High microcystin concentrations occur only at low nitrogen-to-phosphorus ratios in nutrient-rich Canadian lakes

During Bob Sandford’s presentation at the Comox Valley Eco-Asset Management Symposium he stated “cyanobacteria is affecting all lakes in Canada.” I contacted him to ask more about this and he sent me this study. Click here to read the full study and supplemental figures or read its summary below.



In summary, our meta-analysis of microcystins in Canadian fresh waters has revealed that microcystins are now an issue of national concern, as these toxins were detected in every province, and concentrations exceeded water quality guidelines for drinking water, and sometimes for recreational waters, in many eutrophic ecosystems. Consolidating data on microcystin concentrations in water bodies across Canada allowed us to determine nationally relevant minimum thresholds for P and N concentrations corresponding to microcystin exceedences, probabilities of microcystin exceedances at different N:P ratios, and a regression tree partitioning variance in microcystin concentrations based on nutrient concentrations and ratios. Once externally validated, these models may become applicable as screening tools for identifying potentially toxic “hotspots” or “hot times” of unacceptable concentrations of microcystins based on simple chemical parameters that are routinely measured in water quality monitoring programs. Our survey revealed that microcystin concentrations in Canadian fresh waters were elevated only under low mass ratios of N:P, and while this observation is not likely to be disputed, its interpretation and potential ramifications may prove contentious. Is the association between N:P ratios and microcystin concentrations mere coincidence or driven by a biogeochemical mechanism? The present study is a mensurative experiment, and hence we cannot invoke a “cause-and-effect” argument, or identify the causal mechanisms, to explain why high microcystin concentrations coincide with low N:P ratios. We recommend that subsequent experiments should manipulate N:P ratios — at scales relevant to ecosystem management — as the outcome may be germane to the ongoing debate regarding the need for a “dual-nutrient management strategy” (Paerl et al. 2011).


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