Phytoplankton community growth-rate response to nutrient pulses in a shallow turbid estuary

Phytoplankton growth is a physiological process often limited by temperature, nutrients or light, while biomass accumulation is a function of growth rates, grazing and deposition. Although primary productivity measurements are usually used to assess responses to limiting factors, the rates are proportional to biomass and inversely related to grazing pressure during experimental incubations. Alternatively, carbon-specific growth-rate determinations provide insights into physiological responses without the confounding effects of biomass and grazing. The objective of this study was to quantify the growth-rate responses of phytoplankton to enhanced nutrient availability (nitrate and phosphate) over a range of in situ irradiances. Growth rates were determined based on chlorophyll a-specific 14C-uptake rates by phytoplankton. Phytoplankton demonstrated high (24 h) growth rates when exposed to increased concentrations of limiting nutrients, independent of the surface irradiances (12–41%). Growth-rate responses were also compared with the biomass (chlorophyll a) responses and community composition. Observed and estimated phytoplankton biomass changes during the incubations differed, emphasizing the structural role of grazers on the phytoplankton community. The phytoplankton community in Galveston Bay has the potential to instantaneously respond to nutrient pulses, facilitating diatom biomass accumulations in spring and summer and small, flagellated species and cyanobacteria during periods of low nutrient inputs. Thus, Galveston Bay phytoplankton biomass and community composition reflect a dynamic balance between the frequency of nutrient pulsing and grazing intensity