PHYTOPLANKTON RESPONSES TO FRESH-WATER INPUTS IN A SMALL SEMIENCLOSEDGULF (GULF OF TRIESTE, ADRIATIC SEA)

Phytoplankton variations in response to freshwater inputs and water co lumn structure in a shallow (< 25 m) semi-enclosed Mediterranean gulf [Gulf of Trieste, Adriatic Sea) are described. Hydrographic profiles a nd in situ fluorescence were recorded at high resolution, while nutrie nts and phytoplankton were analysed at 5 pre-set depths. Analyses of t he phytoplankton community structure were completed by determinations of the pigment pool using reverse-phase high-performance liquid chroma tography and size-fractionated primary production. The phytoplankton s tanding crop, community structure and primary production were profound ly influenced by freshwater inputs and water column stratification. Th e water column was well mixed and supported modest phytoplankton conce ntrations during winter. A spring diatom bloom (Skeletonema costatum, Nitzschia spp., Chaetoceros spp.) developed in response to major fresh water input to the surface layer; a similar increase in diatoms (Rhizo solenia delicatula, R. fragilissima) followed October-November freshet s. A shift from diatom-dominated winter-spring phytoplankton to a flor a with increased numbers of flagellates in the stratified water column during late spring-summer was reflected in the composition of accesso ry pigments [lower fucoxanthin: 19'-hexanoyloxyfucoxanthin ratio, an i ncrease of chlorophyll b (chi b) and peridinin concentration]. Primary productivity was correlated with chi a biomass and showed surface pea ks in April and June. The 2 to 10 mu m phytoplankton contributed signi ficantly to the total productivity in January and June, while on an an nual basis the most important producers were > 10 mu m. The smallest f raction (< 2 mu m) seemed to contribute little, except in July. Our da ta indicate that the new production resulting from land-born nutrient inputs was the main cause of major phytoplankton blooms, while the nut rient supply from the bottom became comparatively more important durin g dry summer months.