Algal blooms: what they are and what they're not

If you've been on Koonz or Beaulieu in late July, you've probably seen one: the water turns green, a line of "paint" gathers near the shore, and suddenly everyone's asking the same question. Let's take it apart.

What a bloom actually is

Algae are single-celled plants that live in the water and make food from sunlight. Under the right conditions — warm water, plenty of sun, and nutrients (mainly phosphorus) — they multiply fast. When their numbers spike, the water changes color and you can see it. That's a bloom: not a new thing, not a disease, just a very large population of something that's always in the lake.

Most blooms are harmless. The water looks green, it smells a little, and it clears within days when the weather shifts. This is the "green week" that's been part of summer on most Minnesota lakes for as long as there have been lakes.

What a bloom is not

A bloom is not pollution in the instant sense — no one dumped something. It's a response to conditions: warm, still, sunny water with available nutrients.

A bloom is not always the same thing. There are two kinds that matter, and the difference is the whole story:

1. Benign algae (the "green week"). Harmless phytoplankton. The water looks green, maybe a little scummy. No health risk to people or pets. It'll pass.
2. Cyanobacteria (blue-green algae). A group of bacteria that look like algae. Some strains produce toxins. These are the blooms we take seriously.

Cyanobacteria blooms often have tells — a paint-like or foamy texture, a blue-green or "pea soup" color, a scum line that's thick and rubbery, or a distinctive earthy/musty odor. When in doubt, treat it as cyanobacteria until you have reason not to.

What to do if you see a bloom

• Keep people and pets out of the scum. Don't let kids swim in it, and keep dogs from drinking the water or rolling in it. A rinse with clean water afterward is a sensible step.
• Don't drain a boat or live well in the lake if a cyanobacteria bloom is active — you can spread the cells to another lake.
• Report it. Our report a weed or algae form takes a minute. A photo and a location help enormously — it tells us how widespread it is and whether we should sample, notify members, or (for a serious bloom) contact the DNR.
• Wait it out. Blooms on our scale are weather-driven. A cold front, a windy day, or a simple turnover of the water column usually clears most of it within a few days. The die-off that follows can drop dissolved oxygen in the deep water, which is why we keep sampling after a bloom ends.

What a bloom is telling us

A single harmless green week is normal and not a problem. Repeated, early, or cyanobacteria blooms are a signal — they tell us the nutrient load is higher than it should be. That's where the Association's work comes in: tracking the trend (see What the numbers mean), talking to members about fertilizer and runoff, and working with the DNR on the bigger levers.

Bottom line

A bloom is the lake telling you its nutrient budget is running hot. Most are harmless and temporary; some are worth your attention and a quick report. Either way, the best response is the same one: look, note, report, and let the data do the rest.