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.

18 thoughts on “What’s Behind This Extended Phase of Crazy-Clear Water in Lake Mendota?”

  1. Who eats daphnia when spiny numbers are low? Surely something else in the food chain is enjoying the bounty…

    1. Hi Eric! Right now there are plenty of planktivore fish species (like yellow perch, crappie and bluegill) loving the clouds of daphnia. In fact in the late 1980’s CFL researchers and the DNR stocked tons of walleye and pike to eat those fish and take some pressure off of daphnia. It worked & the lake is still managed as a walleye/pike sport fishery.

    1. Hi Margaret, while extreme heat might do some damage to the eggs, spiny water fleas eggs are notoriously tough and these cooler but not cold temps aren’t doing much if anything to them. They can last a year or more. Things like temperature and food availability will impact the amount of eggs a spiny will produce but, one those eggs exist, they’re pretty resilient!

        1. No, the eggs survive summer and winter just fine, unfortunately! (They usually settle down at the bottom of the lake where the temps stay nice and cool). Warmer temps can correspond with lower populations of spiny water flea adults but we’re stuck with them, I’m afraid. still, it’s good to know we have enough daphnia in the lake to still really have a good year when conditions line up perfectly!

        2. Wanted to ask you like that before, it turns out they are still living in the depths of the water. Only at what depth can they live when the summer occurs?

          1. Hi Abang,
            Daphnia and spiny water flea can live pretty much all the way down at the bottom of the lake – as long as there’s enough oxygen. They migrate each day between the surface and the deeper parts of the lake.

  2. Could zebra mussels also be contributing to the water clarity, potentially allowing more light to penetrate and increase weed growth?

    1. Hi Crystal,
      We’re not yet sure if the zebra mussel to lake volume ratio is such that zebra mussels are having a huge clarity impact – that said, they are definitely contributing to this, although long clear-water phases with deep Secchi readings happened commonly before zebra mussels showed up (and before spinies showed up, obviously) so zebra mussels are unlikely to be the driving force behind clear water – at least in the spring time!

  3. Hi Adam,
    Do you know if other Wisconsin lakes known to have spiny water flea populations are experiencing similarly clear water this spring because of the conditions?

  4. Are there any other effects expected later in the year from this, so far as nutrient cycling? For instance, increased plant growth on the lake bottom, feeding other fish (carp?), other aquatic invertebrates, or …?
    The water in Warner Pond (lagoon) is plenty murky already, but of course doesn’t have the same reservoir of cold water beneath the cool surface. Don’t know about the spiny water flea. Lastly, because you’re actually answering these questions (thank you!), do you know of anywhere I could get training on monitoring water quality in Warner Pond and its inflows? I finally have a microscope, and this pond is where I spend a great deal of time foraging and just plain being fascinated by everything. Would like to be able to detect microscopic critters out of balance through the year.

    1. Hi KT,
      We don’t foresee doing much to disrupt other lake processes, other than delaying the inevitable green water of summer (which we’re seeing now). A healthy daphnia population does mean that they are laying plenty of eggs and so the sediment should be loaded with them next year as well (of course they need the right water conditions to really flourish). We are noticing more plant growth on the nearshore bottom of the lake but that’s likely zebra mussel related as zebra mussels concentrate nutrients down at the bottom and provide structure for algae to grow on so as long as sunlight can reach, plants will grow! As far as training, you might check out something like Clean Lakes Alliance’s “Yahara Watershed Academy” – they also train citizen monitors for water clarity so might be of some help! Thanks for reading!

  5. So what happened this weekend? On a canoe ride, the algae was so abundant on Mendota and Monona, that the foul smell ended the trip.

    1. Sorry about the interrupted paddle, Diane! All of those big spring storms, combined with no crops in the ground to hold fertilizer-rich soil in place, meant a big dose of phosphorus for the lakes. Then it finally warmed up….

  6. Is the water also more clear because of the program “Suck the Muck” where phosphorous sediment is being taken out of systems?

    1. Suck the Muck may be having success in slightly lowering phosphorus inputs to the lake, but the clear water we saw this spring was due to a whole lot of daphnia hard at work – there was still plenty of phosphorus in the lake to grow algae – as this summer’s green waters can attest!

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