SANTA CRUZ, CA – A toxin produced by freshwater bacteria is entering the ocean and poisoning sea otters, say Department of Fish and Game (DFG) and UC Santa Cruz scientists.
“This study is significant because it is the first to establish a connection between freshwater contamination by the toxin microcystin and marine mammal mortality,” said Melissa Miller, lead author and senior wildlife veterinarian at the DFG Marine Wildlife Veterinary Care and Research Center in Santa Cruz. “This land-to-sea link has important implications for marine life and human health.”
In a paper published on Sept.10 in PLoS ONE, the researchers reported that the deaths of at least 21 southern sea otters (a federally listed threatened species found only in California) were linked to microcystin. The toxin is produced by a bacteria called Microcystis, also known as blue-green algae, which thrives in warm, stagnant, nutrient-rich water.
Coauthor Raphael Kudela, professor of ocean sciences at UCSC, said the team found high concentrations of microcystin in lakes bordering Monterey Bay and in rivers that flow into the bay. These rivers include the Salinas, Pajaro and San Lorenzo Rivers. The toxin was also detected in ocean water at the Santa Cruz wharf.
Many of the microcystin-poisoned sea otters were recovered near river mouths and harbors. While most of the cases (17) occurred within Monterey Bay, microcystin-poisoned sea otters were also found along the Big Sur and south-central California coastlines. Microcystin poisoning can cause acute liver failure or damage other tissues and can be fatal.
In 2007, Miller began seeing dead and dying sea otters recovered along the shore of Monterey Bay with evidence of acute liver failure. After tissue from the otters tested positive for microcystin, Miller teamed up DFG chemists Dave Crane and Abdu Mekebri, as well as Kudela, to search for potential environmental sources of the toxin. The investigation grew to include scientists from a wide range of agencies, including the U.S. Geological Survey (USGS), the California Department of Public Health, and the State Water Resources Control Board.
In the laboratory, the scientists put oysters, mussels, clams, crabs, and other marine invertebrates that sea otters like to eat into tanks of seawater and added microcystin-contaminated water from a local lake. Subsequent testing showed that tissues from the shellfish had up to 107 times higher concentrations of microcystin than was detected in adjacent seawater. Tissues from shellfish exposed to microcystin continued to test positive for this toxin for at least two weeks after the exposures ended.
The study suggests humans also may be at risk from microcystin poisoning if they consume shellfish harvested near river mouths, especially during or after periods of freshwater runoff. However, the study did not collect and analyze shellfish in these areas and emphasizes that sea otters are likely to have a much higher rate of exposure to the toxin than humans, reflecting their daily shellfish and other marine invertebrate consumption rate of 25 to 30 percent of body weight. Further studies are needed to more accurately assess the potential for risks to human health.
The toxin should not be a concern for commercially harvested shellfish which come from areas unlikely to be contaminated by the toxin. Drinking water systems seek to avoid algal growth in their sources because of foul taste and odor. To date, there have been no known human illnesses linked to microcystin exposure in shellfish or drinking water.
One reason for concern about human health hazards is that no formal surveillance or regulatory system exists for microcystin detection in water or shellfish in most countries, including the United States, although some regions with a history of microcystin blooms have organized monitoring and reporting programs for public health protection such as is currently being done in the Klamath River in northern California.
The health risks associated with recreational use of affected lakes and rivers are still unclear though public health agencies recommend avoiding contact with waters affected by cyanobacteria blooms. Microcystin poisoning may also have caused the deaths of several dogs in the Monterey Bay area, Miller said. “We suspect that dogs are getting poisoned when they drink contaminated runoff or get into the green scum and then lick it off of their fur. However, we haven’t been able to perform postmortem examinations yet to confirm microcystin poisoning in local dogs,” she said. Cyanobacteria toxicity has been implicated in several dog deaths in Humboldt County as well in prior years.
“These findings show the value in closely monitoring sentinel species like sea otters as a way to detect and understand stressors of coastal oceans,” said coauthor Tim Tinker, lead scientist on sea otter studies for the USGS Western Ecological Research Center.
In addition to Miller, Kudela, and Tinker, the authors of the PLoS ONE paper include Dave Crane and Abdu Mekebri, DFG Water Pollution Control Laboratory; David Jessup, Stori Oates, and Sharon Toy-Choutka, DFG Marine Wildlife Veterinary Care and Research Center; Michael Murray and Michelle Staedler, Monterey Bay Aquarium; Woutrina Miller, UC Davis School of Veterinary Medicine; Dane Hardin and Clare Dominik, Applied Marine Sciences in Livermore; Gregg Langlois, California Department of Public Health; and Kim Ward, State Water Resources Control Board, Division of Water Quality.