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The effect of sampling scales on the interpretation of environmental drivers of the cyanotoxin microcystin

Abstract Microcystins (MC), a class of cyanobacterial toxins, were measured in one shallow moderately eutrophic lake over 3 years and compared with results from multi-lake regional studies taken from the literature to understand how the choice of sampling regime affects observed correlations between microcystins and environmental conditions. Our results show that correlations between MC seston content and environmental factors are quite dependent upon the temporal sampling scale. The within-year variation in total MC seston content was high, almost an order of magnitude higher than that of the abiotic variables measured and that of algal biomass (Chl-a) but similar to the variation in the density of cladoceran zooplankton. At the temporal scale of seasons, no significant correlations could be detected between MC seston content and environmental variables, but at the monthly scale, pH and temperature were positively correlated with MC seston content. At the weekly scale, reactive phosphate was weakly correlated with MC seston content (with a lag of 12 d) as detected through time series analysis. Furthermore, the factors previously associated with microcystins at a larger spatial scale in among-lake studies (namely total phosphorus, total nitrogen, and light attenuation) did not explain the temporal MC seston content variation within one lake at any sampling scale.