A Phosphorus Sweet Spot

Farmers, gardeners and people who like to keep lawns growing fast enough to ensure their lawnmowers get regular workouts know that they have to fertilize to get the best results. Plants need nutrients to grow; fertilizer provides the nutrients.

Algae, a rudimentary type of plant life, also needs nutrients to grow. Algae naturally occurs in lake and river water, therefore, nutrients need to be present in the water to make algae grow.

Algae is a major part of the food web of lakes and rivers, a vital component at the bottom of the food chain. In lakes, including the Great Lakes, when algae is able to extract nutrients dissolved in the water and grow, other tiny life forms eat some of the algae, grow and become food for bigger life forms- and so the food chain begins.

Nitrogen, phosphorus and potassium are the major nutrients plants need. There are others, of course, but N, P and K (potassium) are way up here - I’m holding my hand way above my head. The others are way down here - my hand is down around my knees.

Nitrogen is water soluable and the Earth’s air is 78 percent nitrogen so there’s no lack of nitrogen in lake water to grow plants. Without confusing you with biological lingo even I don’t fully understand, there’s plenty of K for the algae in lake water, as well. More than enough. It’s the amount of phosphorus that can be a limiting factor.

It can also be the nutrient, when present in high amounts, transforming a normal crop of algae into a bumper crop - called a bloom. Anyone with a backyard swimming pool knows what an algae bloom looks like. Water is supposed to be mostly clear, not mostly green.

Green, algae-filled bloom-water is yucky. There’s way too much algae for tiny creatures in the lake to eat and the overabundance comes with a multitude of bad consequences. Some algae (called blue-green) actually secretes a toxin dangerous, even fatal, to pets, people, ducks and other creatures. It’s bad stuff made worse when high doses of phosphorus fertilizes a lake and it blooms.

Keeping phosphorus levels low is important in the Great Lakes to ward off unwanted algae blooms. But there-in lies the rub.

Once a lake stratifies in the summer, the deep, cold water doesn’t mix with the warm surface water. The nearshore and upper part of the water column is constantly receiving an influx of fresh nutrients but none of it filters down to the cool depths below the thermocline. The lower, colder part of the water column has to make do with the nutrients trapped there until they are refreshed at the fall turn-over. Creatures living under the ‘cline have to make do with what’s there until the thermocline breaks up when the surface water of the lake cools below 39 degrees in the late autumn.

In short, the lakes have two independent ecosystems most of the summer months. Both ecosystems used to thrive with their own versions of the food chain. Not now.

Now there are a bajillion invasive zebra and quagga mussels coating the bottom of the lakes (not-so-much Lake Superior), each one of them eating algae and even some of the things that eat algae. As the amount of algae is reduced, the algae eating zooplankton starve, the things that eat zooplankton starve and in general, the lake bottom’s food chain is both shortened and weakened.

This causes a conundrum for people trying to do right for the lakes. They've got too much algae in many nearshore zones and they want to reduce phosphorus to solve that problem. But there's not enough phytoplankton in the offshore zone because of the mussels. So if they reduce phosphorus loading in the lake, they could make that offshore problem even worse. A good job done in the surface waters and nearshore can mean there is virtually no food left out there to create a deep water food chain.

For Great Lakes salmon and lake trout fishermen, that’s particularly bad since our preferred fish are cool water loving and spend the majority of the summer swimming in the cold, deep layers of the lake. No food for little fish means no little fish to feed bigger fish.

Projects are underway to learn the "sweet spot" for phosphorus loading. Hopefully researchers will discover the perfect phosphorus load to minimize the nuisance algae while at the same time keeping the offshore ecology alive with enough plankton production.