Spatial and temporal variability of pico-, nano- and microphytoplankton in the offshore waters of the southern Adriatic Sea (Mediterranean Sea)

The spatial and temporal variability of phytoplankton were investigated in the southern Adriatic Sea in the period November 2006–June 2008. Three stations were sampled along the Bari–Dubrovnik transect with a seasonal frequency (except summer 2007 and spring 2008) at four depths (surface, maximum fluorescence depth and two intermediate depths), with the aim of analyzing the structure of phytoplankton community and evaluate the standing stocks of the three size fractions (pico-, nanoand microphytoplankton). Abundances were on average 5.41 � 10 7 7 5.97 � 10 6 cells l � 1 for picophytoplankton, 1.00 � 10 6 7 6.70 � 10 4 cells l � 1 for nanophytoplankton and 2.16 � 10 4 7 7.46 � 10 3 cells l � 1 for microphytoplankton. Total biomass ranged from 8.5 to 80.7 m gCl � 1 . The highest values were recorded in April 2007 (38.475.9 m gCl � 1 ), and the lowest ones in February 2008 (13.87 1.5 m gCl � 1 ). Picophytoplankton was generally the most abundant phototrophic fraction (on average, 96% of the total abundances and 49% of the total biomass), while nano- and microphytoplankton provided a minor contribution (4% and less than 1% of total abundance and 35% and 16% of total biomass for nanophytoplankton and microphytoplankton, respectively). A decreasing gradient from west to east was observed for pico- and nanophytoplankton abundances and for total biomass. The low biomass values and the dominance of the autotrophic picoplankton fraction in the whole area reflected a general oligotrophic condition. The easternmost station (AM9), characterized by the lowest phytoplankton abundances and biomass and low nutrient concentrations, resulted in still more oligotrophic condition and probably influenced by the Ionian Surface Water (ISW), coming from the eastern Mediterranean.