Seasonal variations of nutrients and phytoplankton in the coastal SW Adriatic Sea (1995-1997).

Monitoring cruises were carried out on four coastal transects of the Southern Adriatic Sea over two years to investigate the effects of the hydrodynamic regime on environmental factors and phytoplankton species composition and succession. The concentration of nutrient salts is usually low except for the winter months. Biomass levels (in terms of chlorophyll a) proved to be very low, with values ranging between 0.05 and 1.25 mg Chla L-1. Phytoplankton densities showed specific oligotrophic water values and varied in a wide range from 9 • 103 to 2.9 • 106 cells L-1; the maxima values were reached in January-February mainly because of a bloom of the diatom Skeletonema costatum. Diatoms were present throughout the year and represented the majority of the population (44.3%) together with phytoflagellates (39.6%), while dinoflagellates and coccolithophorids reached, respectively, 10.3% and 5.8%. During the year, diatoms were dominant with high cell densities in winter time, while phytoflagellates had a larger distribution over time and reached the highest values in spring and summer. Species diversity values were typical of temperate coastal areas (H’ = 2.79 ± 0.73). The type of circulation determined the distribution of thermohaline characteristics, abundance and biomass, as well as species composition and succession of phytoplankton in time. However, no great spatial variations were observed.