The Cape Cod Preservation Association with volunteers, pond associations, cities and the state monitors more 100 freshwater ponds for concentrations of cyanobacteria (blue green algae) and toxins produced by bacteria.

These toxins can affect the liver and nerves, and pets have died after drinking water from contaminated ponds on Cape Cod.

When levels are high enough, association staff members alert cities. Cities can then post a ban on swimming and other activities in the pond until the algae population decreases.

Blooms usually occur in late summer, fueled by hot water or high levels of nutrients from fertilizers, runoff, or sewage seeping into the soil.

In some cases, higher concentrations may persist throughout the fall.

Signs warn of dangers at the Santuit pond where a proliferation of cyanobacteria has closed the pond to swimming, from 2021.

Here is a list of ponds currently under alert or monitoring for cyanobacteria:

Santuit Pond in Mashpee. This is based on a sample from August 10, when the association recommended a use restriction when the water temperature was 81 degrees. There was a large accumulation of cyanobacteria of the genus Dolichospermum on the shore. The genus can produce liver and nerve toxins. Blooms of the genus are usually associated with warm, nutrient-rich waters.

Santuit Pond also has high levels of E. coli, according to the Barnstable County Beach Sampling Program.

Mud at the table:Wellfleet Oyster Farms grow shellfish for your plate. Here’s how.

Flume Pond in Falmouth was also reported due to a sample on August 9, when patches of blue-green algae were observed in the mud. In this case, there was a high level of microcystin (32 parts per billion) which is a liver toxin produced by blue-green algae.

North Pond at Barnstable was flagged as of potential concern due to visible scum of cyanobacteria.

The West Reservoir at Harwich had large clumps of cyanobacteria on August 10. Microcystis was the dominant bacterium, but no microcystin was detected.

Nugget the cow:Here’s why a cow munching on algae can help fight climate change

There is still a review for Brewster Lower Mill Pond after a large amount of cyanobacteria was detected on August 5. The water temperature was 81 degrees. Dolichospermum was the dominant species.

This is also advice for School pond in Brewster where there was visible scum on August 10, with Dolichospermum as the dominant species. The water temperature was over 83 degrees. Visibility in the pond was very poor due to high levels of algae.