Microcystis aeruginosa is a single-celled blue green alga, or cyanobacterium, that occurs naturally in surface waters. It can form dense blooms and mats as shown in this photo and it can take other appearances as shown in a recent factsheet from the Office of Environmental Health Hazard Assessment. It typically thrives in warm, turbid, and slow-moving waters that are high in nitrogen or phosphorus. People swimming in dense Microcystis blooms have experienced irritation such as skin rashes, burns, and blistering of the mouth. Ingestion or inhalation can cause vomiting, nausea, headaches, diarrhea, pneumonia, fever and liver damage in humans and animals. No deaths have been reported in humans, but dogs, wildlife and livestock have died following exposure to this toxin.

The Microcystis bloom in 2008 reached comparatively high levels in August and September compared with previous years (IEP Newsletter, Winter 2009, p. 7-8). Nutrient concentrations during the bloom were high as usual. High ammonia values may reduce diatom growth rate by inhibiting the use of nitrate as a nitrogen source but may enhance Microcystis growth (Wilkerson and others, 2006).

Further technical studies are now in process to expand our understanding of the impacts of Microcystis on other parts of the ecosystem in the Delta. For more information on these studies contact Peggy Lehman.