Monitor Mendota: Water Clarity, Daphnia on the Rebound

Algal bloom. Photo: Eddie Heath.
Algal bloom. Photo: Eddie Heath.

Last week on this blog, we wondered if Lake Mendota’s clear water phase was a thing of the past. You see, last year, the algae-eating native zooplankton, daphnia pulicaria were so diminished by predation from the invasive spiny water flea, that their numbers couldn’t grow large enough to keep algae from clouding our waters. Combined with the phosphorus that runs off into our lakes and acts like algal fertilizer, well, you’ve got a recipe for a soupy mess. And this year wasn’t looking much better.
Imagine my surprise, then, when I stepped out onto the Hasler Lab pier this afternoon and saw this…
Lake Mendota clearing up for clear-water phase. The question now isn't if, but how long will it last?
Lake Mendota clearing up for clear-water phase. The question now isn’t if, but how long will it last?

That’s right, that’s the BOTTOM of Lake Mendota!
Lake Mendota May 21, Hasler Pier.
Only a week ago (May 21st) the Secchi depth off of Hasler Lab’s pier was .75 meters. A week later, we hit bottom and head to head out in a boat and got a 5.1 meter (15+ feet) reading.

As a reminder, only last week, the water looked like the picture to your right. That May 21st sample yielded a .75 meter Secchi depth reading.
Today (May 28th), when Ellen Hamann and I went to drop a Secchi disk off the end of the pier, we hit bottom at about 3 meters and could still see the disk.
So, of course, we hopped in a boat and, with assistance from CFL post-doc, Peter Lisi, headed out to water deep enough to get a true reading.
As you can see from the video below, clarity was much improved from last week. In fact, the Secchi disk dropped nearly seven times deeper into the lake before disappearing from sight – a 5.1 meter Secchi depth reading. Each black mark is another meter down the rope…

We also brought along a plankton net to see if we could catch any daphnia in the water column. And, of course, the water was teeming with daphnia, the reason behind the clear-up as they are voracious grazers of algae.

My May 28th plankton tow in Lake Mendota would yield some promising developments. Photo: A. Hinterthuer
My May 28th plankton tow in Lake Mendota would yield some promising developments. Photo: A. Hinterthuer

In one toss over the side, I pulled up a sample full of egg-bearing daphnia – a good sign for the species’ prospects in our lake.
Jake Walsh, a CFL graduate student who probably knows more than anyone else here about the daphnia populations in Lake Mendota, was out on the lake yesterday and saw the same trend. “I haven¹t counted them yet, but it¹s looking better than none at all,” he says. “It¹s definitely a testament to how hardy Daphnia populations are, that they can go from the longest absence in our history to even average densities is pretty crazy. It¹s great that the question isn¹t ‘Will we get a clear water phase?’ but ‘How long will our clear water phase last?’ and ‘How clear will the water get?'”
The plankton tow captured countless zooplankton, many of them the big algae grazing, daphnia pulicaria, a species crucial to early spring water clarity. Photo: A. Hinterthuer
The plankton tow captured countless zooplankton, many of them the big algae grazing, daphnia pulicaria, a species crucial to early spring water clarity. Photo: A. Hinterthuer

But, Jake warns, that doesn’t mean we’re out of the woods regarding long-term water clarity trends.  Since the invasive spiny water flea has been detected in Lake Mendota, we¹ve lost almost a full month of clear water conditions (anything greater than a 4.0 meter Secchi depth) in the spring.  Those clear water conditions are starting 21 days later and ending 10 days earlier.
“If we count today as the first day of clear water conditions,” Walsh says, “it would be the second latest in the past twenty years, with all of the top five latest starts happening in the past 6 years.”
Still, clear water is a thing to be celebrated and, hopefully, this is a sign that daphnia are, at the very least, holding their own in Lake Mendota.
You can still help us monitor Mendota – read how here – and send in pictures of water conditions from wherever you are on the lake. We want to document how long the phase lasts, and just how clear we can get this year! #monitormendota
Seeing the big-bodies of the zooplankton, daphnia pulicaria, in a Lake Mendota water sample was a welcome sight. Many appeared to be carrying eggs. Photo: A. Hinterthuer
Seeing the big-bodies of the zooplankton, daphnia pulicaria, in a Lake Mendota water sample was a welcome sight. Many appeared to be carrying eggs. Photo: A. Hinterthuer

 

5 thoughts on “Monitor Mendota: Water Clarity, Daphnia on the Rebound”

  1. Can you provide any updates to the clear water period and populations of daphnia in Lake Mendota? It’s just amazing to me how fast things change on such a big lake.

    1. Hi Dave, as of yesterday, the Secchi depth was 5.4 meters (meaning you could see 15+ feet below the surface). It is amazing how quickly things change, but that’s what you get when the two main actors (daphnia and spiny water flea) have such short lifespans and can ramp their populations up to billions (if not more) in only a couple of days. No telling how long this clear water phase lasts, but the daphnia we caught all had eggs, so they’re at least holding their own for now! We’re not seeing record numbers of daphnia or anything, but they’re currently far outnumbering spiny water flea!

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