What Causes the Algae Blooms in Madison’s Lakes?


After last week’s massive cyanobacteria (blue-green algae) bloom in Lake Mendota and smaller (but no less unpleasant) blooms reported in Lake Monona and Waubesa, we received all sorts of questions on what causes these blooms, if they are dangerous and how to stop them. Here are some answers to a few of the frequently asked questions. Think of it as “Phosphorus 101.”  
Meet the Problem: Too Much Phosphorus
Phosphorus (atomic number 15 on the periodic table) is an element that is abundant in the Earth’s crust. It gets into ecosystems by the slow weathering of rocks and it plays a crucial structural role in DNA and RNA and is essential to numerous cellular processes. Humans. Animals. Plants. We can’t live without it!
Aquatic ecosystems, especially freshwater ones like lakes and rivers, are often “phosphorus limited.” That means organisms that form the foundation of the food web – phytoplankton and algae – have basically everything they need to grow expect phosphorus. So, when a lot of phosphorus ends up in the lake, they are poised to take advantage and can quickly use it to grow. This is especially true on warm, calm days,as the buoyant algae can float to the surface and grow untroubled by the winds in ideal water temperatures. 
Plants on land need phosphorus too and, through their roots, they take phosphorus out of the soil and incorporate it into their cell structures. The problem with plants on land is often, as with our lawns and the crops we grow,  we then harvest those plants and remove them from the field or bag up our grass clippings. That means plants can’t decompose and release phosphorus back into the soil they grew in so the soil becomes depleted of this essential nutrient. And that’s where fertilizer enters the picture. 
The Primary Source: Agriculture 

Credit: Blake Draper, USGS

In the Yahara Chain of lakes (Mendota, Monona, Waubesa and Kegonsa) agricultural practices are, by far, the main source of phosphorus entering the system. That’s not to pick on agriculture, it’s simply a consequence of how we use our land. And, for these lakes at least, that land is primarily used for agriculture. Sure the lakes are pretty urban and, especially for Mendota and Monona, their shorelines are almost completely surrounded by houses and hotels and lawns and parking lots. But if you move a little further from the lakes, you pretty quickly get into a more agricultural landscape and this tapestry of farm houses and corn and soybean fields and dairy farms makes up most of the land in the Yahara chain of lakes’ watershed. The map on the right is of the watershed and, as you can see by the areas shaded yellow, frming dominates the landscape. 
Making matters worse, farming in Wisconsin creates a lot of phosphorus – both in the form of cow manure from dairy farms and synthetic fertilizers purchased to help grow corn and soybeans. When both kinds of these fertilizers (manure and synthetic) are spread on the soil, they are vulnerable to runoff. One heavy rain can carry loads of phosphorus and phosphorus-laden soils into nearby creeks and streams and, eventually, our lakes. In fact, U.S. Geological Survey stream gauges catch this in action. In streams that run through farmland and into our lakes, the phosphorus levels go way up after heavy rains. 
Other Sources: Cities, Lawns, Soaps
While it is true that urban landscapes can also contribute phosphorus to our lakes, policies and laws enacted over the past several decades have greatly reduced the urban contribution of phosphorus to the system. Wisconsin has banned phosphorus in lawn fertilizers since 2010. Madison’s street cleaning crews do a great job of keeping leaf litter and other sources of phosphorus out of the lake. And, way back in the 70’s, we stopped dumping our untreated wastewater directly into our lakes and, instead, now have a wastewater treatment plant that treats it and returns it to Badfish Creek, downstream of the Yahara Chain of lakes. 
Still, it is important to pay attention to what we do here on the Yahara lakes’ shores. Sources of phosphorus like pet waste or the soap used to wash your car or leaf litter that washes into our storm drains can add small amounts of phosphorus. But, when compared to the amount coming into the system through tributaries that drain agricultural land – like the Yahara River or Sixmile Creek – urban inputs of phosphorus are minimal. 
The Impacts: Gross Beaches, Dead Fish and Health Hazards
By encouraging so much algal growth in our lakes, phosphorus can have a lot of nasty consequences. As blooms of cyanobacteria (or blue-green algae) pile up in nearshore waters or gobs of stringy cladophora wash up on shore, they began to decompose and, well, they can really stink up the joint. 
In the case of blue-green algae, some varieties can produce toxins that are harmful to both pets and humans. Exposure to them can cause skin rashes and, for anything unlucky enough to swallow some cyanobacteria-laced water, it can lead to being very sick and, in rare cases, even death. Pets and small children are especially at risk in these situations. 
Algae blooms also contribute to what are called “dead zones” in our lakes. As algae dies and falls to the bottom of the lake, a bunch of tiny aquatic critters and bacteria start breaking it down and getting it decomposing. These organisms need oxygen and, when an algae bloom is big enough, so many of these tiny organisms can consumer so much oxygen, that the water at the bottom starts to run out of oxygen. When fish can’t escape these conditions in time, they suffocate. Big blooms are often followed by fish kills. 
The Solutions: Less Phosphorus and Less Runoff 
Runoff, from field to stream. Photo: NOAA

