Seasonal dynamics of biotoxins and potentially toxic phytoplankton in three Baltic Sea blue mussel farms

Cyanobacterial blooms are common in the Baltic Sea during summer, and even though several cyanobacteria are toxin producers, many organisms still ingest them as feed. These and other phytoplankton toxins can be detected in blue mussels accumulating over the season, which represents a potential health hazard for shellfish consumers. On a global scale, biotoxins therefore need to be quantified in shellfish before human consumption. We monitored 11 different groups of biotoxins in three blue mussel farms and the composition of 23 potentially toxin producing phytoplankton taxa from March to November 2022. None of the biotoxins were above available health guideline values nor regulated levels. However, the well-known cyanobacterial toxin in the Baltic Sea, nodularin, produced by Nodularia spumigena, was detected in net- and rope-farmed mussels throughout the summer, with the highest concentration of 47 µg kg^-1. In contrast, the less studied toxin cylindrospermopsin was only present in mussels in early spring and late fall (surface water temperature approx. 2–10 °C), with the maximum concentration of 19.7 µg kg^-1 in April, where Aphanizomenon is a potential producer, but yet not confirmed. Further, Dinophysis acuminata, a potential producer of Diarrhetic Shellfish Toxins (DSTs), was observed above warning levels at two sites with up to 2 400 individuals L^-1, although the found concentration of 73 µg kg^-1 is below the regulated level for DST group (160 µg kg^-1). Altogether emphasizing that high species abundance does not necessarily result in a high toxin accumulation. On the other hand, mussels can serve effectively as indicator species, detecting the presence of novel toxins when producer abundance is low. As no period of the year was completely toxin-free, quantitative analysis is recommended when mussels are to be harvested.