Phytoplankton community structure was analysed from 2010 to 2017 at C1-LTER, the coastal Long-Term Ecological Research station located in the Gulf of Trieste, which is the northernmost part of the Mediterranean Sea. Phytoplankton abundance and relevant oceanographic parameters were measured monthly in order to describe the seasonal cycle and interannual variability of the main phytoplankton taxa (diatoms, dinoflagellates, coccolithophores and flagellates) and to analyse their relationship with environmental conditions. Overall, phytoplankton abundances showed a marked seasonal cycle characterised by a bloom in spring, with the peak in May. During the summer, phytoplankton abundances gradually decreased until September, then slightly increased again in October and reached their minima in winter. In general, the phytoplankton community was dominated by flagellates (generally <10 µm) and diatoms co-occurring in the spring bloom. In this period, diatoms were also represented by nano-sized species, gradually replaced by larger species in summer and autumn. Phytoplankton assemblages differed significantly between seasons (Pseudo-F= 9.59;p< 0.01) and temperature and salinity were the best predictor variables explaining the distribution of the multivariate data cloud. At the interannual scale, a strong decrease of the late-winter bloom was observed in recent years with the spring bloom being the main phytoplankton increase of the year.

The composition and seasonal cycle of the cryptomonad community were investigated at a coastal station in the Gulf of Naples (Mediterranean Sea) from March 2002 to March 2004. Serial dilution cultures were established from samples collected fortnightly in surface waters and single species abundances were estimated as most probable number. For species identification, unialgal cultures obtained by further dilution were analysed with light and scanning electron microscopy and spectrophotometry. Additional data collected at the same station and from other Mediterranean areas are also presented. Sixteen different morphs were recognized from 103 cultures. These included Plagioselmis prolonga, Proteomonas sulcata (haplomorph and diplomorph), Rhinomonas pauca, R. reticulata, Rhodomonas salina, R. cf. marina, Storeatula major, Teleaulax cf. acuta. Seven other morphs belonging to the genera Cryptochloris, Hemiselmis, Rhodomonas and Storeatula did not fit the descriptions of any described species, demonstrating a high rate of unknown diversity in the group. Total cryptomonad abundance showed peaks in spring–summer and autumn of both years. Some species were recorded occasionally, others (P. prolonga) throughout the year, still others (Hemiselmis sp., P. sulcata, R. pauca and R. reticulata) only at certain times of the year, the occurrence pattern recurring in both years. The species-specific occurrence patterns and the seasonal succession suggest that these species differ in their ecological requirements.