Microplankton composition, production and upwelling dynamics in Sagres (SW Portugal) during the summer of 2001

Microplankton community, production, and respiration were studied alongside physical and chemical conditions at Sagres (SW Portugal) during the upwelling season, from May to September 2001. The sampling station was 5 km east of the upwelling centre off Cabo S. Vicente, and 2 km west of an offshore installation for bivalve aquaculture. Three major periods were distinguished according to sea surface temperature (SST): period I (Pl; May and June), characterised by high temperature values (17.0 +/- 1.8 degrees C); period 2 (P2; July), characterised by lower temperatures (14.6 +/- 0.3 degrees C), identified as an upwelling-blooming stage; and period 3 (P3; August), characterised by a high temperature pattern (16.25 +/- 1.14 degrees C). Chaetoceros spp., Thalassiosira spp., Lauderia spp., Detonula spp. and Pseudo-nitzschia spp. were the major taxa contributing to the dissimilarities between P2 (July) and the other periods. In July (P2), the average gross production (GP; 52.5 +/- 12.3 mu M O-2 d(-1)) and net community production (NCP; 46.9 +/- 15.3 mu M O-2 d(-1)) peaked with the maximal concentrations of diatom-chl a. Dark community respiration (DCR) remained low and more constant throughout (4.6 +/- 3.6 mu M O-2 d(-1)). The plankton assemblage was dominated by diatoms throughout the survey. Physical events were the primary factors determining the microplankton structure and distribution at this location.