Water currents and zooplankton distributions are described for Pandora Reef, a coralline platform reef topped by a small sand spit, subject to tidal currents within the Great Barrier Reef lagoon (Queensland, Australia). Oncoming tidal currents separated 500 m upstream of the l-km oblong reef. Zooplankton accumulated around the reef variously according to taxonomic group. Copepods and chaetognaths were most abundant directly upstream in an entrained water mass near the reef face and on top of the reef at high tide. Larvaceans were less abundant near the reef and were at low densities downstream. Zooplankton distributions were correlated with flow patterns. Water currents flowed asymmetrically around the reef due to shape of the platform and direction of main tidal flow. Flow separation at one end of the reef created a shear zone and strong gyres that shed periodically downstream. The other end of the reef was without flow separation but generated eddies and a weaker, stationary gyre. Downstream effects of flood and ebb exhibited major differences in flow, due to subtleties of reef morphology, and tidal height. The "island mass effect" of upstream increased primary productivity is in reality a spectrum of biological and physical events upstream, adjacent to, and downstream of islands and reefs. Existing laboratory models and mathematical theory apparently correctly predict flow around islands. At Pandora Reef little upstream water passed directly over the reef top at high tide in calm weather, and prior estimates of coral reef nutrition based on the assumption that nutrient-poor oceanic water crosses the reef may be in error.

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.