Blue-green algal toxin (microcystin) levels in Minnesota lakes

Abstract

Increased interest in blue-green algal toxins in recent years has led to increased monitoring to assess occurrence and levels of toxins in Minnesota lakes. Microcystin (MC), a hepatotoxin, is one of the primary toxins studied in Minnesota and elsewhere in North America. The Minnesota Pollution Control Agency has measured MC in numerous lakes across Minnesota as a part of three separate efforts: (1) A targeted survey in 2006 to assess MC levels in 12 eutrophic lakes in two south central Minnesota counties; (2) A stratified-random survey of 50 lakes in Minnesota as a part of the National Lake Assessment Project; and (3) Incident-based samples from various lakes during 2004–2007 with reports of severe nuisance algal blooms, potential for human health risk and/or documented dog deaths as a result of algal toxins. This investigation focuses primarily on the 2006 study and linkages between MC and other chemical, physical and biological measures. Of 133 MC samples, 94% were above the Method Detection Limit (MDL = 0.15 ug/L). Based on World Health Organization guidelines, 80% of all MC samples ranked in the “low risk” category (<10 μg/L), 8% as “moderate risk” (>10–20 μg/L), 11% as “high risk” (20–2000 μg/L), with an overall maximum of 8400 μg/L. Microcystin exhibited significant positive correlations (Rs) with pH and chlorophyll-a and significant negative correlations with alkalinity and Secchi depth. Data from the other two efforts place the 2006 results in perspective and provide a comprehensive representation of MC concentrations in Minnesota lakes and an improved basis for communicating risk to the public.