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Abstract
Observations of the dinoflagellate Dinophysis norvegica in the Baltic Sea during the summers of 1991–1993 indicate that maximal abundances (c 40–150 × 103 cells l-1) were found at the thermocline, typically at 12°C. Maximum densities were usually between 12 and 15 m where 2·9% and 1·5% of surface photon irradiances, respectively, were measured. No diel vertical migration was observed, and cell densities in the mixed layer were always low. Photosynthesis versus irradiance measurements with an oxygen electrode indicated that these populations had a P max of 2·47 [coefficient of variation (CV) 7·3%] and 3·4 (CV 4·7%) mg O2 mg Chl a -1 h-1, and compensation values of photon irradiance were 16·5 and 83 μmol m-2 s-1 in 1992 and 1993, respectively. Both oxygen electrode and 14C light/dark bottle measurements indicated that D. norvegica had very little net photosynthesis at the depths where it was most abundant; it would have had about 2·5-fold greater capacity at photon irradiances present closer to the surface. Calculated carbon doubling times via photosynthesis averaged 4–11 months. There was no observable diel rhythym of DNA synthesis, suggesting that either D. norvegica was not dividing synchronously (asynchronous division is common in heterotrophs) or not dividing at all. Electron microscopy did not reveal the presence of food vacuoles, but feeding and digestion could have been extracellular. The data suggest that this species is a mixotroph which received its primary nutrition via heterotrophic means during our observation periods in the summers of 1991–1993.
