A mid-October algae bloom shows up on the Lake Mendota shoreline. Photo: Sam Oliver
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.
AN illustration of seasonal lake mixing and stratification. Image: National Geographic
Eventually this upper layer cools to the temperature of the bottom layer and the entire lake mixes, bringing an upwelling of nutrients stored since late spring. And these nutrients act like a fertilizer, promoting the growth of anything green. Continue reading