Fine-scale surface currents in the Whitsunday Islands, Queensland, Australia : effect of tide and topography

Abstract

No data are available on surface currents of the central or northern Great Barrier Reef lagoon. Here we describe the fine-scale surface currents for Cid Harbour, Whitsunday Island group, Queensland, Australia, from 170 drogue releases and extensive aerial photographs. The current regimen is dominated by the strong tidal race to the south through the Whitsunday Passage toward Broad Sound, which experiences an 8-m fall during spring tides. Although the tidal cycle in Whitsunday Passage is symmetrically semi-diurnal, currents in adjacent Cid Harbour are asymmetric, with approximately 8-h southerly floods and 4-h northerly ebbs. This asymmetry is caused by topographic peculiarities of the Whitsunday Archipelago. Headlands and embayments within the harbour generate sharp shear zones, gyres, eddy systems, edge effects, convergences and divergences, and these dominate the fine-scale surface current patterns. In turn these local surface phenomena, particularly shear zones, affect the microdistribution of zooplankton. The flood tide in Cid Harbour is responsible for the depositional pattern of sediments and controls the morphology of coral islands within the habour. Cid Harbour is in the lee of Whitsunday Island and wind does not greatly affect surface currents. The importance of fine-scale current patterns has been underestimated, particularly by planktologists and reef morphologists.