© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1434-2944/06/102-0051 FABRIZIO BERNARDI AUBRY*, FRANCESCO ACRI, MAURO BASTIANINI, ALESSANDRA PUGNETTI and GIORGIO SOCAL C.N.R. ISMAR Istituto di Scienze Marine – Sezione di Venezia, Castello 1364/A, I-30122 Venezia, Italy; email: fabrizio.bernardi@ismar.cnr.it Picophytoplankton Contribution to Phytoplankton Community Structure in the Gulf of Venice (NW Adriatic Sea) key words: picoplankton; Utermöhl phytoplankton; Northern Adriatic Sea; Synechococcus; epifluorescence Abstract The size distribution of the phytoplankton community has been analyzed on a monthly basis in the Gulf of Venice (NW Adriatic Sea), with the aim to assess the seasonal variation of the contribution of autotrophic picoplankton (APP) to total phytoplankton. The investigation was carried out in two stations characterized by different influences of fluvial inputs. APP was mainly made up of Syne- chococcus, the larger fraction (Utermöhl Fraction Phytoplankton, UFP: >2 μm as maximum linear dimension) of diatoms and nanoflagellates. The average APP abundance (46 × 10 3 cells ml –1 ) and bio- mass (8.1 μg C dm –3 ) indicate that APP was a persistent and significant component of the NW Adriatic phytoplankton. The highest APP abundance (up to 270 × 10 3 cells ml –1 ) and biomass (up to 49 μg C dm –3 ) were recorded from summer to autumn, UFP peaked from late winter to late spring. The mean APP contribution to total phytoplankton abundance and biomass was 98% and 31% respectively. The seasonal variation of total phytoplankton biomass was mainly driven by the UFP fraction; how- ever, a seasonal shift in the relative importance of UFP and APP occurred from spring to summer. 1. Introduction The size distribution within the phytoplankton community plays an essential role in deter- mining the direction and the magnitude of carbon and energy fluxes in aquatic ecosystems (BELL and KALFF, 2001). In estuarine and coastal waters of the temperate zone, the marked seasonal changes in light, temperature, nutrient inputs and water column stratification may lead to the coexistence and, sometimes, to the seasonal alternation of smaller and larger algae and, therefore, of the structure of the whole plankton community (RIEGMAN et al., 1993). Moreover, there is growing evidence of the importance of the ecological role of autotrophic picoplankton in estuarine and coastal ecosystems (IRIARTE and PURDIE, 1994; VAQUER et al., 1996; PINCKNEY et al., 1998; PHLIPS et al., 1999; NING et al., 2000; MURREL and LORES, 2004). The Northern Adriatic Sea (Fig. 1) is a shallow basin characterized by the inputs of several rivers, of which the Po, the largest Italian river, and the Adige give the major con- tribution to total freshwater inputs; on the other hand, at its south-eastern end, the Northern Adriatic receives highly saline and oligotrophic waters from the southern basin (FRANCO and MICHELATO, 1992; BOICOURT et al., 1999). A trophic gradient, decreasing from northwest to southeast, is typically observed in the Northern Adriatic Sea, where the nutrient-rich waters Internat. Rev. Hydrobiol. 91 2006 1 51– 70 DOI: 10.1002/iroh.200410787 * Corresponding author