Every summer, some of our bays turn pea-soup green. You can see it from the dock, smell it on a warm afternoon, and feel it when you think twice about letting the kids or the dog in. Here’s the plain picture of what drives it and what tools exist.
What feeds the bloom
Blue-green algae feed on phosphorus and nitrogen. Most of it reaches us from a very large watershed — farm fields, lawns, streets and streams across the Fox-Wolf basin — flowing through Lake Winnebago and down into Little Lake Butte des Morts.
Phosphorus also builds up in lake-bottom sediment and releases back into the water on warm, calm days. Scientists call this “internal loading,” and it’s one reason blooms can return even in years with less runoff.
One tool worth understanding: alum
Aluminum sulfate — “alum” — is the same compound many water utilities use to clarify drinking water. Added to a lake, it binds phosphorus and forms a fluffy layer called a floc that settles to the bottom and seals in phosphorus held in the sediment.
The North American Lake Management Society (NALMS) calls alum a safe and effective lake management tool when the treatment is properly designed. Their key points:
- Treatments typically last 5 to 15 years.
- Alum works best on phosphorus already in the lake. It can also treat phosphorus in incoming water when applied directly to it.
- NALMS treats watershed management as essential, with alum as a tool alongside it — especially where watershed reductions alone won’t come fast enough.
The honest question for our lake
Little Lake Butte des Morts is a flow-through lake. Fresh, nutrient-rich water arrives from Winnebago every day. A bottom treatment addresses phosphorus coming out of the sediment, and in our hardest-hit bays the bigger share may be arriving with the current. Whether alum — or another approach — makes sense here is a question for lake scientists, DNR and the data.
That’s the conversation we want to start, with the people who do this work for a living.
Learn more
- NALMS position on the use of alum
- Aquatic Ecosystem Restoration Foundation (AERF)
- Aquatic Plant Management Society (APMS)
Which bay near you blooms worst? Tell us in the comments on our Facebook page — your observations help us see where to focus first.