus had returned to typical seasonal concentrations. Chlorophyll levels did not rocket up as expected, presumably because tannins and other compounds (CDOM) were blocking light.
Some of the best moments in science come in the form of unexpected results. Even better is when scientists are able to analyze unexpected events and find unexpected results. Minnesota Sea Grant supported such serendipitous research during and after the Solstice Flood of 2012, which drenched the western region of Lake Superior with so much rain that it washed seals out of the zoo.
When 10 inches of rain fell in the Duluth area within 24 hours, no one was planning on measuring how an influx of tons of sediment and debris would affect western Lake Superior. But even before the rain stopped, inquiring minds couldn't help asking, "How will this enormous phosphorus-limited swath of lake water respond to such a large influx of nutrients?" Rapidly, Elizabeth Minor, Associate Professor of Chemistry and Biochemistry and the Large Lakes Observatory at the University of Minnesota Duluth, rallied resources, equipment and people to tackle the question.
The day after the rain stopped, the team was monitoring three sites in the western arm of Lake Superior. Each week thereafter, the scientists and crew aboard the R/V Blue Heron collected water samples from three km beyond the Aerial Lift Bridge, from near the Superior Entry Lighthouse and from about 10 km off-shore from Park Point.
Expecting the extraordinary volume and velocity of water rushing off the land to add a large amount of phosphorus to the lake, it seemed logical that this 500-year flood event would cause an algal bloom ... or at least a spike in productivity. Neither happened.
Yes, after the flood the concentration of Total Suspended Solids (TSS) became orders of magnitude higher than normal. The TSS then dispersed and sank within a month. As anticipated, total phosphorus followed a pattern similar to suspended solids', rising to levels higher than Lake Erie's average in the 1970s within a week and then sinking.
Curiously, however, the concentration of chlorophyll in the water samples did not echo the rise and fall of phosphorus concentrations. Where was the algal bloom? If phosphorus wasn't limiting algae production, something else certainly was. It turns out that the floodwaters dimmed the light.
Measurements indicated that Colored Dissolved Organic Matter (CDOM) also increased dramatically and remained high beyond the span when phosphorus levels were high. CDOM is the name for the natural compounds that make water brownish. It was like the lake was wearing sunglasses; these compounds absorbed sunlight and reduced the amount of light penetrating to the depths where algae tend to dwell.
Observers reported seeing a near-shore algal bloom in the Apostle Islands in summer 2012 but there is no data on nutrient levels or the extent of the bloom.
While the Solstice Flood was an unexpected event, it is unlikely to be an isolated one. Climate change models predict an increased frequency of extreme weather events. Understanding how the lakes and their biota respond to dramatic weather will help us prepare for and maybe mitigate undesirable consequences. To pursue this type of research, it is important to foster nimble and responsive organizations that have on-site resources to tackle unexpected questions when the unexpected occurs. The organizations that helped reveal how light trumped nutrients in Lake Superior after the Solstice Flood include Minnesota Sea Grant, the University of Minnesota Duluth's Large Lakes Observatory and Natural Resources Research Institute, and the Great Lakes Observing System. For more details about this study, see the YouTube presentation (minute 18:33), The affect of the June 2012 flood on dissolved nutrients (C, N, & P) in western Lake Superior.
A journal article about this research is in review.
Brandy Forsman, a teaching assistant with the University of Minnesota Duluth's Chemistry and Biochemistry Department, was awarded the Minnesota Sea Grant/Muskies Inc. Scholarship in 2013. She used the award to share the results of her work with the flood data at the International Association of Great Lakes Research Conference.
By Brandy Forsman