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Phytoplankton abundance in the northern Adriatic during the summer 2008 indicated that the system was highly productive, in spite of low orthophosphate (PO4) concentrations. Mechanisms by which phytoplankton adapted to PO4 deprivation during the summer stratification were studied. In upper, more productive waters, phytoplankton induced high alkaline phosphatase activity (APA) to obtain phosphorus (P) from the dissolved organic pool, and the P turnover time mediated by phytoplankton APA was very short (2 min to 1.5 h). Highaffinity enzymatic activity combined with high hydrolysis rates enabled metabolic flexibility to the phytoplankton in this heterogeneous and fluctuating environment. Another possible mechanism of adaptation to the PO4 deficit during the summer was a shift toward smaller cells. The smaller nanophytoplankton, supported by higher surface:volume ratios, were presumably able to produce more alkaline phosphatase, an exoenzyme bound to the cell surface. Progressive decrease of large cells and increase of smaller cells in the phytoplankton community during summer supported this hypothesis. In upper waters with low PO4 concentrations, phytoplankton reduced their P demand by a preferential synthesis of non-phospholipids. In bottom waters, phytoplankton abundance was markedly lower than in upper waters and growth was probably light limited. In these deeper waters with higher PO4 concentrations, phytoplankton cells did not use APA to obtain P and were able to synthesize more phospholipids. In deeper waters, growth of bigger cells was favored.