Just yesterday, Alice Ogden-Nussbaum, the Center for Limnology’s lead research technician on our long-term ecological monitoring project, headed out to the middle of Lake Mendota to collect some routine data on conditions in the lake. Alice is used to lowering limnology’s oldest tool, the Secchi disk, into the water and having the plate-sized, black and white disk disappear from view fairly quickly. But, each spring, there is a window of time where she gets to watch it slide deeper and deeper into the water.
This may sound unusual, since Lake Mendota is not known for its clear water. In fact, in late summer, a Secchi disk usually only drops one meter into the green, murky water before it disappears. Yet Alice’s Secchi disk finally passed out of sight once it passed 11 meters. The waters were so clear they almost brought to mind the Bahamas.
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!
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. Those conditions occurred right around May 15th this year, which means we’re only two week in to a phase that, from the mid 1990s until 2009 regularly lasted well over 40 days.
Unfortunately, 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.
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.
Until then, temperatures in Lake Mendota are holding at optimal levels for both daphnia pulicaria and the kinds of phytoplankton and algae they like to eat.
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.
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.