Clear Water? Yes. Green Scum? Also yes. What’s Going on Out There?

by Jake Vander Zanden  — You may have noticed that, In the last week or so, the Madison lakes have been crystal clear, with visibility of up to 30 feet recorded in Lake Mendota by Center for Limnology researchers.

Lake Monona in Madison, Wisconsin during clear water phase. Photo: Hilary Dugan

But, in the last two days, (especially if you’ve been near a downwind shore of our lakes) you may have also noticed mats of yellow or green scum floating at the surface, piling up near the shore, and washing up on beaches.

Floating mats of algae form a scum along the Lake Monona shoreline. Photo: Jake Vander Zanden

So what gives? We have a situation in which two contradictory water quality phenomena occur simultaneously. While I am undoubtedly a glass half full kind of guy and excited to see some clear water, it is difficult to ignore the mats of scum washing onto shore!

The Madison lakes are currently experiencing what is known as the spring clear water phase. This occurs for a couple of weeks each spring in our lakes. During this time, the small floating algae (phytoplankton) that normally turn the water green get grazed down by Daphnia (a tiny, native zooplankton or crustacean). Over the last few years, a new addition to our lakes, the invasive zebra mussel (a mollusk) has also taken up residence and is gobbling up algae. The result is a short window of beautiful crystal clear water.

Unfortunately it never lasts long, and the phytoplankton will soon proliferate again.

During the clear water phase, it’s easy to think that everything is alright in our lakes. But it is deceiving, and we need to remember that the Madison lakes are loaded with nutrients – primarily phosphorus that runs off of agricultural fields during spring rains.

A microscopic shot of a green algae called Spirogyras. Image:

When the water is crystal clear, lots of light reaches the bottom of the lake. The combination of lots of light and abundant phosphorus stimulates a luxuriant growth of algal mats on the lake bottom. The mats we are observing in the Madison lakes are dominated by a type of green algae called Spirogyra. These bottom mats of Spirogyra can become dislodged and then float to the surface.

Add a gentle wind like what we’ve seen yesterday and today and the mats stay intact and afloat, and accumulate along the shore. Where exactly these mats appear depends on the wind direction. Yesterday they were along the south shore of Lake Mendota. Today there were thick mats along northeast shore of Lake Monona. In the coming days, these piles of algal will decompose on shores and beaches and create horrible smells and nuisance conditions.

When mats of algae wash up on shore they begin to decompose – creating nuisance conditions and horrible smells. Photo: Jake Vander Zanden

So what to take home from all of this? First, bad water quality can come in many different forms. While we often hear about blue-green algae, other types of algae can also degrade our waterways and cause nuisance conditions. Second, the recent zebra mussel invasion of the Madison lakes is making the problem of floating mats of bottom algae much worse by concentrating nutrients down along the bottom of the lake where the mats like to grow and helping clear the water so that sunlight can reach the bottom and help those mats grow.

And finally, reducing nutrient inputs to the lakes from the surrounding agricultural and urban land is the main thing we can do to reduce excessive algal growth of all types and help keep our lakes in these clear water kinds of conditions more often.

The view along the Mendota shoreline shows the lake in its clear water phase Photo: Adam Hinterthuer