Factors limiting phytoplankton production in a tropical continental shelf ecosystem

The effects of light and nutrients on phyoplankton 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 (<20 m) were well-mixed year-round. Integrated production in well-mixed, deeper (>20 m) waters of the Gulf was low (winter, 557 +/- 351 mg C m(-2) day(-1)) compared with waters which were stratified during the north-west monsoon (summer, 955 +/- 129 mg C m(-2) day(-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 +/- 6.4 mg m(-2)) than in summer (16.8 +/- 3.4 mg m(-2)). While molar N:P ratios were low (2.9), which is indicative of nitrogen limitation, nutrient addition and N-15-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 nitrogen-limited. It is concluded that light, rather than nutrients, limited phytoplankton production during the south-east monsoon in offshore waters.