Effects of N:P:Si ratios and zooplankton grazing on phytoplankton communities in the northern Adriatic Sea.: II.: Phytoplankton species composition

Development of phytoplankton biomass and species composition in northern Adriatic Sea natural plankton communities was investigated in 2 spring/summer (May and June) enclosure experiments. The enclosures (100 l plastic cylinders) were subjected to different concentrations/ratios of nitrate, phosphate and silicate in order to obtain a situation where one of these nutrients would limit phytoplankton biomass in different treatments. The initial phytoplankton community in both experiments was dominated by small (both by cell, numbers and biomass) autotrophic flagellates. In the May experiment the small autotrophic flagellates made up more than 50 % of the initial biomass; this was 95 % in June. The duration of the experiments were 3 and 10 d, respectively. A surplus of nitrogen, phosphorus and silicon favored the development of some fast-growing diatoms, Phosphorus deficiency induced higher phytoplankton biomass than expected from Redfield ratios (Redfield 1958), probably due to fast recycling of phosphorus compared to nitrogen. Despite low silicon concentrations in the silicon-deficient treatments, growth of diatoms was high in the May experiment suggesting a low silicon demand for the dominating small diatom species. The diatom Rhizosolenia fragilissima was particularly favored under phosphorus-deficient conditions whereas under nitrogen deficiency another diatom (Chaetoceros sp.) increased in biomass. Small flagellates were favored by either nitrogen deficiency (May) or by silicon deficiency (June). The increasing dominance of small flagellates in the natural communities during the transition from spring to summer, together with decreasing concentration of inorganic nitrogen, indicates that shortage of inorganic nitrogen can be important for the structure of northern Adriatic phytoplankton communities in this period.