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The inducible enzyme alkaline phosphatase (APA) was used to quantify phosphorus limitation of freshwater phytoplankton growth in situ. Natural phytoplankton communities from oligotrophic–eutrophic Lake Memphremagog were grown at phosphorus-limited rates of 0.06–0.93 per day in continuous, lake water cultures. APA varied inversely with phosphorus-limited growth rate and was little influenced by the community composition changes that occurred in culture. Compared to cultures, average in situ APA in Lake Memphremagog during the ice-free season was low, and suggested phosphorus-limited growth rates greater than 0.6 per day in even the most oligotrophic parts of the lake. Although algal abundance varies with total phosphorus among most north-temperate zone lakes, including Lake Memphremagog, our data show that average community growth rates are normally high and change relatively little …