The Lake Where You Live

Phosphorus is the main cause of excess growth of vegetation and algae blooms in our lakes. But where does the phosphorus (chemical symbol P) come from?

At last month’s Wisconsin Lakes and Rivers Convention William James, director of the Center for Limnological Research and Rehabilitation at UW Stout, provided useful insights and some information about one remedy.

We know that phosphorus gets into lakes with runoff from the land. It’s attached to soil particles carried by water from heavy rainfall. It’s in pet waste and fireplace ashes. It’s in autumn’s dead leaves and in effluent from failing septic systems.

But, according to James, it’s also already in the lake, in the sediment on the bottom. While thus buried it does no harm, but if it finds its way back into the water in dissolved (reactive) form, it will nourish water plants and algae. The effect of just a little phosphorus can be significant. The Algae Adage says that a single pound of added phosphorus can foster growth of 500 pounds of algae or aquatic vegetation.

A lake’s bottom can include three feet or more of phosphorus-containing sediment, and that phosphorus is naturally recycled over time. Much of the phosphorus is tightly bound up in the sediment, particles, but the reactive form can get into the water column in various ways. 

One route is simple diffusion. Another is by way of aquatic plants, which take up the phosphorus into their tissues and eventually die and decay, liberating the nutrient once again. Another route is by agitation of the sediment, whether by natural wind and wave action, or through human disturbance, such as from boat traffic.

Collectively, these mechanisms are called phosphorus resuspension, and in some lakes it is a major contributor to nutrient enrichment of the water. So, if resuspension is causing problems in a lake, is there a remedy for it? James would respond in the affirmative.

He advocates treatment with alum (aluminum sulfate). Alum is the same chemical used in a final settling stage in municipal drinking water treatment. When added to a treatment basin, the alum forms a fluffy material called floc that slowly sinks to the bottom and carries suspended solid particles with it.

When added to a lake, alum immobilizes phosphorus in two ways. First, the floc has a strong capacity to bind with and trap phosphorus as it settles down. Second, once on the bottom, the floc covers the sediment like a blanket, keeping phosphorus from being released into the water.

Alum is safe for humans and aquatic life and, according to James, treatments can remain effective for 10 to 20 years. It has been used in numerous lakes to mitigate what otherwise could be severe and annually repeated blooms of algae, including noxious cyanobacteria (commonly called blue-green algae).

The main issue with alum is that it treats a symptom rather than the cause of nutrient enrichment. A better approach is to keep phosphorus out of the lake in the first place through best practices in preserving natural shorelines.

But for lakes with sediment already high in phosphorus, alum treatment can be an affordable and workable solution. 

Ted Rulseh is a writer, author and lake advocate who lives on Birch Lake in Oneida County. His new book, “Ripple Effects,” has been released by UW Press. You can learn about it by visiting his website at https://thelakeguy.net.