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.

Topographically Controlled Fronts in the Ocean and Their Biological Influence

“…Eddies are formed downstream of headlands, islands, and reefs (5)(6)(7)(8)(9)(10)(11). These eddies can take various forms, from stable, island-size eddies with strong, recirculating currents, to unstable eddies that generate meanders far downstream in the prevailing current.…”

“…These flows can significantly influence the local distribution of benthic and pelagic organisms (3)(4)(5)(6)(7)(8). The effects of fine-scale physical phenomena on the biology of pelagic organisms have been of concern to plankton biologists for more than a century.…”

Entrainment and advection in an island's tidal wake, as revealed by light attenuance, zooplankton, and ichthyoplankton

“…Swirls of turbidity in the wakes of small islands are distinctive features within shallow, tidally energetic areas such as the Great Barrier Reef lagoon (Hamner and Hauri 1977; Wolanski et al 1984a,b). Wakes of this nature may extend downstream several island diameters within the semidiurnal tidal excursion, and they may exist for periods of 4-5 h (Black and Gay 1986;Signell and Geyer 1991).…”

“…Neuston net collections across the wake revealed two larval fish assemblages that were correlated with either the small surface zooplankton or with the deeper, medium-sized zooplankton, which included epibenthic taxa. If EA is a common process for patch formation in tidal waters, then the geometry of the associated plumes may predict larval settlement (recruitment ''hotspots'') in shallow tidal waters.Swirls of turbidity in the wakes of small islands are distinctive features within shallow, tidally energetic areas such as the Great Barrier Reef lagoon (Hamner and Hauri 1977; Wolanski et al 1984a,b). Wakes of this nature may extend downstream several island diameters within the semidiurnal tidal excursion, and they may exist for periods of 4-5 h (Black and Gay 1986;Signell and Geyer 1991).…”

“…enhance mixing (Townsend et al 1983) and modify sediment transport, thus influencing the bathymetry. Wakes can also have biological consequences that affect species-diversity gradients and plankton patchiness Hauri 1977, 1981), as well as the dispersal and settlement of reef fishes (Kingsford et al 1991;Kingsford and Suthers 1996). For example, zooplankton can be entrained within convergence zones (Kingsford et al 1991) or become concentrated within the reduced turbulence zones of gyrelike features (Aldredge and Hamner 1980;Murdoch et al 1990).…”