If you spend enough time on Wisconsin lakes, eventually you're going to see something in the water that makes you stop and ask:

What in the world is THAT?

Recently, while we were out on Castle Rock Lake, we came across a heavy green accumulation in a small, protected cove with very little water movement. The Wisconsin DNR confirmed that what we were seeing was blue-green algae.

And before anyone reads that sentence and thinks Castle Rock Lake is covered in blue-green algae — it isn't.

That's actually one of the most important things to understand about algae blooms.

A Bloom in One Cove Does NOT Mean the Whole Lake Is Affected

The conditions we observed were very localized.

The accumulation was concentrated inside a protected cove where there was little current or water movement. Out on the main lake and in areas with more moving water, conditions looked dramatically different.

Blue-green algae can be suspended at different depths and can move toward the surface depending on light, nutrients and weather conditions. According to the Wisconsin DNR, surface scum can even appear overnight and may remain until wind and waves disperse it.

That's why one portion of a large lake can look completely different from another.

On a lake and river system as large and dynamic as Castle Rock, conditions should be evaluated where you actually are — not based solely on what someone saw miles away.

What Is Blue-Green Algae?

Despite the name, blue-green algae aren't actually true algae.

They're cyanobacteria, naturally occurring photosynthetic bacteria found in Wisconsin waters. They tend to grow particularly well when water is warm and contains nutrients such as phosphorus and nitrogen. Under favorable conditions, they can multiply rapidly and form a visible bloom.

Blooms may look like:

  • Pea soup
  • Spilled green or blue paint
  • Floating mats or scum
  • Puffy blobs on the surface
  • Thick green accumulations near shore

And here's the tricky part: you can't determine whether a particular bloom is producing toxins just by looking at it. Laboratory analysis is necessary to know whether toxins are present.

Why Protected Coves Can Look Worse

Castle Rock Lake is part of the Wisconsin River flowage system, which means much of the lake experiences water movement from river flow, wind and waves.

But protected bays and coves can be different.

Where there's less wind and water movement, floating algae can accumulate rather than being dispersed.

That can create a dramatic-looking patch of green water in one location while another part of the lake looks perfectly normal.

That's exactly what we observed when these photos were taken.

The takeaway isn't “Castle Rock Lake has blue-green algae everywhere.”

It's:

Lake conditions can vary tremendously from one location to another, and boaters, swimmers and pet owners should pay attention to the water immediately around them.

When in Doubt, Stay Out

This is the part worth remembering.

The Wisconsin DNR recommends avoiding water when you see a scum layer or floating mat. People should avoid swimming, wading, water skiing and similar activities in affected water, and children should not play with algae accumulating along shore.

Pets deserve extra attention.

Dogs can ingest contaminated water while drinking or swallow algae after it sticks to their fur. The DNR advises keeping pets and livestock from swimming in or drinking water experiencing a blue-green algae bloom.

So the simple rule is a pretty good one:

When in doubt, stay out.

Don't panic about the entire lake.

Just move to another area where the bloom isn't present.

Conditions Can Change Quickly

What you saw yesterday isn't necessarily what you'll see today.

And what someone reports from one bay isn't necessarily what's happening five miles away.

Weather, wind, temperature, sunlight, nutrients and water movement can all influence where cyanobacteria accumulate and how visible a bloom becomes.

The DNR notes that Wisconsin blue-green algae blooms generally occur from mid-June through late September, although they can occur outside that window.

That's another reason lake users should simply get into the habit of looking at the water before getting into it.

Protecting Castle Rock & Petenwell Is a Community Effort

There isn't a magic overnight solution for blue-green algae.

Reducing nutrients entering our waterways is one of the long-term pieces of improving water quality. The Wisconsin DNR specifically points to reducing nutrient runoff, maintaining shoreline vegetation, minimizing erosion, addressing leaking septic systems and reducing unnecessary fertilizer use as ways communities and property owners can help.

And we have people working on that right here.

Petenwell and Castle Rock Stewards (PACRS) is a local organization focused on our waters and gives residents, property owners and others who care about Castle Rock and Petenwell opportunities to get involved.

Learn how to get involved with Petenwell and Castle Rock Stewards (PACRS)

Because loving our lakes isn't only about enjoying them.

It's also about protecting them.

Learn More About Blue-Green Algae in Wisconsin

For reliable information rather than Facebook rumor-mill science — because we all know how that can go 😂 — these are excellent resources:

Wisconsin DNR: Blue-Green Algae Information

Wisconsin Department of Health Services: Blue-Green Algae & Health

Wisconsin Blue-Green Algae Educational Video

Petenwell & Castle Rock Stewards: Get Involved

Love Your Lake.

Living on or recreating around Castle Rock Lake means understanding the lake — not just enjoying the view.

Seeing a localized algae bloom doesn't mean the entire lake is unsafe or experiencing the same conditions. It means pay attention to what's happening where you are.

Know what blue-green algae can look like.

Avoid areas where it's accumulating.

Keep kids and pets away from suspicious water.

And support the organizations and people working to protect the waters we're lucky enough to enjoy.

 

Love your lake. Protect what we love.