Physiological Indicators of Nutrient Deficiency in Lake Phytoplankton

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Abstract
Physiological indicators of N and P deficiency were measured in summer populations of several central Canadian lakes. Measurements included six composition ratios (P/C, N/C, N/P, ATP/C, chlorophyll a/C, and protein/carbohydrate) and three metabolic indicators (alkaline phosphatase activity and P and N debts expressed per unit particulate ATP). Overall results show P deficiency is generally greater than N deficiency in all the small lakes studied, embracing a wide range of maximum chlorophyll concentrations and N/P loading ratios. In two larger, very turbid lakes, neither P nor N deficiency appeared important, and light limitation is suggested. In a series of lakes of known N and P loading rates, indicators of P deficiency increased but those of N deficiency showed little trend with increasing N/P loading ratio. Some indicators of P deficiency appeared to provide a sensitive means of following the movement of river water, and loss of P from it, through a large lake. Some comparisons of net phytoplankton with total seston showed surprisingly little interference from detritus in these measurements.Key words: deficiency, nitrogen, phosphorus, physiological indicators, phytoplankton