Factors Limiting Phytoplankton Production in a Tropical Continental Shelf Ecosystem

Abstract

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 nitrogen-limited. It is concluded that light, rather than nutrients, limited phytoplankton production during the south-east monsoon in offshore waters.