Fall 2023


Trina McMahon standing in Lake Mendota with a beaker of lake water in her hand.
Trina McMahon wades in the shallows of Lake Mendota. Photo by Sharon Vanorny


In the fall 2017 issue of Grow, Erik Ness highlighted bacteriologist Katherine (Trina) McMahon and her quest to understand Lake Mendota through its microbial species. Since then, her long-term research program has analyzed decades of data from lake samples. One recent study found that two previously identified incursions by invasive species — a zooplankton known as the spiny water flea in 2009 and the early stages of a zebra mussel infestation in 2015 — had profound effects on water quality, toxicity, and algae blooms in Mendota.

McMahon, former graduate student Robin Rohwer, and their team found that in the years following both invasions, cyanobacteria (blue-green algae) began appearing in the lake during late spring, when the water used to be crystal clear.

This extended the length of the seasons in which harmful algae blooms persisted in subsequent years. In addition, they discovered that a far more diverse range of cyanobacteria thrived in the lake during the summers following the invasions.

The proportions of many other kinds of bacteria also changed during this time. The dominant species grew while the smaller populations dwindled. This finding shocked microbiologists because heterotrophic bacteria (those that, unlike cyanobacteria, get nutrients from plant or animal matter) were thought to be less sensitive to changes in their surrounding ecosystems.

Another finding: Following the zebra mussel invasion, toxic water conditions in Mendota rose, and the toxin production season lengthened. Specifically, detectable levels of microcystin, a common toxin produced by cyanobacteria, increased.

Thanks to this long-term data collection and analysis, researchers now have a deeper understanding of the interconnectedness of Lake Mendota’s microbial communities and how they are subject to long-term environmental change.

This article was posted in Basic Science, Changing Climate, Fall 2023, Follow-Up, Healthy Ecosystems and tagged , , , , , , , , , , , .