Specific affinity for phosphate uptake and specific alkaline phosphatase activity as diagnostic tools for detecting phosphorus-limited phytoplankton and bacteria

We analyzed responses of soluble reactive phosphorus (SRP), bioavailable phosphate (PO4), Particulate phosphorus, turnover time of orthophosphate (T-t), and alkaline phosphatase activity (APA) to varying degrees of nutrient stress. The nutrient stress was evoked by different treatments in concentration and combination of inorganic nitrogen (N) and phosphorus (P), and labile organic carbon (glucose) to mesocosms in experiments carried out in eutrophic southern (Odense Fjord, Denmark) and northern (Tvarminne Archipelago, Finland) coastal zones of the Baltic Sea. Despite seasonal and geographical differences, similar responses were observed in both experiments. Low SRP (< 100 nmol I-1), short T-t (< 10 h), and increased levels of APA were observed in both N + P balanced and P deficient treatments, while the opposite trend was observed in P replete treatments. The shortest T-t and the highest APA were found when glucose was combined with N treatment. Bioavailable PO4 was estimated using T-t and P uptake rates as derived from stoichiometric conversion of carbon based primary and bacterial production. With shorter T-t, the PO4 pool declined to < 1 nmol-P I-1, whereas the SRP background pool (difference between SRP and PO4) remained relatively constant (c. 50 nmol I-1). APA was inversely related to PO4 but not to SRP. Responses of specific APA and specific affinity for PO4 uptake, which are APA and PO4 uptake rates (inverse of T-t), respectively, normalized to the summed P biomass of phytoplankton and bacteria, responded consistently to the pattern and magnitude of nutrient limitation evoked in our experiments. Our results, together with a literature survey, suggest that both parameters can be useful in examining PO4 availability for the natural phytoplankton and bacteria community in P starved aquatic systems.