The view along the Mendota shoreline shows the lake in its Spring "clear water" phase Photo: Adam Hinterthuer

If you head down to the shore of Lake Mendota today, you’ll notice you can see right down to the bottom. In fact, the current Secchi reading is seven meters, meaning you can get a clear view of Lake Mendota’s depths more than 20 feet down.

At first glance, it might seem that there’s just not much going on down there, but Lake Mendota is actually teeming with life and right in the middle of an algae bloom.

So what gives on the clear water?

The secret to our currently crystal-clear lake is a tiny zooplankton called Daphnia pulicaria.

While conditions are ideal for some species of algae, like fast-growing diatoms, to thrive, the current cool, highly-oxygenated water is also perfect for Daphnia pulicaria, which are voracious grazers of these kinds of algae.

Daphnia pulicaria Photo: The Wilson Lab at Auburn University

According to CFL research specialist, Ted Bier,  this kind of algae means good eats for daphnia and, right now, “they’re gobbling it up as fast as it’s growing.”

Bier says that, in its current state, the lake’s food web is humming right along. Nutrients in the water are consumed by the algae, which are then eaten by Daphnia that then become food for fish, efficiently passing nutrients right up the chain.

But, Bier says, there’s no way to know how long it’ll last. “Two years ago clear water only lasted 36 hours,” he says, thanks to a big rainstorm followed by baking temperatures. “Last year it was two weeks. We’ve had it last as long as two months.”

From left to right: water samples from March 15, April 1 and April 15 show the spring daphnia pulicaria population bloom in Lake Mendota Photo: Adam Hinterthuer

The current clear-water state is happening a bit earlier than average. Bier’s been taking samples each spring for ten years and the first big lake clearing is usually sometime around mid May.

Thanks to this year’s early lake warming and the last couple of weeks of cool, windy, dry weather – conditions are perfect for the annual early algae bloom and subsequent daphnia pulicaria feast. But, if we have a week of high temperatures or a big rain event that flushes a lot of nutrients into the lakes, a different kind of algae, called blue green or cyanobacteria, will begin to take over and we can kiss the clear water phase goodbye. Daphnia just don’t graze on blue green algae with the same relish and head to cooler, deeper waters once the lake warms.

Whatever window of clear water we do get this year, we can thank a little tiny zooplankton that’s a crucial component to our lakes’ water quality and is currently teeming right before our eyes – even if we can’t quite see it.