Algae Blooms in Fall Mean Lake Mendota Is Mixed Up

Lake Mendota on November 2nd, 2018. Photo: Natalie Schmer 

Originally Published October, 2014 – Earlier this month, anyone down on the shores of Madison’s lakes may have noticed the water tinged the green hue of an algae bloom, something we normally associate with the warmer summer months. But a final fall bloom isn’t all that unusual – It just means that the lakes are, literally, all mixed up.

During the summer our lakes stratify, meaning that a warm upper layer of water forms and settles on top of the denser, colder water below. Throughout this period, only that upper warm layer of water, or epilimnion, gets mixed up, bringing anything floating at the bottom of the layer to the surface and sending things at the top down. But nutrients that fall into the lower, colder layer of water, the hypolimnion, are trapped and begin to build up over time.

When the weather (and the lake) begins to cool, the lake enters a phase called “turnover.” During turnover, the epilimnion cools off, grows denser and pushes into the hypolimnion, shrinking that cold, bottom layer and bringing whatever is in it back into the mixing cycle.

Illustration of seasonal lake mixing and stratification. Image: National Geographic

Eventually this upper layer cools to the temperature of the bottom layer and the entire lake mixes, bringing an upwelling of nutrients stored since late spring. And these nutrients act like a fertilizer, promoting the growth of anything green.

Center for Limnology grad student, Luke Loken, was recently out taking samples on Lake Mendota and, crunching his data back in the lab, discovered that he had managed to catch the turnover in action. Loken has devised a scientific instrument that takes real-time measurements of different parameters as he speeds around the lake in one of our boats.

Below is a map of Lake Mendota’s chlorophyll readings on October 10th. Chlorophyll measurements indicate, essentially, how green the water is. The greener the water, the higher the concentration of algae. As you can see, Loken’s instruments began registering low chlorophyll readings on October 10th, except for the hotspot where the Yahara River flows into the lake, bringing upstream nutrients with it.

Aside from the nutrients being carried into the lake by the Yahara River and promoting algae growth, Lake Mendota is relatively quiet on October 10th. Image: Luke Loken

But, compare the above map to the one below. On October 23rd, only thirteen days later, Luke was out on Lake Mendota as it was in the middle of a bloom, with chlorophyll concentrations highest near-shore. The lake still wasn’t fully mixed, but more and more water (and the nutrients stored in it) was suddenly brought back into the mixing cycle as the epilimnion grew and its boundary dropped deeper into the lake.

By October 23rd, chlorophyll A readings in Lake Mendota were on the rise, as nutrients from below were suddenly available to algae near the surface. Image: Luke Loken

It’s a pretty cool snapshot of a major lake process in action. And, as Loken notes, it also shows how variable conditions can be on a lake. While the buoy deployed in the middle of the lake may not have registered much of a change, those of us on shore could see the results of the turnover right before our eyes!