What’s Behind This Extended Phase of Crazy-Clear Water in Lake Mendota?

Last Friday, Center for Limnology lab technician, Petra Wakker, headed out to the middle of Lake Mendota to collect some routine data on conditions in the lake. As she unwound the rope holding limnology’s oldest tool, the Secchi disk, and lowered it over the side of the boat, something strange happened – the plate-sized, black and white disk slid deeper and deeper into the water, but Wakker could still see it.

This is unusual because Lake Mendota is not known for its clear water. In fact, in summer, the average depth a Secchi disk drops into the green water in the middle of Lake Mendota is somewhere between 3.5 and 7 feet before it disappears from view. Yet Wakker’s Secchi disk finally passed out of sight once it passed 11 meters, not feet. Petra was out on Lake Mendota in waters so clear they brought to mind the Bahamas. It was, she says, “crazy” watching the Secchi disk dangle 33 feet below her boat.

Center for Limnology postdoctoral researcher, Jake Walsh, agrees with her assessment. He’s been studying Lake Mendota for the better part of a decade and has never seen water this clear.

Water clarity last reached 11 meters in Lake Mendota was 2019. 

If you’re looking for someone to thank for this clear water, look no further than one of Lake Mendota’s tiniest residents. A little native zooplankton called Daphnia pulicaria is eating even tinier green plants like phytoplankton and algae almost as fast as they can grow. And there are a LOT of daphnia.

Toss a plankton net into any part of Lake Mendota at the moment and you’ll end up with a sample that essentially looks like the video below.

Let’s pretend that sample reflects the amount of daphnia in a cubic meter of water (it’s awkward to talk about “boxes” of water, but imagine a square section of the lake that’s 3 feet by 3 feet). There are roughly 500 million of those cubic meters of water in Lake Mendota. So multiply the number of daphnia in the sample above by 500 million and you can see how quickly the numbers add up!

What’s more, Walsh says, is that this year’s extremely clear water is lasting quite a bit longer than what we’ve seen in recent years.

Clear water phase begins when springtime water is clear enough on Lake Mendota that a Secchi disk can be seen down at least 4 meters. That places the rough start date to this year’s phase at May 1st, which means we’ve enjoyed a month and a half of water clarity. While that easily beats anything we’ve seen in the last several years, it’s not all that unusual if you look back far enough. From the mid 1990s until 2009, Mendota’s clear water phase regularly lasted well over 40 days.

This images models the length of clear water phases since 1995. Figure: Jake Walsh

Then, in 2009, another tiny zooplankton entered the picture. This one, called the spiny water flea, is invasive. And it loves to eat daphnia. After the spiny water flea invasion, what we would commonly see in spring is shorter clear water phases as the spiny water flea took advantage of the springtime daphnia buffet. In other words, they ate the things that eat the algae. Daphnia numbers would explode and the lake would clear up and then the spiny water flea population would boom, pushing daphnia numbers down.

Spiny water flea. Photo: Jake Walsh

So far this year, spiny water fleas aren’t showing up in our nets. That doesn’t mean they aren’t here- there are millions upon millions of their eggs resting in the lake sediment, just waiting for the right conditions to hatch.

In the meantime,though, this extended cool spring has held the surface of Lake Mendota at optimal temperatures for both daphnia pulicaria and the kinds of phytoplankton and algae they like to eat. As you can see from the graph below, Lake Mendota’s surface water temperatures have been cooler for this time of year than most springs from the past decade.

Image courtesy Hilary Dugan.

Eventually, as we move into summer and the lake warms, the spiny water flea will enter the picture and start eating daphnia, the remaining daphnia will head deeper in the lake looking for cooler waters and the warm surface of the lake will favor the growth of things like cyanobacteria, or blue green algae.

While it’s unfortunate to know that murkier, greener waters are just around the corner, it is nice to see that daphnia pulicaria are doing well enough in Lake Mendota to take advantage when conditions in the lake suit them. While clear water phases like the one we’re seeing now are getting much more rare, it at least means that they’re not gone for good.

Common carp check out the CFL boatslip during clear water phase. Photo: A. Hinterthuer

Editor’s Note: As with all of our posts on how lakes have changed over time, this wouldn’t be possible without an NSF-funded project called the North Temperate Lakes Long-Term Ecological Research program. NTL-LTER has monitored Lake Mendota and ten other Wisconsin lakes and bogs since the 1980s.