Changes in the tintinnid community were studied in the neritic area of the middle Adriatic Sea from January 1998 until November 1999. Variability in tintinnid abundance showed a clear seasonal pattern with winter–spring and autumn maxima. The vertical distribution of tintinnid populations was affected by their taxonomic structure. A total of 51 taxa belonging to 11 families and 20 genera were identified. Helicostomella subulata, Codonellopsis schabi, Stenosemella nivalis and Salpingella decurtata were the dominant species. Species richness was positively related to overall abundance temporally at each station, and was also affected by environmental trophic state. The total tintinnid abundance, the dominant species abundance and the dominance grade increased significantly with increasing trophic state, while the diversity indices decreased. Hence we conclude that abundance and dominance may be straight indicators of ecosystem trophic state. In addition to environmental trophic state and food availability, tintinnid species distribution was strongly affected by temperature, which was the most important environmental factor determining their seasonality.

Flow cytometry was used to describe seasonal cycles of Prochlorococcus (Prochl), Synechococcus (Syn), picoeukaryotes and heterotrophic bacteria in the central Adriatic Sea along the trophic gradient from January to December 2010. All picoplankton parameters decreased from eutrophic to oligotrophic areas, while the biomass ratio of bacterial to autotrophic picoplankton showed an increase along the trophic gradient. Bacterial biomass ranged from 5.28 to 21.20 μg C l-1. Increased values were present during warmer seasons with the domination of low nucleic acid (LNA) group of bacteria. The high nucleic acid (HNA) bacterial group dominated during the winter and the spring. Bacterial production ranged from 0.09 -0.45 × 104 cells ml-1 h-1 .At coastal stations increased production was present during the winter and the spring and was more or less uniform at open sea stations. Biomasses of Syn and Prochl ranged from 0.16 to 11.47 µg C-1 l-1 and from 0.01 to 3.08 µg C l-1, respectively. They were elevated during the summer and the autumn at coastal stations and during the late winter at the open sea. Syn biomass always dominated over Prochl participating with 61.6-97.2% in biomass of cyanobacteria. Biomass of picoeukaryotes ranged from 1.21 to 21.85 µg C l-1 and was the highest during the winter. Their biomass notably prevailed in autotrophic picoplankton (APP) biomass over that of picocyanobacteria during the whole year. Autotrophic components (Prochl, Syn and picoeukaryotes) made greater contribution to the picoplankton biomass in mesotrophic and eutrophic areas, while heterotrophic bacteria became more important under oligotrophic conditions.

Bacterial abundance, biomass, volume and morphological diversity were studied in sediments collected in the eastern Adriatic Sea (Kaštela Bay) in order to investigate their relationship with changes in environmental parameters. To asses the changes in the investigated parameters on a temporal scale and between sediment layers, the sediment samples were collected monthly in 2002 with a piston corer from the sediment surface to a depth of 10 cm. The concentrations of organic matter (OM), chloroplastic pigment equivalents (CPE), chlorophyll a (chl a) and phaeopigments (PHAEO) were used as indicators of substrate concentrations in sediments. Sediment depth was a significant factor influencing the distribution of bacterial abundance, biomass and volume. Granulometric properties of the sediment had no effect on the distribution of bacteria. Bacterial abundance, biomass and volume were strongly related to the indicators of substrate concentrations on both scales. The accumulation of labile OM in deeper layers also had a profound effect on the size and structure of bacteria. High amounts of OM and the low proportion of labile organic fraction (CPE; chl a and PHAEO) indicate that this environment acts as a sink for accumulation of detrital material.

Bacteria are an active and diverse component of pelagic communities. The identification of main factors governing microbial diversity and spatial distribution requires advanced mathematical analyses. Here, the bacterial community composition was analysed, along with a depth profile, in the open Adriatic Sea using amplicon sequencing of bacterial 16S rRNA and the Neural gas algorithm. The performed analysis classified the sample into four best matching units representing heterogenic patterns of the bacterial community composition. The observed parameters were more differentiated by depth than by area, with temperature and identified salinity as important environmental variables. The highest diversity was observed at the deep chlorophyll maximum, while bacterial abundance and production peaked in the upper layers. The most of the identified genera belonged to Proteobacteria, with uncultured AEGEAN-169 and SAR116 lineages being dominant Alphaproteobacteria, and OM60 (NOR5) and SAR86 being dominant Gammaproteobacteria. Marine Synechococcus and Cyanobium-related species were predominant in the shallow layer, while Prochlorococcus MIT 9313 formed a higher portion below 50 m depth. Bacteroidota were represented mostly by uncultured lineages (NS4, NS5 and NS9 marine lineages). In contrast, Actinobacteriota were dominated by a candidatus genus Ca. Actinomarina. A large contribution of Nitrospinae was evident at the deepest investigated layer. Our results document that neural network analysis of environmental data may provide a novel insight into factors affecting picoplankton in the open sea environment.