Abstract
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 1 (P1; May and June), characterised by high temperature values (17.0±1.8°C); period 2 (P2; July), characterised by lower temperatures (14.6±0.3°C), identified as an upwelling-blooming stage; and period 3 (P3; August), characterised by a high temperature pattern (16.25±1.14°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 µM O 2 d -1 ) and net community production (NCP; 46.9±15.3 µM 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 µM 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.
Highlights
Coastal fertilisation by cold nutrient-rich upwelled waters stimulates productivity and phytoplankton blooms (Barber and Smith, 1981).These blooms are dominated initially by nonmotile diatoms (Officer and Ryther, 1980) that are preferentially selected under the turbulent conditions produced by strong winds, which are responsible for the upwelling
Three major periods were distinguished according to sea surface temperature (SST): period 1 (P1; May and June), characterised by high temperature values (17.0±1.8oC); period 2 (P2; July), characterised by lower temperatures (14.6±0.3oC), identified as an upwelling-blooming stage; and period 3 (P3; August), characterised by a high temperature pattern (16.25±1.14oC)
Three periods were distinguished on the basis of the changes in SST during the survey (Table 1): period 1 (P1), from 24 May to July, corresponded to a high temperature stage prior to a persistent upwelling event; period 2 (P2), from July to 31
Summary
Coastal fertilisation by cold nutrient-rich upwelled waters stimulates productivity and phytoplankton blooms (Barber and Smith, 1981). These blooms are dominated initially by nonmotile diatoms (Officer and Ryther, 1980) that are preferentially selected under the turbulent conditions produced by strong winds, which are responsible for the upwelling. The ocean biota is sustained by the balance between the autotrophic (i.e. production) and heterotrophic (i.e. respiration) processes (e.g. Williams, 1984, 1998). In coastal systems where inputs from terrestrial sources are limited, such as the studied location, phytoplankton primary production represents the main source of organic matter. Size fractionation studies (Williams, 1981)
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