Cyanobacteria blooms have long been described for Chesapeake Bay nontidal and tidal waters, but measurable toxin has only been recently recorded. During September 2000, the earliest tidal-water records of cyanotoxins in the Bay identified microcystin from a Microcystis-dominated bloom on the Sassafras River. Between 2000 and 2006, opportunistic samples collected from cyanobacteria blooms were analyzed for toxin concentration to better inform natural resource, agriculture, and human-health management agencies on potential bloom-related health risks. The hepatotoxin microcystin was detected most frequently and over a range of concentrations from 2.9 × 10−2 to 6.58 × 102 μg L−1. Microcystin levels exceeded literature-based chronic drinking-water guidance values of 1 μg L−1 and recreational safety guidance for children of 10 μg L−1 in 71% and 31% of samples, respectively. Samples from tidal fresh and oligohaline habitats showed a log-normal distribution of toxin concentrations, and microcystin had positive log-linear relationship with Microcystis aeruginosa cell counts (r2 = 0.42). A subset of the samples positive for microcystin was also tested for neurotoxins and showed anatoxin-a as the next-most common toxin encountered (46% of samples tested) at concentrations from 3 × 10−3 to 3 μg L−1. Saxitoxin (PSP-toxin) was present in trace amounts (3 × 10−3 μg L−1) in one sample. Cylindrospermopsis raciborskii has occasionally been found in abundance, but all tests for cylindrospermopsin were negative. Microcystin and anatoxin-a have been identified in association with fish kills, bird kills, and human-health events. Virginia and Maryland state management agencies conducted beach closures during 2000, 2003, and 2004 and provided waterway health advisories in 2005 and 2006 in response to the findings.
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