PHYTOPLANKTON PRODUCTIVITY RESPONSE TO NUTRIENT CONCENTRATIONS, LIGHTAVAILABILITY AND TEMPERATURE ALONG AN AUSTRALIAN ESTUARINE GRADIENT

Phytoplankton productivity and the factors that influence it were stud ied in the Logan River and southern Moreton Bay, a large embayment on the east coast of Australia. Phytoplankton productivity, dissolved and total nutrient concentrations, and turbidity were determined througho ut high and low rainfall periods to characterize light and nutrient in fluences on productivity. Turbidity and nutrient concentrations were h ighest at upriver sites, but productivity was highest at the river mou th and within the river plume. Phytoplankton productivity peaked after rainfall events (>150 mg C m(-3) h(-1)), commensurate with a decrease in dissolved nitrogen concentrations. Productivity responses to incre ased nutrient concentrations and light availability were determined in laboratory incubations. During summer, productivities at the bay site s were stimulated by nitrogen (N) enrichment, while productivities at upriver sites were stimulated by phosphorus (P) addition. Light stimul ation of productivities was more pronounced at upriver sites than bay sites. The relative magnitude of nutrient and light stimulation of pro ductivities indicate a predominance of light limitation upriver, signi ficant N limitation within the Logan River plume, and little effect of light, N, or P at sites beyond the Logan River plume. Productivity de creased with seasonal decreases in temperature. Lower water temperatur es in winter probably helped determine maximum rates of phytoplankton productivity. The combination of light and N limitation of productivit y during summer, and temperature limitation during winter, account for low areal productivities (<0.6 g C m(-2) d(-1)), compared with other rivers and estuaries worldwide.