Harmful compounds also occur naturally in the sea. These include toxins produced by certain algae species and hydrogen sulfide found in anoxic seafloor. Human activities indirectly increase the amount of these compounds through eutrophication
When assessing the state of the Baltic Sea, the concentration of liver toxins produced by cyanobacteria in surface water, phytoplankton, and herring is also monitored. Currently, the total liver toxin concentration in surface water is below the threshold set by the World Health Organization (WHO) in all Finnish marine areas.
No limit value has yet been defined for the liver toxin concentration in phytoplankton and herring, but algal toxins have been found in phytoplankton in all Finnish marine areas. In the Bothnian Bay, the liver toxin concentrations in herring are lower than in other marine areas.
Cyanobacteria have been present in the Baltic Sea for millennia. They produce two peptide liver toxins, Nodularin-R and Microcystin-LR.
As their name suggests, liver toxins produced by cyanobacteria accumulate primarily in liver tissue and only to a small extent in muscle. High concentrations of liver toxins have been found in flounder, three-spined stickleback, and eider. In contrast, the concentrations in herring, salmon, and cod have been lower.
Cyanobacterial toxins have occasionally caused poisonings in pets and livestock, sometimes leading to death. However, such severe effects are rare. These cases have become even rarer in recent years due to the reduction of cyanobacterial blooms.
In addition to cyanobacteria, some dinoflagellates also produce toxins. The most well-known are the diarrhoea-causing dinophysistoxins and okadaic acid produced by dinoflagellates of the genus Dinophysis.
Okadaic acid causes symptoms similar to food poisoning. This condition is called DSP syndrome (diarrhoeic shellfish poisoning). Okadaic acid has been found in mussels and flounder, for example.
PST compounds, on the other hand, are neurotoxins that cause PSP syndrome (paralytic shellfish poisoning). An example of a PST compound is saxitoxin. In the Baltic Sea, saxitoxin is produced by the dinoflagellate Alexandrium ostenfeldii. In the Baltic Sea, PST compounds are particularly found in the Föglö area of Åland. In Föglö, PST concentrations exceeding the World Health Organization (WHO) standards have been measured in mussels and fish viscera.
In the Baltic Sea, algal toxins affect marine organisms more than humans. Neurotoxins produced by algae can be transferred to organisms that feed on algae. The toxins can accumulate in benthic communities, such as mussels and the organisms that eat them.
Consuming organisms containing neurotoxins can, at worst, lead to respiratory paralysis and death. Therefore, the growth conditions and PST concentrations of marine organisms cultivated for human consumption are closely monitored worldwide.
