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 coasta l 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 +/- 3 51 mg C m(-2) day(-1)) compared with waters which were stratified during th e north-west monsoon (summer, 955 +/- 129 mg C m(-2) day(-1)). The lower pr oduction was due to high turbidity and light attenuation in the water colum n; 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 sum mer (16.8 +/- 3.4 mg m(-2)). While molar N:P ratios were low (2.9), which i s indicative of nitrogen limitation, nutrient addition and N-15-nitrogen up take experiments showed no such limitation. There was also little or no ind ication 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 s outh-east monsoon in offshore waters.