Phyto- and bacterioplankton abundance and viability and their relationship with phosphorus across the Mediterranean Sea

In a synoptic cruise performed during early summer, the composition and viability of pelagic communities of the Mediterranean Sea were studied across the different sub-basins and straits in relation to nutrient regimes and hydrological conditions. The picoplankton fraction dominated the pelagic community across the study region. Bacterioplankton was the most abundant (mean +/- SE = 7.73 +/- 0.39 x 10(5) cells ml(-1)) component of the plankton, reaching the highest abundance in the Marmara and Black Seas, and its abundance was positively related to phosphate concentration. Synechococcus spp. was abundant (3.70 +/- 0.87 x 10(4) cells ml(-1)), and was most frequent in the surface layers of the West Basin and the Black Sea. Two populations of Synechococcus (Synechococcus sp. 1 and sp. 2) were identified, with contrasting distributions along the Mediterranean Sea. Prochlorococcus spp. was also present (3.77 +/- 0.36 x 10(4) cells ml(-1)) and was associated with the deep chlorophyll maximum. Diatoms dominated the microphytoplankton except in the Black Sea, where dinoflagellates and flagellates became predominant. The percentage of living cells (%LC) of Synechococcus sp. 1 displayed a negative relationship with temperature, whereas that of Synechococcus sp. 2 increased with increasing temperature. Prochlorococcus spp. %LC was negatively related to inorganic nitrogen, but positively related to salinity. The viability of heterotrophic bacteria, dinoflagellates and Synechococcus sp. 1 increased with increasing phosphate, underlining the important role of phosphorus in the Mediterranean Sea. Our in situ observations relating survival to environmental forcing allowed the identification of competition and niche segregation within the coexisting populations.