Factors Limiting Phytoplankton Production in a Tropical Continental Shelf Ecosystem

The effects of light and nutrients on phytoplankton production were examined during both the south-east monsoon (winter) and north-west monsoon (summer) in the northern Gulf of Carpentaria, Australia. Integrated production and chlorophyll values were similar between summer and winter in Albatross Bay in the north-east Gulf over 4 years where shallow waters within the coastal boundary layer (<20m) were well-mixed year-round. Integrated production in well-mixed, deeper (>20m) waters of the Gulf was low (winter, 557±351mgCm-2day-1) compared with waters which were stratified during the north-west monsoon (summer, 955±129mgCm-2day-1). The lower production was due to high turbidity and light attenuation in the water column; coccoliths from algal detritus were the main cause. However, chlorophyll a concentrations were higher in winter (32· 9±6·4mgm-2) than in summer (16·8±3·4mgm-2). While molar N:P ratios were low (2·9), which is indicative of nitrogen limitation, nutrient addition and15N-nitrogen uptake experiments showed no such limitation. There was also little or no indication of silicate or phosphate deficiency. This contrasts with many other continental shelf systems that are nitrogenlimited. It is concluded that light, rather than nutrients, limited phytoplankton production during the south-east monsoon in offshore waters.