Phytoplankton Productivity Response to Nutrient Light Availability and Temperature Along an Australian Estuarine Gradient Concentrations

Phytoplankton productivity and the factors that influence it were studied 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 throughout high and low rainfall periods to characterize light and nutrient influences on productivity. Turbidity and nutrient concentrations were highest at upriver sites, but productivitywas highest at the river mouth and within the river plume. Phytoplankton productivity peaked after rainfall events (>150 mg C m -3 h-l), commensurate with a decrease in dissolved nitrogen concentrations. Productivity responses to increased nutrient concentrations and light availability were determined in laboratory incubations. During summer, productivities at the bay sites were stimulated by nitrogen (N) enrichment, while productivities at upriver sites were stimulated by phosphorus (P) addition. Light stimulation of productivities was more pronounced at upriver sites than bay sites. The relative mag- nitude of nutrient and fight stimulation of productivities indicate a predominance of light limitation upriver, significant N limitation within the Logan River plume, and little effect of fight, N, or P at sites beyond the Logan River plume. Productivity dccrcascd with seasonal decreases in temperature. Lower water temperatures in winter probably helped determine maximum rates of phytoplankton productivity. The combination of light and N limitation of productivity during summer, and temperature limitation during winter, account for low areal productivities (<0.6 g C m -~ d-l), compared with other rivers and estuaries worldwide.