Temporal succession of phytoplankton in the northern adriatic

Abstract

The seasonal succession of marine phytoplankton was studied along a transect across the Northern Adriatic from the Istrian peninsula, Yugoslavia, to the Po delta, Italy. The phytoplankton communities were dominated by the nannoplankton (74 to 88%), with seasonal blooms resulting primarily from inceases in the microplankton component. Phytoplankton communities exhibited a 4 to 6× gradient in cell densities along the transect, associated with oceanographic conditions created by Po river discharge. Ranges in cell densities increased westward, and a marked bimodality of cell density distribution occurred at those stations close to the Po delta. The annual cycle was composed of “winter”, “late winter”, “spring” and “summer” periods, starting with a clearly defined microplankton diatom species succession in the “winter” and ending with dinoflagellate dominated communities during the “summer”. The nannoplankton group succession was similar along the transect, being dominated by microflagellates. Maxima of the diatom and dinoflagellate components coincided with maxima in the same components in the microplankton. Regional differences in microplankton successional series occurred with Skeletonema costatum and Nitzschia seriata being dominant at western stations, and warmer water species (characteristic of the Central Adriatic) being relatively more common at eastern stations. Changes in community structure and similarity indexes were indicative of major changes in the oceanographic environment, in particular the transition from stratified to vertically mixed periods.