Dynamics of Phytoplankton and Zooplankton Communities on the Continental Shelf of Northern Portugal

In order to study simultaneously the communities of phytoplankton and zooplankton on the NW coast of Portugal, four cruises were carried out in 2002 and 2003, sampling the water column in a cross-shelf transect. Temporal variability of phytoplankton presented a seasonal pattern characteristic of temperate regions - minimum in winter and maximum concentrations in spring and late summer. Spatial distribution of phytoplankton seemed to be forced by the winds stress over the water surface layer, with possible transport of organisms. The temporal variability of zooplankton close to shore followed the seasonal pattern of phytoplankton, while offshore the same was not observed. Zooplankton was concentrated in the vicinity of phytoplankton patches but was less influenced by wind forcing. Very low concentrations of both phytoplankton and zooplankton were repeatedly observed at the outer-shelf, possibly caused by an upwelling-associated feature of surface waters deepening. The phytoplankton showed a seasonal succession of species that is common for the region, with diatoms (e.g. Guinardia delicatula, Leptocylindrus danicus) dominating mixed and upwelled waters whereas dinoflagellates (e.g. Ceratium fusus, Dinophysis spp.) prevailed in thermally stratified waters. The zooplankton community structure reflected an important interannual variability (with substitution of dominant species) that superimposed the seasonal succession. Seven copepod taxa accounted for nearly half of the zooplankton community. No differences on the relative composition of zooplankton were found with depth, while across the section two groups were identified: a nearshore cluster (e.g. Temora longicornis, Podon spp.) and a widespread cluster that included offshore taxa (e.g. Clausocalanus spp., Paracalanus spp., Ctenocalanus vanus). The interactions between the assemblages of zooplankton and the phytoplankton groups were evaluated and grazing “hot spots” were detected in thermally stratified conditions.
In 2003, under highly stratified conditions, a bloom of the dinoflagellate Dinophysis acuta (toxin producer, responsible for DSP) was detected in a relatively thin layer of 5m (around 18-20m deep), reaching 24x103 cells L-1. Important numbers of different life cycle stages (small and intermediate forms) of D. acuta were observed together in that thin layer, which indicates that this type of thin layer bloom can embody a strategy to ensure successful gamete mating during sexual reproduction. During this D. acuta bloom, a field evaluation of the importance of this dinoflagellate on the diet of five abundant copepod species was conducted. The average presence of D. acuta in the copepods digestive contents was low (0.3 cells ind-1). Only the two largest copepods (namely Centropages chierchiae) seemed to be capable of important grazing on D. acuta, although restricted to locations with more than 9x103 cells L-1. Since the three smaller copepods accounted for 57% of all mesozooplankton, we speculate that the bloom of D. acuta was not being top-down controlled by copepods.