General patterns in the size scaling of phytoplankton abundance in coastal waters during a 10-year time series

Abstract

To ascertain the general patterns in phytoplankton size structure of a temperate, coastal ecosystem, we determined the scaling relationship between total abundance and cell size (size spectrum) for nano- and micro-phytoplankton in a shelf station off NW Iberian Peninsula on a monthly basis during the period 1993 –2002. The inverse linear relationship between log abundance and log cell size was persistent throughout the water column and across seasonal and inter-annual time scales. In addition, and despite the high productivity and marked temporal variability in water column structure at our study site, departures from linearity in the size spectra were rare. The slope (20.96) of the overall size spectrum for the entire time series indicated that roughly equal amounts of biomass were present over different logarithmic size classes in the size range considered. The phytoplankton size spectra had similar average slopes during winter mixing, early upwelling, summer stratification and autumn downwelling, suggesting that, under these oceanographic conditions, both nano- and micro-phytoplankton respond similarly to environmental variability. In contrast, significantly less negative slopes were observed during upwelling relaxation, indicating an increased importance of larger cells. Our results illustrate the utility of individual size distributions to provide a synthetic description of phytoplankton community structure in dynamic, non steady-state marine ecosystems.