Is Lake Monona A Sign That Lake Mendota’s Clear Water Phase Is On Its Way?

Earlier this week, Center for Limnology director, Jake Vander Zanden, noted a startling difference in Madison’s two largest lakes, Mendota and Monona. The waters of Lake Monona, near where he lives, were crystal clear, while the waters along the shoreline of Lake Mendota, where he works, were green and murky. The two lakes are connected by the Yahara River and barely a half mile apart so what gives? Why would they be so unalike?
“it used to be that Lake Mendota got way clearer than Monona in the spring,” says Jake Walsh, a post-doctoral research at the Center for Limnology. But, now, “Mendota’s clear water phase is weaker, shorter and later.”
At least part of this dramatic change is a story about an off-kilter food web. 
Walsh has been keeping an eye on Lake Mendota’s water clarity for the past eight years and is often out on the lake studying how native and invasive zooplankton dynamics impact water clarity. 

Spiny water fleas can be distinguished from other zooplankton by their exceptionally long tails. Photo: Jake Walsh

Before the invasive spiny water flea moved in, Walsh says, Lake Mendota would enter what’s called “clear water phase” sometime in mid-May to early June as the water reached the perfect temperature for populations of a native zooplankton, called daphina pulicaria, to explode. Millions upon millions of the tiny little animals would eat algae nearly as fast as it could grow, keeping the water clear and free of its green tinge. Then, as the lake got warmer and conditions became less favorable for daphnia, the lake would grow more murky. 
But, in 2009, the spiny water flea exploded on the scene.
Like daphina, the spiny water flea is a zooplankton. But it’s not from around here. Originally transported into the Great Lakes accidentally in the ballast water of big ocean-going cargo ships, the spiny water flea has spread to other water bodies.
And it isn’t interested in algae. What it loves to eat are the big (for a zooplankton) and slow-moving daphnia. With daphnia numbers diminished, algae thrived.
Over in Lake Monona, Walsh says, the spiny water flea is present but not nearly in the abundance that we see in Lake Mendota. This means more daphnia are around, gobbling up algae and, for now at least, keeping the water clearer. 
But we haven’t completely lost the clear water phase in Lake Mendota, Walsh says. While it now starts sometime in late May or even well into June, there is still a window in the spring when conditions favor daphnia and spiny water flea populations haven’t yet reached super high abundance. In fact, Walsh says, that time may be right about now. 
Thanks to data from the North Temperate Lakes Long-Term Ecological Research program, we can see how water clarity trends changed before and after spiny water flea invasion. Image: Jake Walsh

“I feel like we may just be entering our window to have a clear water phase, if we’re going to get one,” Walsh says. “There are daphinia out there, just not a lot of them.”
With all of our spring storms, he notes, tons of water – along with fertilizers and other forms nutrient pollution – has been pouring into Lake Mendota all month. That feeds algae (turns out fertilizer is just as good at growing algae as it is growing corn) and the heavy flows from tributaries and strong winds associated with storms keep the lake mixed up and lots of sediment suspended in the water. 
Now that we’re entering a little calmer stretch of the forecast, it will be interesting to see how Lake Mendota responds. Daphnia pulicaria could rise up and give us a clearer window into our lake or, Walsh cautions, the kind of algae or, more accurately, cyanobacteria, that’s currently growing in Lake Mendota may add a different wrinkle to the story.
Often called blue-green algae, “it’s not something that daphnia is going to eat,” he says. Nor will zebra mussels, the lake’s newest invaders. But that’s a tale for another time. 
For now, stay tuned and keep an eye on Lake Mendota. We may be in for a pleasant surprise. 

Top Photo: Lake Monona’s already doing it, why aren’t Mendota’s waters this clear? Credit: A. Hinterthuer