APCC Cyanobacteria Info Series #2

Cyanobacteria blooms can look like green paint, pea soup, floating or submerged mats, small particles in the water, or a scum line on the shore. Cyanobacteria blooms are often blue or green, or sometimes brown, red, or white. Not every green particle in the water is cyanobacteria—pollen, sediment, and other naturally occurring organisms can look similar. But if the water looks suspicious, especially if children or pets will be in the water, choosing another pond is the safest option.

A thick green surface scum that looks like spilled paint is one of the most obvious signs of a cyanobacteria bloom (see Photo 1 below). However, blooms don’t always form a surface scum. As sunlight, wind, and water conditions change throughout the day, cyanobacteria may collect at the surface to form a scum or become mixed throughout the water, so a pond can look very different over the course of a day.

Cyanobacteria can also appear as many tiny green dots or particles suspended throughout the water (see Photos 2–5 below) and can make the water look green or cloudy. These particles are easier to spot in the water column against a dark background, like a stick or sediment, rather than light sand (see Photo 6 below).

Cyanobacteria can also form benthic mats that can cover the bottom of a pond. Pieces of these benthic mats can come loose and float to the surface, which can be especially attractive, and therefore dangerous, to curious dogs. See photos 7 and 8 below for examples of what these pieces of benthic mat can look like.

If you notice any of these signs, it’s best to choose another pond that day.

The Massachusetts Department of Health offers this advice: When in doubt, stay out. We agree.