Monitoring of saxitoxin production in lakes in Denmark by molecular, chromatographic and microscopic approaches

•qPCR assay targeting the sxtA gene in saxitoxin-producing cyanobacteria was designed.•Densities of potential saxitoxin-producing cyanobacteria were monitored in five lakes.•qPCR cell numbers agreed with densities measured by microscopy and pigment analysis.•Intracellular saxitoxin concentrations covaried with numbers of sxtA gene copies.•Saxitoxins might cause health concern in one of the lakes due to commercial fishing.