EFFECTS OF TEMPERATURE,SALINITY, IRRADIANCE AND DIURNAL PERIODICITY ON GROWTH AND PHOTOSYNTHESIS IN THE DIATOM NITZSCHIA AMERICANA; LIGHT-SATURATED GROWTH1

The physiological response of an estuarine clone of Nitzschia americana Fryx3ell was measured under experimental conditions of temperature and salinity which represent the average range of these variables in the Cape Fear River Estuary, North Carolina. The influence of temperature (10, 15, 20, 25, 30°C) and salinity (8, 15, 20, 26, 32‰) on specific growth rates, μ, and parameters of photosynthesis-irradiance curves, α, and P_max were measured during maximum and minimum rates of diurnal photosynthesis using axenic semi-continuous batch cultures maintained at an irradiance saturating for photosynthesis (140 μE m^-2·s-1). There was an increase in μ with increasing temperature up to a broad uptimum (25 ± 2.5°C), above which μ gradually declined. At the predicted optimum temperature of 25°C, μ increased as a linear function of salinity. oth light-limited (α) amd light-saturated (P^max) rates of photosynthesis increased as salinity decreased. The effect of temperature on a and P^max was complex and dependent on salinity. P^max exhibited a diurnal periodicity, whereas estimates of a were not significantly different between sampling periods. Growth efficiencey opf N. americana, calculated as the ratio between specific growth rates and rates of gross photosynthesis, increased with an increase in salinity with a maximum at the predicted optimum temperature and salinity of 25°C and 32‰, suggesting and uncoupling between photosynthesis and growth at nonoptimum growth conditions.