Data for summer chlorophyll and spring total phosphorus concentration were collected from 19 lakes in southern Ontario and combined with data reported in the literature for other North American lakes to produce a regression line that can bc used to predict the average summer chlorophyll concentration from a single measurement of phosphorus concentration at spring overturn. This equation is not significantly different from a previously published phosphorus-chlorophyll relationship derived for a number of Japanese lakes.Before adcquatc watershed management policy can bc implcmcntcd, the necessary quantitative predictive ecological theories must be developed and tested. Very few useful models or theories arc available to the planner at present because ecology has been largely a descriptive science with too little attention given to predictability at the field level. We feel that some emphasis should be diverted to the development of models whose value will bc judged on their predictive ability alone rather than their causal or heuristic properties. Using this criterion, empirical models are as valid as "realistic" models. Either could allow us to make predictions about the effects of various stresses on the environment, but empirical models are easier to product, bccause one tries to optimize only predictive ability.With respect to lake eutrophication we know that incrcascd nutrient loading frcquently results in increased phytoplankton standing crop which, in turn, may lead to increased hypolimnetic oxygen deficit, dccreased water clarity, and changes in species composition. However, we cannot say how much phytoplankton standing crop will increase with a given change in nutril ContributionNo.

The total phosphorus budgets for a number of lakes in the Haliburton–Kawartha region of southern Ontario were measured over a 20-mo period. These data, combined with the lakes' morphometry and water budgets, were used to test a simple nutrient budget model similar to that proposed by Vollenweider (1969) purporting to predict the total phosphorus concentration in lakes. Except in the case of two very shallow lakes [Formula: see text], the concentrations predicted by the model were very close to those measured in the lakes at spring overturn. Additional data from the literature supported the belief that this model could be used effectively for oligotrophic and mesotrophic lakes. Its value lies in the fact that quantitative changes in phosphorus loading can be interpreted in terms of changes in phosphorus concentration, which in turn, can be related to changes in parameters that reflect the lake's trophic state such as summer chlorophyll a concentration.

A general technique is presented for calculating the capacity of a lake for development based on quantifiable relationships between nutrient inputs and water quality parameters reflecting lake trophic status. Use of the technique for southern Ontario lakes is described. From the land use and geological formations prevalent in a lake’s drainage basin, the phosphorus exported to the lake in runoff water can be calculated, which, when combined with the input directly to the lake’s surface in precipitation and dry fallout, gives a measure of the natural total phosphorus load. From the population around the lake, the maximum artificial phosphorus load to the lake can be calculated and, if necessary, modified according to sewage disposal facilities used. The sum of the natural and artificial loads can be combined with a measure of the lake’s morphometry expressed as the mean depth, the lake’s water budget expressed as the lake’s flushing rate, and the phosphorus retention coefficient of the lake, a parameter dependent on both the lake’s morphometry and water budget, to predict springtime total phosphorus concentration in the lake. Long-term average runoff per unit of land area, precipitation, and lake evaporation data for Ontario provide a means of calculating the necessary water budget parameters without expensive and time-consuming field measurements. The predicted spring total phosphorus concentration can be used to predict the average chlorophyll a concentration in the lake in the summer, and this, in turn, can be used to estimate the Secchi disc transparency. Thus, the effects of an increase in development on a lake’s water quality can be predicted. Conversely, by setting limits for the "permissible" summer average chlorophyll a concentration or Secchi disc transparency, the "permissible" total phosphorus concentration at spring overturn can be calculated. This can be translated into "permissible" artificial load, which can then be expressed as total allowable development. This figure can be compared to the current quantity of development and recommendations made concerning the desirability of further development on the lake.

The feeding rate of Daphnia magna was studied by measuring the radioactivity of animals fed on pure cultures of Escherichia coli, Saccharomyces cerevisiae, Chlorella vulgaris, and Tetrahymena pyriformis labeled with radioactive phosphorus. Below a certain concentration of each food, the feeding rate is proportional to concentration of food. Above that concentration, feeding rate is independent of concentration. Starved animals, when placed in a nonlimiting concentration of food, behave temporarily as if it were limiting and for a few minutes filter at the maximum rate. Although the maximum volume of the various foods eaten in unit time is not the same, it is probably determined more by digestibility than by size of the food cells. Filtering efficiency of Daphnia magna is independent of the size of food cells between 0.9µ3 and 1.8×104µ3. Log‐phase Chlorella vulgaris was not observed to inhibit feeding, but senescent cells caused Daphnia magna to decrease the filtering rate and its maximum feeding rate.

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