Seasonality of diarrhetic shellfish poisoning at a coastal lagoon in Portugal: rainfall patterns and folk wisdom

Of the three types of toxicity known so far in Portuguese shellfish, only diarrhetic shellfish poisoning (DSP) and amnesic shellfish poisoning (ASP) are produced by microalgae that seem to have been present in the last decades or centuries. The most important paralytic shellfish poisoning (PSP) producer, Gymnodinium catenatum, is hypothesised to have been introduced quite recently as only in 1976 PSP toxicity was detected for the first time in shellfish from Galicia, NW Iberian Peninsula. While ASP presents very short episodes of contamination, the concentration of DSP toxins in some years surpasses human safety values for much longer periods. It is traditionally stated that shellfish should be consumed in ‘months with R’ (September–April). A retrospective study of the maximum monthly DSP levels attained in mussels from a coastal lagoon—Ria de Aveiro—between 1994 and 2001, showed that the highest frequency of months with concentrations surpassing the safety level of 2 μg/g digestive glands were found in June–September, followed by May and October. These months correspond with the months of lowest historical average rainfall in the period 1941–1998. Oscillations in the rainfall pattern coincided with earliest (or latest) detection by HPLC of DSP toxins in mussel in the years studied. In a semi-closed lagunar environment prone to in situ growth of DSP-producer microalgae, like Dinophysis acuminata, rainfall affects river output, lowering salinity and disrupting water column stability that favours Dinophysis growth. The seasonality of DSP recurrence may be connected to the folk adage on safety of shellfish consumption, after many years of empirical observations by coastal populations of diarrhoea episodes in summertime.