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Phytoplankton succession and sinking rates were studied from January to December 2003 at a coastal station in the Gulf of Trieste (northern Adriatic Sea), 200 m offshore, in a relatively undisturbed area. A conical sediment trap, moored at 15 m depth (water depth 17 m), was used. The hypothesis if the presence of benthic and epiphytic diatoms can lead to an overestimation of the vertical fluxes was tested. To evaluate primary and secondary sedimentation contributions, planktonic, benthic and epiphytic diatoms were distinguished. Benthic species abundance varied throughout the year and it was related to resuspension that strongly influenced sinking rates. All over the year, diatoms were the prevailing class in the trap material accounting for 75. 32% of the settled cells, while flagellates represented 24. 11%. Dinophyceae and resting cells constituted minor components, accounting for 0. 43% and 0. 14%, respectively. The gross sedimentation rates ranged from 0. 006 × 108 cell m−2 d−1 in the second week of May to 6. 30 × 108 cell m−2 d−1 in the third week of January with a mean annual value of 1. 09 ± 1. 43 × 108 cell m−2 d−1. To the primary sedimentation rate Pseudo-nitzschia seriata of the group “Nitzschia seriata complex” contributed for 49. 77% followed by Chaetoceros spp. (23. 88%). The major contributor to the secondary sedimentation rate was the diatom Paralia sulcata, accounting for 24. 76%. Epiphytic diatoms contributed for 11. 19% and 12. 27% on annual average gross abundance and biomass, respectively, reaching even 72. 04% of gross abundance and 56. 06% of gross biomass in the second week of August. The correlation between temperature and the logarithm of the epiphytic biomass was statistically significant, with r = 0. 66 and P < 0. 001. Both in the cluster analysis and in the PCA four main groups were formed, where benthic and epiphytic species were separately gathered. Planktonic, benthic and epiphytic forms accounted for 50. 78%, 36. 95% and 12. 27%, respectively, calculated on the annual average biomass. Therefore, vertical fluxes can be overestimated of 50% or more if benthic and epiphytic species are not rejected. [ABSTRACT FROM AUTHOR]

Primary production of the microphytobenthic community and carbohydrates concentrations were studied in the lagoonal system of Grado and Marano, located in the Northern Adriatic coast. Sediment samples were collected along a salinity gradient. Abundance and species composition of the microphytobenthic communities were analysed and the benthic microalgal biomass was estimated as Chlorophyll a (Chl a). Primary production of benthic diatoms was estimated using 14C-tracer. Extracellular carbohydrates were extracted from the sediment and separated in two operationally defined fractions (colloidal and EDTA-extractable). Salinity was higher in the Grado lagoon, where the benthic microalgal community was mainly composed of marine diatoms. In the Marano lagoon, which has a lower salinity, freshwater species were also found. In both lagoons, photosynthetic efficiency showed an inverse relationship with salinity and a direct relationship with the main biological variables. Photosynthetic activity was directly related to Chl a and abundance of benthic microalgae, suggesting that in the benthic system microalgal community is responsible for primary production. Overall, salinity was also influent on the microphytobenthic primary production, which was greater in the more saline Grado lagoon. [ABSTRACT FROM AUTHOR]