Upwelling as a Source of Nutrients for the Great Barrier Reef Ecosystems: A Solution to Darwin's Question?

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Abstract
The Great Barrier Reef shelf ecosystem was examined for nutrient enrichment from within the seasonal thermocline of the adjacent Coral Sea using moored current and temperature recorders and chemical data from a year of hydrology cruises at 3-5 wk intervals. The East Australian Current pulsates in strength over the continental slope with a period near 90 days and to pump cold, saline, nutrient rich water up the slope to the shelf break. The nutrients are then pumped inshore in a bottom Ekman layer forced by periodic reversals in the longshore wind component. The period of this cycle is 12-25 days in summer (30 days year round average) and the bottom surges had an alternating onshore-offshore speed up to 10 cm s-1. Upwelling intrusions tend to be confined near the bottom and phytoplankton development quickly takes place inshore of the shelf break. There are return surface flows which preserve the mass budget and carry silicate rich Lagoon water offshore while N rich shelf break water is carried onshore. Upwelling intrusions penetrate across the entire zone of reefs, but rarely into the Lagoon. Nutrition is delivered out of the shelf thermocline to the living coral of reefs by localized upwelling induced by the reefs. Bottom chlorophyll concentrations average 0.4 mg m-3 at the inner reefs and 0.8 mg m-3 near the shelf break while surface concentrations average 0.3 mg m-3; annual top and bottom variances are 0.5 and 1.9 mg m-3, respectively, and there was no apparent seasonal cycle. The estimated onshore N flux in a 10 m thick bottom layer gives an annual N input of 20 .mu.g at l-1 throughout the water column in a 50 km zone of reefs, an enormous value for tropical waters.