Fish Fry Day: Daphnia Update & Perch (H2O) Purifiers?

Lake Monona is crystal clear, while Mendota stays murky and, on Wednesday, we asked you to help us monitor Lake Mendota as we wait to see if the native zooplankton, daphnia pulicaria, can rally and clear up the situation after being decimated by a tiny invasive predator called the spiny water flea. Read that previous post here. And see coverage of the issue courtesy of NBC 15’s nightly news!

Lake Mendota, May 21st, from the Hasler Lab pier. Secchi depth .75 meters. Photo: Adam Hinterthuer

Yesterday, CFL graduate student, Jake Walsh sent in these updates on the current conditions on both Mendota and Monona, as well as his thoughts about how we might go about tackling the spiny water flea problem. Fortunately for us, it involves our very favorite fish fry species! 

Thursday 5/21/15, Deep Hole of Lake Mendota: Daphnia are officially detectable again, however their densities are still very low: 472 per cubic meter. At this time of year before we found the spiny water flea in the lake, we’d expect 103,000 Daphnia per cubic meter. Even after the invasion we’d expect almost 49,000 per cubic meter at this time. As you might expect, water clarity is still very low in Lake Mendota: 3.9 feet.
Thursday 5/21/15, Deep Hole of Lake Monona: After my time on Lake Mendota, I drove through Tenney Locks, down the Yahara and into Lake Monona – it was a gorgeous day for putzing down the Yahara River (it’s a no wake zone, I wasn’t lingering). The clear water phase hit Lake Monona sometime in the past couple weeks and it’s still looking really good with a water clarity of 10.5 feet. I also found a lot of large Daphnia in my tow, there are many many more large Daphnia in Monona than Mendota right now.
Could these yellow perch in Lake Mendota help keep the water clear by eating invasive spiny water fleas? Photo: Ted Bier
Jake went on to offer up an idea or two about how we might help daphnia “fight back” against spiny water flea (SWF) and get back to clearer waters.
There are currently no ways to manage or eradicate SWF. I think there’s a lot of potential in the food web/fishery to reduce the direct impact of SWF, but I think manipulating the food web is just as likely to do something totally unexpected. Either way, it’s a testament to the long-term basic research that goes on in this lake via the Wisconsin Department of Natural Resources and our own Long-Term Ecological Research project, that we even have a chance at managing these things.
Our best bet is to manage water clarity by reducing phosphorus loading into the lake. Phosphorus sets the bounds of what “can” happen with water clarity. Zooplankton, primarily Daphnia, determine what “does” happen. With a strong Daphnia population, we were in the best possible scenario of what can happen, now that it’s weakened and other zooplankton are more abundant, we’re much closer to the worst possible scenario. Because of this, we have to change what can happen in water clarity by reducing the amount of phosphorus in the lake. We’re already doing this to some degree, by limiting the fertilizer that runs off of farm fields, for example.
One other possibility to manage SWF, would be to look at Lake Mendota’s fishery.For example, Monona has a stronger “planktivore” (zooplankton eating fish) community than Mendota. And it also has far fewer SWF, and, currently, better water clarity.I’ll admit that it’s a pretty fuzzy/loose hypothesis, but its a start for further research for sure.
We think the most likely “knob” that we could turn in the Mendota food web is the yellow perch population. Perch eat a ton of SWF and there’s evidence they prefer SWF over other zooplankton. However, perch also eat a ton of Daphnia, so there’s a strong possibility that turning that knob backfires. It’s still an interesting option to ponder and, at the very least, it would lead to better fish fry Fridays!