Studies on Freshwater Micro-Organisms: Phosphatase Activity in Lakes of Differing Degrees of Eutrophication

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Abstract
Sixteen lakes in the English Lake District were examined during the period of thermal stratification. Epilimnetic samples were analysed for total phosphorus, total soluble phosphorus and soluble inorganic phosphorus (Pi) concentrations, chlorophyll a, viable bacteria and alkaline phosphatase activity. Temperature and dissolved oxygen readings enabled the position of the thermocline to be determined and the degree of hypolimnetic oxygen deficit to be assessed. The lakes were placed in a series in order of increasing degree of eutrophication. Total phosphorus and chlorophyll a concentrations, bacterial numbers, hypolimnetic oxygen deficit and phosphatase activity measurements appeared to fit this series and so supported this classification which was based mainly on that of Pearsall (1921). Two methods were used to assess phosphatase activity, one based on the use of an artificial substrate, p-nitrophenol phosphate (PNP-PO4) and the other on the natural release of Pi in stored water samples. Both methods produced results which were significantly correlated with total phosphorus levels of the water samples and also with microbial biomass. Repression and/or inhibition of alkaline phosphatase in relation to phosphorus levels found in the lakes was not satisfactorily demonstrated, but the variety of factors affecting these two control mechanisms (particularly inhibition) is such that a more detailed survey of each water body would be required. The high degree of correlation between the two variables indicated that the enzyme was a good indicator of biomass present and degree of eutrophication, except in samples where very high numbers of the dinoflagellate Ceratium sp. were present. The organism was found to contain significantly lower levels of the enzyme than those found in other micro-organisms.

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