Alkaline phosphatase activity and cellular phosphorus as an index of the phosphorus status of the phytoplankton in Minnesota lakes

Nine lakes were investigated during 1975 in Clearwater and Hubbard Counties, Minnesota, one of them (Lake Josephine) throughout the year and the others during straification in late summer and early autumn. Total alkaline phosphatase activity is inversely related to cellular phosphorus in the particulate organic matter on waters above and below the thermocline in the nine lakes. Diagnostic criteria for assessing surplus and limiting levels of phosphorus, when applied to seasonal changes in phosphorus status in Lake Josephine, demonstrate severe phosphorus starvation in late summer and autumn. (Baker-IVI)