The structure of the microbial food web was studied in six estuary areas along the eastern Adriatic coast during March, July and October 2012. Limitation by phosphorus, not nitrogen, was a common feature for all studied estuaries. Heterotrophic bacteria and autotrophic picoplankton (APP) (particularly picoeukaryotes and Synechococcus) can reach notable abundances and biomasses, suggesting potential importance of the picoplankton community in P-limited estuarine environments. The main features of the microbial community structure in these environments included: (1) higher heterotrophic biomass in comparison to autotrophic biomass within the picoplankton community; (2) general domination of picoeukaryotes within the APP community, and increase of absolute and relative biomass of prokaryotic autotrophs (Prochlorococcus and Synechococcus) in the total APP in P-limited conditions; (3) domination of Synechococcus over Prochlorococcus biomass in all studied conditions, and common spatial distribution of these two groups of cyanobacteria, which was mostly determined by concentration of phosphorus; (4) relatively high contribution (about 50%) of LNA bacteria in the total bacterial abundance; and (5) relatively high contribution (about 33%) of heterotrophic pico-flagellates in the total flagellate abundance.

Two contrasting high nutrient/low chlorophyll regions having different conditions that control phytoplankton production, and separated by an area of blooming, are found during summer in the vicinity of the South Shetland Islands (Antarctica). Low chlorophyll conditions occur either in Fe-rich, deeply mixed and high salinity Weddell Sea shelf waters, or the Fe-poor, shoaled and low salinity Drake Passage Antarctic Circumpolar Current waters, while phytoplankton blooms are located between in mid salinity water. Contrasting phytoplankton communities were found to populate these different biogeochemical provinces. In data from six field seasons (1999–2007), nanoplankton (2–20 μm) were found to be dominant in the phytoplankton populations from light-controlled coastal waters, including blooms, with most chlorophyll found in the 2–5 μm size class. In contrast, the adjacent and presumably Fe-controlled Drake Passage waters were dominated by the microplankton (> 20 μm) size class. The asymmetrical distribution of phytoplankton size classes across the salinity gradient, when analysed independently of total chlorophyll concentration, supports the hypothesis that the different food web grazing dynamics are dependent upon biogeochemical provinces.