Growth and phosphorus status of limnetic phytoplankton and bacteria

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Abstract
The phosphorus status [total phosphorus (P), surplus P, and alkaline phosphatase activity (APA)] of phytoplankton and bacteria were followed in a eutrophic Norwegian lake. On average 36% (range 20–60) of the particulate P and 57% (range 10–100) of the surplus P were bound by bacteria. The P : C ratio of phytoplankton and bacteria varied between 2.5 and 55 (median 5.1) and 34 and 360 (median 89) µg P (mg C)−1, indicating that bacteria have a substantially higher P requirement than do phytoplankton.The inverse relation found between surplus P : C and APA : C ratios during a Microcystis aeruginosa bloom and the positive relation between P : C ratio and growth rate of the cryptophyte‐dominated community in summer indicate that the phytoplankton were P limited during most of the ice‐free period. This is supported by the fact that the data from the cryptophyte‐dominated community could be described by the Droop model for nutrient‐limited growth. For the bacteria a positive relation was found between growth rate and cellular P (i.e. the P : C and the surplus P : C ratios), as predicted by the Droop model. Although the bacteria obviously were P subsaturated, it cannot be concluded that they were P limited since C may have been supplied at an even lower rate. Because of their high P requirements the bacteria acted as consumers of inorganic P and their net consumption of P was four times higher than that of the phytoplankton.