The combined stress of nutrient limitation and suboptimal temperature on growth was studied with turbidostat and chemostat cultures of Scenedesmus sp. and Asterionella formosa. The combined effects were greater than the sum of individual effects and were not multiplicative.In N-and P-limited Scenedesmus sp. and A. formosa the cell quotas (4) of both limiting and nonlimiting nutrients increased with decreasing temperature. At a given temperature cell quotas of limiting nutrients ,also increased with the growth rate (p) and followed a saturation function. Higher values of the minimum cell quota (9J at lower temperatures show that cells require more nutrient with decreasing temperature.The change of q,, with temperature varies with the type of limiting nutrient. This change for N and P in Scenedesmus sp. suggests that their optimum ratio, the ratio at which one limitation changes over to the other, is higher at suboptimal temperatures.Cell quotas of nutrient-sufficient cultures (qm) for C, N, and P and cellular chlorophyll a concentration increased with decreasing temperature. The quota of each nonlimiting nutrient in nutrient-limited cultures had the same value as 9m. RNA decreased with temperature.

The optimum atomic ratio of N to P, the ratio at which one nutrient limitation changes over to the other, was determined in seven species of freshwater planktonic algae. The ratio varied over a wide range among species; the average for these species was 17. If the cellular nutrient ratios in marine species are comparable with those in freshwater organisms, Redfield's ratio of 15 is remarkably close to the average. Cellular N:P ratios varied over a 24‐h period under a light:dark cycle. The variation of the optimum ratio between species and diel change in cellular N:P ratios within a species could play an important role in population dynamics by enhancing the probability of coexistence of species.

The effects of simultaneous limitations of light and nutrient on growth were investigated with turbidostat and chemostat cultures of Scenedesmus sp. and Fragilaria crotonensis. The combined effects were greater than the sum of individual effects and were not multiplicative. Under nitrate-limited conditions the cell quota of the limiting nutrient for a constant growth rate and the subsistancc quota (qO) increased as irradiance decreased. Within a certain limit of growth rates, light and qN can compensate for each other in maintaining growth rate. Under nutrient-sufficient conditions, cell quotas of C, N, and P, and cellular chlorophyll a concentration increased as light decreased below saturation. The cell quotas are the maximum level (y,,[) that can be achieved at a given irradiance. Cell quotas of nonlimiting nutrients in nutrient-limited cultures are the same as these (I~? values. The increasing levels of g, and q. with decreasing irradiance indicate that nutrient requirements increase as irradiance decreases.

A comparative kinetic study of phosphate‐limited growth and phosphate uptake was carried out in chemostat cultures of Anabaena flos‐aquae Lyng. Bréb., Ankistrodesmus falcatus (Corda) Ralfs, Asterionella formosa Hass., Fragilaria crotonensis Kitt., and Microcystis sp. Lemm. For each test organism the growth rate (μ) can be described as a function of total cell phosphorus (P), i.e. cell quota (q), or hot‐water‐extractable P (qsp). Phosphate uptake rate in each species examined was a function of external P concentration (S) and intracellular P levels. The substrate parameter for uptake (Km) remained constant at all growth rates, but the apparent maximum uptake rate (Vm') increased with decreasing μ, or q, in all test organisms. At low growth rates Vm' was greater than the calculated steady‐state uptake rate (μq) by two to three orders of magnitude because Vm' is the uptake rate at S ≫ Km and μq is the rate at S ≪ Km. The difference between Vm' and μm decreased and virtually disappeared as μ approached the physiological maximum growth rate (μm), where q = qm and S ≫ Km. An inverse relationship existed between Vm' and qsp which could be described by a function that mathematically resembles noncompetitive inhibition in enzyme kinetics. The inverse relation between Vm' and (q – q0) was described by a three‐parameter function. This empirical function provided a good description of this inverse relation over the range from Vm' at q approaching q0 up to and including calculated net, maximum uptake rates at q = qm.

Persons in counties with high dead crow densities had elevated risk for disease.