SIZE DISTRIBUTION OF CHLOROPHYLL-A BIOMASS AND PRIMARY PRODUCTION IN A TEMPERATE ESTUARY (SOUTHAMPTON WATER) - THE CONTRIBUTION OF PHOTOSYNTHETIC PICOPLANKTON

Size-fractionated chlorophyll a (chi a) biomass and primary production rate (> 3 mu m 1-3 mu m and < 1 mu m) and picophytoplankton abundance (both prokaryotic and eukaryotic) were investigated in Southampton Wa ter (south coast of England), with routine sampling at 2 stations, rep resentative of conditions in the mid and outer estuary. In the mid est uary the cycle of chi a biomass in the > 3 mu m size fraction was char acterized by a small spring peak and a more intense summer peak. In th e outer estuary a chl a maximum in the > 3 mu m size fraction occurred in spring; phytoplankton in this size fraction may have been nutrient limited in summer at this station. Phycoerythrin-containing picocyano bacteria, eukaryotic picophytoplankton cell numbers, and chi a biomass and primary production rate by the < 1 mu m size fraction all showed a positive correlation with temperature and peaked during summer at bo th stations. The 1-3 and < 1 mu m size fractions contributed around 14 and 6 %, respectively, to the estimated annual rate of depth-integrat ed plankton community primary production. These results suggest that t he impact of the photosynthetic picoplankton diminishes in an increasi ng eutrophication gradient, from offshore (> 50 %) to coastal (ca 20 % ) and estuarine waters (< 10 %). This pattern in relation to a eutroph ication gradient was also apparent on a seasonal basis, the < 1 mu m f raction having its maximum significance at times of lowest overall chi a concentration. It is thus suggested that factors limiting growth an d accumulation of larger phytoplankton are the primary cause of an inc rease in the relative significance of picophytoplankton.