Associations between chlorophyll a and various microcystin-LR

Cyanobacteria harmful algal blooms (cHABs) are associated with a wide
range of adverse health effects that stem mostly from the presence of
cyanotoxins. To help protect against these impacts, several health
advisory levels have been set for some toxins. In particular, one of the
more common toxins, microcystin-LR, has several advisory levels set for
drinking water and recreational use. However, compared to other water
quality measures, field measurements of microcystin-LR are not commonly
available due to cost and advanced understanding required to interpret
results. Addressing these issues will take time and resources. Thus,
there is utility in finding indicators of microcystin-LR that are
already widely available, can be estimated quickly and in situ, and used
as a first defense against high concentrations of microcystin-LR.
Chlorophyll a is commonly measured, can be estimated in
situ, and has been shown to be positively associated with
microcystin-LR. In this paper, we use this association to provide
estimates of chlorophyll a concentrations that are indicative of
a higher probability of exceeding select health advisory concentrations
for microcystin-LR. Using the 2007 National Lakes Assessment and a
conditional probability approach, we identify chlorophyll a
concentrations that are more likely than not to be associated with an
exceedance of a microcystin-LR health advisory level. We look at the
recent US EPA health advisories for drinking water as well as the World
Health Organization levels for drinking water and recreational use and
identify a range of chlorophyll a thresholds. A 50\% chance of
exceeding one of the specific advisory microcystin-LR concentrations of
0.3, 1, 1.6, and 2 g/L is associatied with chlorophyll a
concentration thresholds of 24.64, 65.6, 89.71, and 94.93, respectively.
When managing for these various microcystin-LR levels, exceeding these
reported chlorophyll a concentrations should be a trigger for
further testing and possible management action.