Massive Blue-Green Algae Bloom Stretches Across Lake Mendota

A couple of weeks ago, after yet another round of intense rain in the Madison area, we headed over to the website of the Wisconsin State Climatology office, curious if we were seeing more rain than normal. The answer was, yes, yes we were. In fact, our little corner of Dane County had received anywhere from 4 to 5 inches of rain more than the long-term average for the month of May. 
And we knew what that meant – conditions were ripe for a blue-green algae bloom. 
Today, almost a year exactly from the big Father’s Day bloom of 2017, Lake Mendota is glowing a seemingly unnatural shade of green all the way from our lab to where our buoy is anchored in the middle of the lake. 
So what’s going on? To borrow from last year’s report on the bloom
Lake Mendota sits in a landscape dominated by agriculture. And some elements of this agriculture, especially the manure produced by dairy operations and synthetic fertilizers used to help corn and soybeans grow, is loaded with phosphorus. This wouldn’t be a huge problem if things would just stay where they’re put.
But rain, especially the “gullywashers,” carry tons of phosphorus-laden soil into nearby creeks and streams, where it eventually ends up in our lakes and is just as good at growing algae as it is soybeans.
Then the weather got warm. 
“So we had perfect conditions for blue greens because they like it warmer than other algae and they grow fast in warm water,” Steve Carpenter, professor emeritus at the Center for Limnology says. As soon as it got hot, “we had this incredible spin up of cyanobacteria (blue-green algae) in the lake surface water and then the wind stopped, and these kinds of algae are buoyant and they just floated to the top in this awful scum.”
So there you go. We should expect this to dissipate fairly quickly, but not before it starts stinking and potentially produces a fish kill. If you want to learn more about what’s causing these conditions and their impacts, you can start with last year’s article.
The take home message is that what we do on land ends up in our water. Oh, and that you (and your pets) shouldn’t be going in the water during a bloom as blue-green algae (more accurately known as cyanobacteria) can produce harmful, even deadly, toxins. 
So please just watch this show from the safety of the shore! 


7 thoughts on “Massive Blue-Green Algae Bloom Stretches Across Lake Mendota”

    1. Hi Cyndy,
      While most of the Yahara Lakes are physically in cities and have lots of urban development around them, their watersheds are predominantly agricultural landscapes. You can see on this map (the yellow indicates land that is cultivated crops). It is true that some urban runoff can contribute to algae blooms, but just by the nature of the watershed, the vast majority of our runoff into the lake is coming from farm fields. Also in Wisconsin, we have a state-wide ban on phosphorus in lawn fertilizer that has dramatically reduced the contribution lawns make to a lake’s phospohrus level. Our research has shown that phosphorus from agricultural sources is overwhelmingly the biggest contributor to these problems.

      1. Hey Adam and Cyndy,
        It looks like there’s not a map of the Yahara Watershed in Adam’s reply, but it’s really very interesting to see where the water in our lakes comes from. The map here: shows a map that highlights the entire watershed and shows the land use. Any rain that falls in this highlighted area, which north of Deforest, WI and south into northern Rock County will end up in the Yahara River. Of course, anything that falls downstream of the Madison lakes will not end up in these lakes, but regardless, it affects everything downstream.
        This map shows exactly what Adam was talking about — most of the land in the Yahara Watershed is agricultural land, so the fertilizers used on fields will have a large impact on what makes its way into Lake Mendota, Lake Mendota, et al. That’s not to say that residential property doesn’t matter — it does, of course! So it’s also a good idea to make conscientious choices about lawn treatments and to dispose of pet waste properly.

        1. Thanks, Rob for the link and the reply. We agree – what we do on land ends up in our lakes so lawns, parking lots, etc. it all matters. It’s just that, when we talk about algae blooms, that is primarily a problem caused by the over-enrichment of our lakes by phosphorus and the vast majority of that phosporus comes from agricultural sources.

  1. I think it’s important to also note that this can be fixed. I grew up on the East Coast and when I was a child I swam in grean slime. After 20-plus years of intensive watershed management, the water is clear for my kids to swim in. I am hoping now that I live in the Northwoods of Wisconsin we will stop the pollution before it gets to this point. And I hope the greater Madison area makes the effort to fix Lake Mendota.

  2. The manure and phosphorus runnoff into Lake Mendota has been a major water quality crisis since the early seventies. Its now, due to large dairy cattle operations, a major ecological embarassment for the State and Madison.
    When will the dairy farms be required to digestor process their manure on their property and stop putting the burden on the Watershed, Land, and Lake Mendota.
    The dairy farming operations are distributing costs of manure processing to the water resources and the tax payers in Wisconsin. It understandable, but way overdue for correction. How many more children will be sickened when swimming in the Lake, how many fish and wildlife will die directly from the algae blooms….
    Madison is a nationwide laughing stock for the yearly water, wildlife and fishery loss…..

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