Learn Your Lakes: Spring Runoff and Nutrient Loading

This weekend, my family and I went on a drive. These days, any chance to leave the house while still practicing safe social distancing feels like a blessing and this drive felt especially so. There wasn’t a cloud in the sky and hints of the lush, green landscape Wisconsin will soon become were everywhere – buds on the trees, daffodils swaying in the breeze, tractors crawling across acres and acres of fields studded with the stalks of last year’s corn.

Spring manure application. Photo: Cornell Cooperative Extension

But, with the windows down, there was another reminder in the air. One that hints at a soupy, green summer for our lakes – the pungent, overwhelming smell of cow manure.

Today, we are on our third consecutive day of rain, and that scene from our Sunday drive keeping flashing across my mind. Because here in America’s dairyland, April showers don’t just bring May flowers. They also bring June’s algae blooms.

You see, big rain events end up carrying a lot of top soil off of farm fields and into nearby waterways. And the fertilizers that farmers use to grow corn and soybeans (like the manure currently being applied to spring fields) are just as good at growing algae as they are cash crops.

We dusted off an old post from 2018 to help explain:

Meet the Problem: Too Much Phosphorus

Nitrogen is also a culprit here, especially in marine ecosystems, but for Wisconsin’s lakes the prime suspect in algae issues is phosphorus.

Phosphorus (atomic number 15 on the periodic table) is a naturally occurring element that is abundant in the Earth’s crust. It gets into ecosystems by the slow weathering of rocks and 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 of summer, 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, every year, the cycle starts again and we have to add fertilizer to get phosphorus back into the soil.

The Primary Source: Agriculture 

Heavy rains mean lots of farm runoff into Wisconsin waters. Photo: The Yahara Project

In the Yahara Chain of lakes (Mendota, Monona, Waubesa and Kegonsa) agricultural practices are, by far, the main source of phosphorus entering the system. We are not trying to pick on agriculture, it’s simply a consequence of how we use our land.

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 landscape where the water still runs into our lakes, but the land is covered in a tapestry of farm houses and corn and soybean fields and dairy farms. This agricultural landscape makes up most of the land in our watershed.

The map on the right is of the watershed and, as you can see by the areas shaded yellow, farming 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

Urban landscapes can also contribute phosphorus to our lakes, but policies and laws enacted over the past several decades have greatly reduced their 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 – the urban contribution of phosphorus is a small part of the larger problem.

The Impacts: Gross Beaches, Dead Fish and Health Hazards

A cyanobacteria bloom on Lake Mendota. Photo: David Tenenbaum, UW-Madison

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 a stringy algae called 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 because they’re not quite as dilligent about not swallowing the water.

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 in a process called “decomposition.” These organisms need oxygen and, when an algae bloom is big enough, so many of these tiny organisms can use up so much oxygen, that the water at the bottom starts to run out of oxygen. Certain bottom-loving fish can’t escape these conditions in time, and suffocate. Big blooms are often followed by fish kills.

The Solutions: Less Phosphorus and Less Runoff 

Buffer strips along waterways are one way farmers reduce fertilizer runoff. Photo: USDA NRCS

The primary way to get clearer and cleaner water 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.

There are many farmers 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 “flash” algae blooms in summer, where a strong rain moves tons of phosphorus into our lakes and then warm, still conditions provide algae with a big phosphorus buffet.

If you’ve read this far, congratulations! You’ve passed “Algae Blooms 101” and can now share this most important message with everyone you know – what we do on our land ends up in our waters.

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