The effect of hydrodynamics on the phytoplankton primary production and species composition at the entrance to the Gulf of Finland (Baltic Sea) in July 1996

. A 10-day multidisciplinary experiment with the aim to study the development of late summer phytoplankton bloom and the controlling hydrodynamic mechanisms was carried out at the entrance to the Gulf of Finland in July 1996. Clearly defined physical phenomena (cid:150) a downwelling, near-slope jets, and a mesoscale anticyclonic eddy (cid:150) were observed in the area during the study period. Due to the relatively strong currents connected to the jets and eddy periphery (current velocity up to 35 cm s (cid:150)1 ), the advection of different water masses through the sampling area gave rise to a noteworthy variability of biological parameters. However, the formation of the eddy was reflected in horizontal patterns of the patchiness of phytoplankton productivity and species composition in the upper layer. The largest value of primary production was observed in the water mass trapped inside the anticyclonic eddy. The flagellates abundance was the highest inside the eddy, while the proportion of cyanobacteria in the plankton community was the largest outside. Our conclusion is that the vertical motions associated with the mesoscale eddy and occasional mixing events on the background of low concentrations of dissolved inorganic N and P created horizontally different conditions, which favoured simultaneous development of different groups of phytoplankton in a small spatial scale.