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Size-fractionated chlorophyll a (chl a) biomass and primary production rate (>3 pm, 1-3 pm and < l pm) and picophytoplankton abundance (both prokaryotic and eukaryotic) were investigated in Southampton Water (south coast of England), with routine sampling at 2 stations, representative of conditions in the mid and outer estuary. In the mid estuary the cycle of chl a biomass in the >3 pm size fraction was characterized by a small spring peak and a more intense summer peak. In the outer estuary a chl a maximum in the >3 pm size fraction occurred in spring; phytoplankton in this size fraction may have been nutrient limited in summer at this station. Phycoerythrin-containing picocyanobacteria, eukaryotic picophytoplankton cell numbers, and chl a biomass and primary production rate by the < l pm size fraction all showed a positive correlation with temperature and peaked during summer at both stations. The 1-3 and <l pm size fractions contributed around 14 and 6%, respectively, to the estimated annual rate of depth-integrated plankton community primary production. These results suggest that the impact of the photosynthetic picoplankton diminishes in an increasing eutrophication gradient, from offshore (>50%) to coastal (ca 20%) and estuarine waters (<10%). This pattern in relation to a eutrophication gradient was also apparent on a seasonal basis, the <l pm fraction having its maximum significance at times of lowest overall chl a concentration. It is thus suggested that factors limiting growth and accumulation of larger phytoplankton are the primary cause of an increase in the relative significance of picophytoplankton.

marine ecology. progress series

Size distribution of chlorophyll a biomass and primary production in a temperate estuary (Southampton Water): the contribution of photosynthetic picoplankton