School of Civil, Environmental and Mining Engineering

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Cecile Rousseaux


Start date

Feb 2007

Submission date

Feb 2011

Cecile Rousseaux

Thesis

Oceanographic forcing of phytoplankton dynamics in the waters off north Western Australia

Summary

Recent studies have shown that coral reefs can rely heavily on the delivery of offshore particulate matter as nutrient sources to sustain their high productivity. Off north west Australia, the Leeuwin Current, an anomalous poleward flowing eastern boundary current, flows adjacent to Ningaloo Reef, Australia's longest fringing coral reef. Using chlorophyll a estimated from satellite-derived ocean colour and in situ field observations we identified the existence of a autumn phytoplankton bloom in the waters off north west Australia. In autumn, a combination of the accelerating Leeuwin Current and net surface cooling lead to a significant deepening of the mixed layer depth down to ~100 m. This deepening also coincided with increased nutrient and chlorophyll a concentrations in the euphotic zone. We conclude that the MLD deepening is the mechanism driving the phytoplankton dynamics in the waters off Ningaloo Reef through the replenishment of nutrients in the surface waters autumn. In both spring and autumn the size-fractionated phytoplankton concentrations and community diversity, as well as nutrient uptake rates, were quantified. In autumn, the concentration of largeĀ­ sized phytoplankton increased. Ammonium uptake was always greater than nitrate uptake. In autumn, nitrate uptake was relatively great at 30 % of the total dissolved inorganic nitrogen uptake. For a bloom to develop nutrients must be available at sufficient concentrations; however, grazing by predators can also control phytoplankton populations suppressing the development of a bloom. Theory suggests that a deepening mixed layer may result in a reduction in grazing pressure through dilution. The findings of study showed that grazing rates did not decrease with the deepening of the mixed layer depth and therefore this theory did not hold for the waters off Western Australia. Instead growth rates were positively correlated with the mixed layer depth which further suggests that the mixed layer depth deepening is the key mechanism allowing net phytoplankton growth. The findings of this study show that there is likely to be substantial seasonal variation in the net production of offshore particulate matter and that this is likely to impact on the productivity of Ningaloo Reef itself.

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