This photo, taken earlier this month, shows the state of Pinto Lake, a small body of water in Santa Cruz County that suffers a seasonal bloom of microcystis. The strain of freshwater cyanobacteria is on the rise around the world in warmer, stagnant waters that are enriched by nitrogen, phosphorus and other nutrients from agricultural and urban runoff.

One member of a team of California Department of Fish and Game researchers described this fluorescent-green lake water as “gnarly” when she found it during an investigation that tied this annual bloom to the poisoning deaths of sea otters. This pond scum has a bite. It produces a deadly biotoxin that is known to kill cows and dogs that drink from farm ponds. And now marine mammals, the sentinels of ocean health, appear to be vulnerable.

The toxin made its way down the Pajaro River to Monterey Bay, where it was likely picked up by clams and mussels and crabs, magnified in potentcy and then consumed by hungry sea otters, which eat about 25% of their bodyweight a day in shellfish.

So far, public-health officials do not test shellfish for such freshwater biotoxins, unlike their marine cousins that cause paralytic shellfish poisoning and other ailments. It raises the question of whether such tests will be needed in the future for human health as these and other primitive organisms make a global resurgence that one researcher calls “rise of slime.”

Wayne Carmichael, a cyanobacteria expert from Wright State University in Daton, Ohio, said science had never documented a case of human microcystin poisoning from something a person ate or drank. But there have been deaths.

He once flew to Brazil to investigate such an occurrence. He discovered that the drinking-water supply provided to a dialysis center was contaminated by microcystin nearly 20 times the level deemed safe by the World Health Organization. “The patients thought they were having their kidneys cleaned and instead they were dosed by microcystin intravenously.” He said 100 patients died and 76 had liver failure linked to microcystin exposure.

Read more in the LA Times on the deadly challenge to sea otters.