A bloom of Dinophysis acuta in a thin layer off North-West Portugal

Abstract

Dinophysis acuta, which is responsible for diarrhetic shellfish poisoning, reached particularly high concentrations on the north-west coast of Portugal in 2003. In the Ría de Aveiro (40°41'N), the species reached a maximum concentration of 5.0 × 10⁴ cells l⁻¹ on 8 September, the highest value in a 17-year record of monitoring. The bloom followed a brief period of upwelling-favourable winds, at the end of an extremely hot summer marked by weak upwelling, thereby favouring the development of highly stratified conditions. In mid-September, during a cruise transecting the shelf 30km south of Aveiro, a subsurface maximum of D. acuta was identified by fluorescence, with cell concentrations exceeding 2.4 × 10⁴ cells l⁻¹. The species was restricted to a relatively thin layer of 5m (with maxima between 18m and 20m depth) within the pycnocline extending 30km offshore. Cross-shelf distributions revealed the presence of two smaller forms of D. acuta, the smallest of which was identified as D. dens. Their coincident distribution with that of D. acuta reinforced the supposition that these smaller forms correspond to different life-cycle stages of D. acuta, with D. dens representing a gamete of D. acuta. The high cell concentrations in the thin layer are thought to embody a species' strategy to ensure successful gamete mating during sexual reproduction.