Export–import dynamics of zooplankton on a coral reef in Palau

Although coral reefs are widely viewed as oases in an oceanic desert, we demonstrate that the oceans around coral reefs in Palau are not deserts devoid of food, nor are coral reefs oases isolated from the surrounding sea. On flood tides, oceanic zooplankton advects onto the Palau barrier reef, where it is consumed by a wall of planktivorous fish. Similarly, on ebb tides lagoon zooplankton is consumed as it advects across the reef flat. Coral reef planktivores ingest zooplankton imported from both the sea and the lagoon, but coral reef fish also export meroplanktonic fish eggs that disperse offshore. Every day about 30 species of reef fishes, some forming spawning aggregations (parrotfish and surgeonfish), spawn at the forereef when the tide turns, releasing clouds of buoyant eggs and sperm into the ebb-tide waters that flow off the reef flat. Fish eggs disperse seaward, but remain entrained within several kilometers of the reef in a tidal, neritic water mass, separated from oceanic currents by tidal fronts. Flood tides return fish embryos and offshore zooplankton to the reef. The flux of materials between oceanic, neritic, and lagoon waters near coral reefs must be reexamined.