Earlier this month, anyone down on the shores of Madison’s lakes may have noticed the water tinged the green hue of an algae bloom, something we normally associate with the warmer summer months. But a final fall bloom isn’t all that unusual – It just means that the lakes are, literally, all mixed up.
During the summer our lakes stratify, meaning that a warm upper layer of water forms and settles on top of the denser, colder water below. Throughout this period, only that upper warm layer of water, or epilimnion, gets mixed up, bringing anything floating at the bottom of the layer to the surface and sending things at the top down. But nutrients that fall into the lower, colder layer of water, the hypolimnion, are trapped and begin to build up over time.
When the weather (and the lake) begins to cool, the lake enters a phase called “turnover.” During turnover, the epilimnion cools off, grows denser and pushes into the hypolimnion, shrinking that cold, bottom layer and bringing whatever is in it back into the mixing cycle.
The latest issue of Madison’s Capital Times has an excellent cover story exploring a new effort to reduce the amount of phosphorus that ends up in Madison lakes. As the article points out – each pound of phosphorus that enters our waterways can fertilize the growth of up to 500 pounds of blue-green algae.
Yahara Pride Farms is an affiliate of the Clean Lakes Alliance and, in our opinion, is on the right track to addressing the impaired water quality of our lakes. Agriculture is, far and away, the leading contributor of nutrient pollution to Wisconsin waterways. And, according to the article, of those farms, 30 percent of the fields are responsible for contributing 70 percent of the phosphorus. Helping those farmers reduce their runoff by updating nutrient management plans, planting cover crops and changing tilling practices will go a long way toward meeting clean water goals.
Sure, the state’s surface water may currently seem to be frozen solid, but it’s nice to know that there are folks out there working to ensure that we’ll continue to get to enjoy our lakes once they’re back in liquid form.
If you head down to the shore of Lake Mendota today, you’ll notice you can see right down to the bottom. In fact, the current Secchi reading is seven meters, meaning you can get a clear view of Lake Mendota’s depths more than 20 feet down.
At first glance, it might seem that there’s just not much going on down there, but Lake Mendota is actually teeming with life and right in the middle of an algae bloom.
So what gives on the clear water?
The secret to our currently crystal-clear lake is a tiny zooplankton called Daphnia pulicaria.
While conditions are ideal for some species of algae, like fast-growing diatoms, to thrive, the current cool, highly-oxygenated water is also perfect for Daphnia pulicaria, which are voracious grazers of these kinds of algae.
According to CFL research specialist, Ted Bier, this kind of algae means good eats for daphnia and, right now, “they’re gobbling it up as fast as it’s growing.”
Bier says that, in its current state, the lake’s food web is humming right along. Nutrients in the water are consumed by the algae, which are then eaten by Daphnia that then become food for fish, efficiently passing nutrients right up the chain.
But, Bier says, there’s no way to know how long it’ll last. “Two years ago clear water only lasted 36 hours,” he says, thanks to a big rainstorm followed by baking temperatures. “Last year it was two weeks. We’ve had it last as long as two months.”
The current clear-water state is happening a bit earlier than average. Bier’s been taking samples each spring for ten years and the first big lake clearing is usually sometime around mid May.
Thanks to this year’s early lake warming and the last couple of weeks of cool, windy, dry weather – conditions are perfect for the annual early algae bloom and subsequent daphnia pulicaria feast. But, if we have a week of high temperatures or a big rain event that flushes a lot of nutrients into the lakes, a different kind of algae, called blue green or cyanobacteria, will begin to take over and we can kiss the clear water phase goodbye. Daphnia just don’t graze on blue green algae with the same relish and head to cooler, deeper waters once the lake warms.
Whatever window of clear water we do get this year, we can thank a little tiny zooplankton that’s a crucial component to our lakes’ water quality and is currently teeming right before our eyes – even if we can’t quite see it.