The prevalence of cyanobacterial blooms in freshwater systems worldwide is well known, and they become more and more present in countries with temperate climates, due to global climate change. Moreover, some blooms contain toxin-producing cyanobacteria species. Among them, the hepatotoxic microcystin congeners (MCs) are commonly observed in European blooms. Yet, the dissemination of these MCs in the ecosystem, food web and human food chain are insufficiently studied. A lack of suitable and sufficiently robust analytical methods for different matrices should still be addressed. For instance, for fish, multiple studies report the accumulation of MCs in diverse species, without accounting for the possible differences in the matrix during the development and validation of the analytical method. Accurate assessing the accumulation of MCs in different fish species is vital to evaluate the effect of the blooms on the ecosystem and the potential human health risk after consumption.

Therefore, a method was validated to enable robust MCs analysis in multiple fish species from freshwater ecosystems. After multiple optimization steps due to the differences in matrix effect, the method was validated for eight MCs and nodularin. Method validation included five fish species, commonly consumed in Europe. The method was developed to determine MCs concentration both in muscle and liver. Thereafter, it was applied to multiple fish samples, collected from Belgian waters.

A concentration of up to 88.3 µg kg^-1 sum of MCs was found in perch and sander liver tissue. Moreover, a perch muscle sample contained 6 µg kg^-1 MC-LR, while MC-RR was only detected, below the limit of quantification. Consumption of contaminated fish muscle could contribute to human exposure to MCs or accumulate in the food web. There is still a need for comprehensive research on the potential harm to humans and the ecosystem from cyanotoxins, using a One-Health approach