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Factors controlling phytoplankton size class distribution in the upwelling area of the Galician continental shelf (NW Spain)
Manuel Varela, Guillermo Díaz del Río, Mª Teresa Álvarez-Ossorio, Eduardo Costas

Changes inphytoplankton size class distribution in relation to upwelling were studied inthe Galician continental shelf (NW Spain), in two transects off Cape Villano, and off the Ría (fjord-like estuary) de Arosa. Studies were carried out attwo-day intervals during a ten-day period at each station in July-August 1985.Changes in size class distribution were related to upwelling and downwelling ofcold and nutrient-rich North Atlantic Central Water (NACW) and zooplanktongrazing. When NACW emerges into the photic zone, phytoplankton biomassincreases and size class distribution shifts to greater sizes. On the otherhand, when downwelling occurs, phytoplankton biomass rapidly decreases and sizeclass distribution shifts to smaller sizes. Small form biomass remains fairlyconstant, regardless of whether upwelling or downwelling occurs. However, largeforms (mainly diatoms) show greater variations in biomass in relation tohydrological events. Strong downwelling causes a decrease in largephytoplankton biomass higher than expected an if only through passivesedimentation and/or zooplankton grazing. It is suggested that the decrease isdue mainly to the very sinking of the water mass which drags down populationsout of the photic zone. Moreover, the upwelled water can play an extremelyimportant “seeding” role in the photic layer by providing not only nutrientsbut also previously and recently settled phytoplankton populations. In this wayan important increase in the biomass would be produced, an increase farsuperior to what we would expect if the upwelling only provided nutrientswithin the photic layer. Therefore, upwelling and downwelling not only causequantitative changes (increasing or decreasing phytoplankton biomass), but alsoqualitative ones (changes in phytoplankton size).

Keywords: phytoplankton, size class(es), upwelling, zooplankton, grazing, Galicia, NW Spain.
Contents of this volume Sci. Mar. 55(3) : 505-518 Back PDF
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