Cyanobacteria have taken center stage in many newscasts in the last couple of years, and have gotten more attention from the USEPA and state environmental agencies in the last 3 years or so than in the two decades before that. Cyanobacteria are not new; they are among the oldest organisms on the face of the earth, and are adapted to life under a wide range of conditions. But they do best in warm, freshwater with elevated phosphorus levels and a low ratio of nitrogen to phosphorus. Urbanization and agricultural use of land tends to create conditions that favor cyanobacteria, and the warmer temperatures related to climate change are fostering increases in blooms. All of this has been well documented in scientific literature with minimal bias; all political wrangling aside, we understand why cyanobacteria are becoming a more common problem and recognize the risks associated with these organisms.
But how is that risk assessed? New England states have a range of systems in place, but most fall short of addressing the issue to the extent necessary to allow appropriate response. The World Health Organization came up with thresholds of 20,000 and 100,000 cells/mL for low, moderate, and high risk, but cell counts are approximate and do not directly translate into toxin concentrations, which is where the risk really lies. Most states adopted a compromise concentration of 70,000 cells/mL, but it is certainly not true that 69,999 cells/mL is acceptable while 70,001 cells/mL represents extreme hazard. Thresholds create hard boundaries along a gradient with a lot of important variation. Some states now encourage toxin testing, but the ability to economically and rapidly test for the range of known toxins is very limited. Consequently, lakes get posted with warnings to avoid contact when cyanobacteria levels are elevated without actual confirmation of a hazard. Warnings do not legally prevent lake use, but rational people usually take those warnings seriously.
So what is the actual risk? This is a tough question. There is no doubt that some cyanobacteria produce toxins, including liver, nerve and skin toxins, but many species do not produce toxins. We know which species potentially produce toxins, and have identified the genes that allow toxin production, but because a species can produce toxins does not mean that it will. Many very dense blooms contain no toxins. Major die off of a bloom of toxin-producing cyanobacteria can result in elevated toxins in the water without a high cell concentration, although rarely for more than a few days. Risk is a matter of toxicity and exposure, and we don’t know enough about either to make definitive statements in many cases. Posting a lake for elevated cyanobacteria can help control exposure, but failure to follow up on toxicity leaves open questions and impairs our future ability to predict risk. We still have a lot of work to do to characterize cyanobacterial risk.