What’s bloomin’ these days in Lake Mendota?
By Jackie Lemaire– Last week we had our first big, noticeable, bloom of Lake Mendota of the season. Every year, as expected, we see a similar succession of greenness overtake the lake. However, it can be hard to predict the exact timing and the exact culprits. At the Center for Limnology, researchers in Trina (Katherine) McMahon’s and Jake Vander Zanden’s labs collect water samples twice a week to study the microorganisms in the water to track long-term trends of who is there; how they are interacting (ecology) and how they are changing (evolution). We now have 20-years of microbiome data published (Rohwer, Nature Microbiology, 2025).
Unfortunately there is a long lag-time in the process to identify the microbes and we often want more immediate information about which blue-green algae are out there. So, we also collect samples using a net and examine the samples under the microscope. PhD graduate student Jackie Lemaire and summer researchers Brita Lake and Mars Giehtbrock have had their boots in the water collecting samples all summer to help address and answer both immediate and long-term questions.
Many of the blue-green algae (more scientifically accurately called Cyanobacteria) in Lake Mendota are known to us from our two-decade long time-series and some even have distinct shapes and sizes. While it can be difficult to distinguish among many species of algae, these Cyanobacteria are easier to identify from the surface waters. After the ice melts in the spring, the green algae and diatom populations boom. Zooplankton (like Daphnia) eat all these organisms and grow to high abundances causing a clearwater phase where the water clarity is very high.
Once the waters are warm enough, the Cyanobacteria begin to grow. To identify which Cyanobacteria are here now, we do microscopy on the water to visualize and count what is present. Usually, Aphanizomenon is the first to grow when the waters are still cool in late May. These Cyanobacteria look like grass clippings in the water and form bright green surface blooms. This year we did not have a large surface bloom of Aphanizomenon, although it was still there in smaller numbers.
In early June this year, we found numerous different species of Cyanobacteria in the water. Along with the Aphanizomenon, we saw Dolichospermum (previously named Anabaena), Microcystis, and Gloeotrichia. Each has its own unique characteristics. Some can take nitrogen from the atmosphere and turn it into more available forms for themselves or other organisms in the water. Many produce toxins (e.g. microcystin, cylindrospermopsin, anatoxin-a) that impact the health of the system as well as the humans and animals who use it. Dolichospermum produces a volatile taste and odor compound (Geosmin) that has a very strong smell.
We know that phosphorus runoff from the surrounding agricultural and urban watershed feeds the algae and the Cyanobacteria, leading to Cyanobacterial blooms every year. However, the exact timing and growth of organisms varies on a year-to-year basis due to seasonal factors like rain, sun and wind. We will continue to track these changes and shifts in variability to establish important long-term trends in these freshwater systems that are vital to both our local and global communities.