PRESENCE OF THE PHYCOTOXIN OKADAIC ACID IN MUSSELS (MYTILUS-EDULIS) IN RELATION TO NUTRIENT COMPOSITION IN A SWEDISH COASTAL WATER

The concentration of the phycotoxin okadaic acid (OA) in mussels (Myti lus edulis), collected from the mussel farming district on the Swedish west coast, displayed great geographic variations. The geographical p attern of the variations, however, did not change much during the 5-yr long investigation which started in 1989. Monthly measurements during a 3-yr period of dissolved inorganic nitrogen (DIN), phosphorus (DIP) and dissolved reactive silicate (DSi) were compared with weekly measu rements of OA in mussels. Low OA concentrations were found only in a r ather isolated fjord system where the highest concentrations of DIN an d DIP were also found. The deep water in this fjord was always rich in DSi, and supplied the photic zone with enough DSi to support primary production dominated by diatoms. The DIN/DSi and DIP/DSi ratios in thi s fjord system were the lowest in the whole mussel farming district. A reas with low DIN/DSi and DIP/DSi ratios during the end of the summer coincided with low OA concentrations in mussels during autumn and wint er. High OA concentrations in mussels occurred in areas where DSi was almost totally depleted in July and remained low during the rest of th e production season. It is hypothesized that presence of silicate prom otes growth of diatoms which are non-toxic here, whereas absence of si licate favors dinoflagellates including DSP-producing Dinophysis speci es.