SAN JOSE — The city has closed Almaden and Cunningham lakes to boating, fishing and other water activities following the discovery of a blue-green algae that can sicken humans and kill dogs.

Elevated levels of cyanobacteria were detected during routine testing, and guidelines set by the California Department of Health Services prohibit water activities when counts exceed recommended levels, according to the city.

“We’ve taken this step to protect the health of our residents and their pets,” San Jose Department of Parks, Recreation and Neighborhood Services Director Angel Rios Jr. said Wednesday. “We’re optimistic that this closure will be temporary, and we will continue regular testing of water quality to determine when we can reopen the lakes.”

The algae, which can cause skin irritation and gastrointestinal issues in humans, is lethal to dogs.

Rios said algae blooms are common toward the end of summer. Cyanobacteria grows thicker and faster when lake levels are low, temperatures are high and runoff provides more nutrients to feed it, according to the city.

The city said it will continue testing water quality at least twice weekly, and will not reopen the lakes to water activities until two weeks after results show that they have returned to safe levels.

The parks where the lakes are located will remain open. Swimming is already not allowed in either lake, but recreational fishing, boating and San Jose State University classes will be affected, according to the city.