APCC Cyanobacteria Info Series #3

In our last installment of APCC’s cyanobacteria info series, we explained that blooms don’t always appear as clear, visual green surface scums. Cyanobacteria can also be present as tiny particles or larger clumps suspended throughout the water, and a pond’s appearance can change throughout the day as sunlight, wind, and water conditions shift. Because of this, it’s important to remember that what you see at first glance on the surface doesn’t always tell the whole story.

In fact, the U.S. Environmental Protection Agency notes that the absence of a visible bloom doesn’t necessarily mean cyanotoxins are absent and recommends laboratory testing to determine whether toxins are present. We saw this in our own monitoring data as well.

During the 2024 and 2025 monitoring seasons, APCC reviewed data from 49 sampling events where toxin samples were analyzed from ponds where no surface scum was observed at the time of sampling. Of those, 44 had detectable levels of the cyanotoxins microcystin or anatoxin.

While most detections were well below Massachusetts recreational guidance thresholds, these findings reinforce that visible surface scum is only one indicator of bloom conditions (see Figure 1 below).

This is one reason APCC’s monitoring program doesn’t rely solely on what can be seen from shore. We combine visual observations with water sample collection and analysis, including microscopy and phycocyanin (a cyanobacteria pigment) measurements, and where indicated conduct cyanotoxin testing initially using rapid response test strips and then positive tests for analysis at the Barnstable County lab, to provide a more complete picture of pond conditions.

We hope this series helps you better understand what’s happening in our ponds so that you can make informed decisions and continue enjoying Cape Cod’s freshwater resources with confidence.