Bloom dynamics of Dinophysis acuta in an upwelling system: In situ growth versus transport

Blooms of Dinophysis spp. associated with lipophilic shellfish toxin outbreaks are common in Northwestern Iberia waters from spring to autumn. Blooms of Dinophysis acuta are very seasonal (late summer-early autumn); they start earlier in Northern Portuguese waters during the upwelling season, and reach their maximum values in the Galician Rías during downwelling events at the end of the upwelling season. There is controversy about whether sudden increases in cell concentrations in the rías result from cross-shelf transport of populations previously established in adjacent shelf waters, or are due to longshore transport that brings populations located off Portugal to the North. In 2005, record concentrations of D. acuta were observed in Portuguese waters (14 × 104 cell L−1) off Aveiro in early September, while concentrations off the Galician coast were very moderate (102–103 cell L−1). During the autumn transition from upwelling- to downwelling-favourable winds, D. acuta declined abruptly off Portugal while the annual maximum (up to 22 × 103 cell L−1) was found in the Galician Rías. A new approach was used that combined physical observations (SST, current measurements); weekly observations from monitoring programmes in Galicia and Portugal; weekly division rate (μmin) estimates of D. acuta in Ría de Vigo, together with monthly transects and additional ad hoc sampling in the Ría. During August and early September, division rates were high in Ría de Vigo but concentrations were low, whereas higher SST values in Portugal (a proxy for thermal stratification) seemed to promote the build up of high densities of D. acuta. During the last week of October and the first week of November, populations declined, whereas in Galicia, maximum concentrations were reached while division rate estimates were almost zero. Results presented here confirm that increased numbers in the Galician Rías are not due to intrinsic growth but to physically driven accumulation; a simple cell concentration budget calculated during the accumulation period suggests that the high net growth observed during downwelling, in the absence of cellular division, must be due to cells imported by longshore transport.