The primary way to get results is by reducing phosphorus loading in our lakes. And the only way to do that is to use less of it and make sure it stays where it’s put when it is used. 
Many farmers are working hard to keep their soil where they want it – on their land – and reduce runoff into our waterways. In fact, the innovative and collaborative Yahara Pride project is helping farmers in the Madison lakes’ watershed implement runoff reduction strategies like buffer strips, cover crops and other soil conservation efforts to reduce agriculture’s contribution to the phosphorus load. With dozens of farms enrolled in the project, Yahara Pride has documented reductions in phosphorus runoff from its partnering farms. Unfortunately, many farmers aren’t as engaged as the Yahara Pride farmers and are still using generations-old practices that do little to slow runoff from their farms. 
Of course the people who live around the Madison lakes can also play a role by cleaning up your pet waste and getting leaves and yard waste to the curb for pick up and looking for phosphorus-free soaps to wash cars. 
All of these things add up, but they are not going to move the needle on our bigger phosphorus problems. Perhaps the best thing a concerned citizen can do is to support the farmers who are committed to reducing their phosphorus contributions to the lakes and support any policies or programs that aim to increase participation in such practices.  
This won’t fix the problem overnight – phosphorus has an extremely long cycle and our soils in Wisconsin are saturated with it. But it could move the needle in the right direction and it could also reduce the number of these “flash” algae blooms, where a strong rain moves tons of phosphorus into our lakes and then warm, still conditions allow algae to hit that unexpected phosphorus buffet and take off.  
If we want our future to be one of addressing the problem and watching water quality slowly improve, then we’ll need to remember that what we do on land ends up in our waters. 

8 thoughts on “What Causes the Algae Blooms in Madison’s Lakes?”

  1. Residential & commercial properties still plant lawns & use cosmetic chemicals despite “bird city” etc, but agribusiness gets all the blame…. prohibit use of all cosmetic chemicals. Lawns don’t grow food.

    1. Hi Thomas, thanks for your comment. Agriculture is, indeed, a far more crucial land-use to human well-being than lawns. That said, current agricultural practices use a whole lot of phosphorus to grow that food. While phosphorus does sometimes enter Wisconsin waterways through residential and commercial use on lawns, the vast majority of it was taken out of that equation with the 2010 phosphorus ban in lawn fertilizer. The amount of fertilizer used by agriculture (and produced by dairy cows) FAR outweighs the amount applied to lawns (even before the ban). So, if we really want to see change in our lakes, we have to tackle the biggest source of phosphorus and that is, hands down, agribusiness related. We aren’t trying to “blame” anyone but, to tackle a problem, you need to know the scope of it – and phosphorus is largely entering our lakes in the form of runoff from farm fields where it has been used to fertilize crops. We started to address the problem with lawns through legislation like the phosphorus ban. Now we need to figure out ways to help farmers keep their soil on their fields and more efficiently use phosphorus-based fertilizers if we want to move the needle on this issue.

      1. Adam, you start with “agriculture” and then move to “agribusiness.” Apart from eliminating cosmetic chemical loads into lakes, which urban landowners have already made great progress doing with their use of native plants, consumers can be educated to make choices to prefer those agriCultural ops which manage their nutrients appropriately…. when I was on the Lake Belle View restoration committee in the early 1980s, WDNR had surveyed upstream land uses and of 151 farms, 3 were causing most of the problems — at that time — that was, of course, before CAFOs and consolidation. As the Sugar River figures above demonstrate, painting “ag” and “agribusiness” with large brush, is exposing those producers who are responsible, to unneeded and expensive regulations and oversight — of course CAFOs are easier to manage by enforcement agencies, and can more easily offset the cost of regulatory oversight through small cost additions per unit…. also, it’s *obviously* not just P but N and K and NH4, etc, doing damage, e.g. recent federal lands study showing 35% of all federal lands were exposed to excessive N drift. I’m sure the public understands there is not one bullet solution for one problem, but rather a web of things — like IES used to teach.

        1. Agreed, Thomas. There are plenty of farms – large and small – working to be good stewards of the land. In my post I took great care to mention the work of programs like Yahara Pride where several dozen Yahara watershed farms are working hard to improve the runoff situation. I didn’t mean to lump all agriculture together but, as the number of farms shrink and the size of the remaining farms grow, agribusinesses like CAFO’s and large corn/soybean plantings are quickly becoming the dominant model on the landscape.

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