SEMINAR: Oceanographic forcing of phytoplankton dynamics in the waters off Ningaloo Reef, Western Australia

Recent studies have shown that coral reefs rely heavily on the delivery of offshore particulate matter to sustain their high productivity. Off North West Australia, the Leeuwin Current, an anomalous eastern boundary current flows adjacent to Ningaloo Reef, Australia’s longest fringing reef. Using chlorophyll a estimated from satellite-derived ocean colour and in situ field observations we identified the existence of an autumn bloom adjacent to the Reef.

In autumn, a combination of the accelerating Leeuwin Current and net surface cooling leads to a significant deepening of the mixed layer depth down to ~100 m. This deepening also coincides with increased nutrient concentrations in the euphotic zone. Size-fractionated phytoplankton abundance and diversity, as well as nutrient uptake rates were quantified for both autumn and summer. In autumn, the phytoplankton community was dominated by cyanobacteria and large diatoms and was less diverse than in summer when very few diatoms were observed. Ammonium uptake was always greater than nitrate uptake. Autumn nitrate uptake was 30% of total dissolved inorganic nitrogen uptake, considerably greater than the 10% normal for the broader, oligotrophic region. For a bloom to develop nutrients must be available at sufficient concentrations; however, if grazing by predators is greater than, or equal to, the growth of phytoplankton, phytoplankton populations will be kept under control, i.e. the bloom will be suppressed. In autumn, a decoupling between the growth of phytoplankton and the grazing bymicrozooplankton was observed off Ningaloo. Off Western Australia the degree of decoupling was significantly correlated with the mixed layer depth.

Theory suggests that a deepening mixed layer may result in a reduction in grazing pressure through dilution, thus two conditions necessary for a bloom are present: greater nutrient concentrations and a decrease in grazing pressure. The findings of this study show that there is likely to be substantial seasonalvariation in offshore particulate matter production and this is likely to impact the productivity of Ningaloo Reef itself.