which increased the taxonomic assignment success from 23.7 to 50.5 %. When the communities were studied along with environmental variables, similar spatial and temporal trends of taxonomic diversity were observed for metabarcoding and microscopic studies of zooplankton, but not for phytoplankton. This is most likely attributable to the lack of representative sequences for phytoplankton species in current databases. In addition, there was high correspondence in community composition when comparing abundances estimated from metabarcoding and microscopy, suggesting semiquantitative potential for metabarcoding. Furthermore, metabarcoding allowed the detection and identification of two non-indigenous species (NIS) found in the study area at abundances hardly detectable by microscopy. Overall, our results indicate that metabarcoding is a powerful approach with excellent possibilities for use in plankton monitoring, early detection of NIS and plankton biodiversity shifts.

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.

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