Growth at the edge of the niche: An experimental study of the harmful alga Prymnesium parvum

The haptophyte Prymnesium parvum forms harmful blooms toxic to fish in coastal and inland waters. Its growth in relation to niche factors is poorly characterized for the low salinities found in the inland waters in which P. parvum blooms have occurred. The specific growth rate of P. parvum as a function of temperature and salinity was determined in nutrient‐sufficient cultures with low salinity. Additionally, phosphorus‐limited growth was determined at low salinity and temperatures at or below 20°C. In nutrient‐sufficient cultures, decreasing salinity from 4 g L−1 to 0.5 g L−1 reduced the growth rate of P. parvum. The estimated optimal temperature for growth decreased with decreasing salinity from about 27uC at salinities above 10 g L−1, to about 24°C at 4 g L−1, to about 22°C at 0.5 g L−1. In phosphorus‐limited experiments, the half‐saturation concentration for growth was less than 0.02 µmol L−1 under most conditions. The phosphorus‐saturated growth rate was 0.84 d21 at 4 g L−1 salinity and 20°C, and it was reduced at lower salinities and temperatures. The salinity‐temperature interaction found here weakens the negative effect of low temperature on growth at low salinity and might partially explain why blooms of P. parvum occur in the winter months in inland waters of the southwestern United States. However, the relatively slow growth of P. parvum at low temperature and salinity suggests that additional factors should be examined, such as reduced effects of competitors, pathogens, and grazers during winter.