The mean carbon, nitrogen, and phosphorus contents of particulate material for 5 1 lakes or lake basins, extending from arctic to tropical climatic regions, including small lakes as well as the largest lakes in the world, indicate that Redfield ratios are the exception rather than the rule in freshwater. The C: P and N : P ratios are more variable for lake particles but generally higher than marine particles, and the mean molar C: N, C: P, and N : P ratios are substantially higher than the Redfield ratio of 106 : 16: 1. On average, lower C : N, C : P, and N : P ratios occur in subarctic lakes while higher ratios occur in the tropics and in temperate, oligotrophic lakes on the Canadian Shield. In shield lakes with long residence times (>6 months) the high ratios of C : N, C : P, and N : P do not originate from streamborne or atmospherically deposited particles but arise from in-lake processes. Regression analysis demonstrates that small lakes are generally more N and P deficient than large lakes. In freshwaters, particulate composition ratios imply that a wide variety of conditions exists in lakes, including N and P deficiency, as well as N and P sufficiency. In the Experimental Lakes Area of Canada, independent physiological nutrient status indicators generally agree with the status indicated by seston ratios. The relative uniformity of marine C : N : P composition (compared to lakes) at the Redfield ratio suggests that marine plankton cannot be as severely, or as frequently, limited by N and P as lake plankton. Consequently, the paradigm of N limitation in the oceans requires qualification.Based on particulate comDosition, it is more correct to say that ocean plankton is noi as N grid P deficient as lake-plankton.The composition of marine particulate matter is relatively uniform. Redfield (1934Redfield ( , 1958 noted the near constancy of the ratio of C : N : P in marine plankton and the similarity of the N : P ratio of plankton to the oceanic deepwater ratio of nitrate to phosphate. As early as 1940, the C : N : P molar composition ratio of marine plankton was accepted to be 106 : 16 : 1 (Redfield et al. 1963); this ratio is now referred to as the Redfield ratio. The ratio has withstood the test of time, and the ever-growing number of analyses of marine particles and nutrient regeneration profiles, with relatively AcknowledgmentsWe thank G. J. Bumskill and H. E. Welch for permission to publish particulate C, N, and P data on Lake Winnipeg and the Saqvaqjuac lakes, respectively. Dana Cruikshank and his field assistants sampled the ELA lakes for us.

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

Five freshwater algae (Chlamydomonas reinhardi, Scenedesmus quadricauda, Anabaena variabilis, Pseudoanabaena catenata, and Cryptomonas erosa) were grown in batch culture without nutrient limitation and with various degrees of P and N limitation. They were analysed for several aspects of composition and some metabolic activities of potential use as indicators of nutrient deficiency to aid in establishing values indicative of the presence or absence of nutrient deficiency. Ratios of N/C, P/C, N/P, protein/C, carbohydrate/C, and protein/carbohydrate most clearly differentiated between no, P, and N deficiency. The initial saturated rate of phosphate uptake, P debt, and alkaline phosphatase activity increased distinctly in P deficiency in some of the algae but not in others. Similarly, the initial saturated rate of ammonium uptake and N debt rose in some but not all N-deficient algae. Cryptomonas erosa tended to form walled resting cells (cysts) during growth into P or N deficiency. Key words: algae, nitrogen, nutrient deficiency, nutrient uptake, phosphatase, phosphorus

No abstract

SUMMARY. Using both cultures of algae and natural populations, alkaline phosphatase activity located on the cell surface has been measured by a fluorometric procedure. This was done in order to establish optimum standard conditions for the measurement of this activity as an indicator of phosphorus deficiency and to provide a means of interpreting alkaline phosphatase measurements on natural phytoplankton populations. A concentration of 10 μM o‐methylfluorescein phosphate saturates or nearly saturates the reaction in a variety of situations. In most trials, rates increased with temperature to or beyond 35°C. Optimum pH values in the range 7–10 were observed. In six of the algae examined, maximum alkaline phosphatase activities were dependent on external calcium at 100 μM or more. One alga, Synura uvella, showed acid phosphatase activity, peaking at pH 5–6, induced by phosphorus deficiency. Based on comparisons between P‐sufficient and P‐deficient cultures, alkaline phosphatase activities in excess of 0.1 μmol o‐methylfluorescein phosphate hydrolysed per mg dry weight per h or 0.1 μmol per μg ATP per h are suggested as indicative of phosphorus deficiency.