Abstract
Surface‐water microbial populations were investigated in the northwest Mediterranean for possible indicators of phosphate deficiency and limitation. Low phosphorus availability was suggested by short turnover time (min. observed 0.68 h), high‐alkaline phosphatase activity ( V max = 28 nM hydrolyzed h −1 ), subsaturation of phosphate uptake (2.6–9% of V max ), and high‐pulse uptake capacity of added orthophosphate. Based on high pulse uptake capacity and subsaturated uptake in both the >1‐µm and in the 0.2–1‐µm size fractions, P deficiency is suggested for both phytoplankton and heterotrophic bacteria. P limitation of heterotrophic bacteria was also supported by fast positive responses after phosphate addition in both thymidine incorporation in whole‐water samples and increased bacterial cell numbers in predator‐free water. No effects were found after addition of carbon or nitrogen sources alone. Combined with other published evidence, we suggest that the growth rates of not only phytoplankton, but also of heterotrophic bacteria, are P limited in this environment in summer. The finding has important implications for the dynamics of accumulation of dissolved organic carbon in the photic zone and thus for the carbon cycle of oceans.