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Phytoplankton responses to freshwater inputs in a small semienclosed gulf (Gulf of Trieste, Adriatic Sea)

Phytoplankton variations in response to freshwater inputs and water column structure in a shallow (< 25 m) semi-enclosed Mediterranean gulf [Gulf of Trieste, Adriatic Sea) are described. Hydrographic profiles and in situ fluorescence were recorded at high resolution, while nutrients and phytoplankton were analysed at 5 pre-set depths. Analyses of the phytoplankton community structure were completed by determinations of the pigment pool using reverse-phase high-performance liquid chromatography and size-fractionated primary production. The phytoplankton standing crop, community structure and primary production were profoundly influenced by freshwater inputs and water column stratification. The water column was well mixed and supported modest phytoplankton concentrations during winter. A spring diatom bloom (Skeletonema costatum, Nitzschia spp., Chaetoceros spp.) developed in response to major freshwater input to the surface layer ; a similar increase in diatoms (Rhizosolenia delicatula, R. fragilissima) followed October-November freshets. A shift from diatom-dominated winter-spring phytoplankton to a flora with increased numbers of flagellates in the stratified water column during late spring-summer was reflected in the composition of accessory pigments [lower fucoxanthin: 19'-hexanoyloxyfucoxanthin ratio, an increase of chlorophyll b (chi b) and peridinin concentration]. Primary productivity was correlated with chi a biomass and showed surface peaks in April and June. The 2 to 10 mu m phytoplankton contributed significantly to the total productivity in January and June, while on an annual basis the most important producers were > 10 mu m. The smallest fraction (< 2 mu m) seemed to contribute little, except in July. Our data indicate that the new production resulting from land-born nutrient inputs was the main cause of major phytoplankton blooms, while the nutrient supply from the bottom became comparatively more important during dry summer months

Phytoplankton ; Primary production ; Nutrients ; Freshwater inputs ; Photosynthetic pigments ; Adriatic Sea

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