by Adam Hinterthuer
On a beautiful calm Friday back in June, CFL 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 was unusual because Lake Mendota is not known for 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 three-and-a-half and (if you get lucky) seven feet before it disappears from view. Yet Wakker’s Secchi disk only passed out of sight once it passed eleven. And that was meters, not feet. Lake Mendota looked more like the waters of the Bahamas than a eutrophic lake in southern Wisconsin. It was “crazy,” Wakker said, watching the Secchi disk dangle 33 feet below her boat.
The “clear water phase” of Madison’s lakes is a normal rite of spring. But this year, that “normal” clear-water phase became exceptional. Not only did Secchi readings reach remarkable depths, but this year’s extremely clear water lasted 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 four meters. That places the rough start date to this year’s phase at May 1st and, by the time waters ‘greened up’ this year, we’d enjoyed more than six weeks of clear water. While that easily beats anything we’ve seen in the last decade, 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.
Much of this trend can be tied to our favorite native zooplankton – Daphnia pulicaria. In spring, as ice comes off and wind mixes the lakes and the sun begins warming them back up, the water reaches a point where conditions are optimal for daphnia. And, when their populations boom, they gobble up the phytoplankton and algae that usually give our lakes their green hue.
In 2009, however, another 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.
For some reason this year, spiny water fleas took a long time to show up in the samples we regularly take on Madison’s lakes and the extended cool spring held the surface of Lake Mendota at optimal temperatures for both daphnia pulicaria and the kinds of phytoplankton and algae they like to eat.
Of course, nothing lasts forever and our lakes did get green again this summer once spiny water flea populations finally got going and began eating daphnia and the warm surface of our lakes became coated with cyanobacteria, or blue green algae.
While it’s unfortunate to know that murkier, greener summer waters are just as much a part of our lakes’ cycle as spring’s clear water phase, it was nice to see that daphnia pulicaria are doing well enough in Lake Mendota to take advantage when conditions favor them. Clear water phases like the one we saw this June appear to be getting rare, but as this spring showed us, they’re not gone for good.