Over the last two weeks, Lake Monona, Lake Waubesa, and Lake Kegonsa have all entered into their annual rite of spring’s clear-water phase. Lake Mendota, however, remains a murky mystery. Why are the downstream Yahara lakes so clear, when the lake at the top of the chain isn’t? Perhaps more important, are we going to miss Mendota’s clear-water window this year?
Not necessarily, says Ted Bier, research specialist for the North Temperate Lakes Long-Term Ecological Research program. “Mendota was right on the cusp of moving into a clear-water phase over Memorial Day weekend,” Bier says. “But then it got slammed by that last storm.”
If you need a refresher (or aren”t here in Madison) that storm dumped 1 to 2 inches of rain on our fair city in a two-hour span. And all of that water ended up in our lakes, carrying sediments and nutrients that can cloud the water. Sediments can take several days to settle out to the bottom, especially if windy conditions keep them suspended, says Bier. And that sudden influx of nutrients can cause algal blooms.
People around the lake have been noticing. My office, for example, usually affords me a view of carp congregating just off shore for their annual spawning run (million-dollar view, I know). This year, all I can see are ripples on the green surface. The Clean Lakes Alliance recently shared a picture one of their members took of a greenish brown boat wake in Mendota – the result of algae being churned up to the surface by its propellers.
Still, Bier says, if the weather stays mild, we may yet see a clearer Lake Mendota this spring. And that all has to do with a tiny crustecean called Daphnia.
Daphnia’s role in clear water has been covered before in this blog, so read here to get the full story. The quick hit version is that they are a zooplankton that loves eating algae – especially the kind of algae growing in our lakes in the spring. But daphnia are picky, and they only really get going once the water reaches their optimal temperature.
“Daphnia populations start ramping up when the surface temperature is about 55 degrees [Fahrenheit],” Bier says. “They’re going full bore once it’s about 65.”
What does “full bore” mean for Daphnia populations in Lake Mendota? Think of it this way, says Center for Limnology faculty member, Jake Vander Zanden. “There are, roughly, 500 million cubic meters of water in Lake Mendota.” When Daphnia are at their peak, he says, “there can be 10,000 or more in a single cubic meter of water.”
Once those billions upon billions of daphnia get going, they eat algae nearly as fast as it can grow, keeping the water crystal clear. Lake Mendota was slow to warm this year, thanks to a relatively late ice out and mild spring temperatures. Last week, Bier’s sampling showed Lake Mendota’s surface at right about 60 Fahrenheit, near-optimal for Daphnia but then, of course, the rains came.
Our current calm weather should let Lake Mendota settle down and the forecast is good for a clear water phase, Bier says. “Days in the mid seventies with nights in the fifties is perfect to keep the lake temperatures right where Daphnia like it,” he says. “But, the minute we get lows in the seventies and eighty-degree days, it’s over.” Then Daphnia head deeper, chasing the 60-degree water.
Right now, Lake Mendota’s surface temp is right in Daphnia’s sweet spot, while Lake Monona, Waubesa and Kegonsa are closer to the 70’s. That means their clear water is on it’s way out, while, hopefully, Mendota is just getting started.
In fact, Bier reports, the Secchi depth in the middle of Lake Mendota yesterday was only 1.4 meters. Today, it’s already more than two meters. Maybe we haven’t missed clear-water just yet.