Numerical models are used to simulate 3-dimensional circulation and dispersal of material, such as larvae of marine organisms, on Davies Reef in the central section of Australia's Great Barner Reef. Residence times on and around this reef are determined for well-mixed material and for material which resides at the surface, sea bed and at mid-depth. Results i n d c a t e order-of-magnitude differences in the residence times of material at different levels in the water column. They confirm prevlous 2-dimensional modelling which indicated that residence times are often comparable to the duration of the planktonic larval life of many coral reef species. Results reveal a potential for the m a~n t e n a n c e of local populations of vaiious coral reef organisms by self-seeding, and allow reinterpretation of the connectedness of the coral reef ecosystem.
Numerical hydrodynamic and particle dispersion simulations showed that neutrally buoyant larvae are subject to a complex circulation pattern which induces a spatial variability in their relative numbers. The interaction of tidal, wind and gradient-driven flows with the reef were found to cause some areas within individual reefs to retain higher numbers of larvae than others. These locations were compared with diver observations of the abundance of Acanthasterplanci (L.) at the beginnings of outbreaks and a clear correspondence was found in all 6 simulated cases. Results suggest that hydrodynamics have a major influence on not only the dispersal of larvae of A. planci but also perhaps the location of their initial recruitment. Surveillance of these locations may provide an 'early warning' strategy for monitoring and controlling future outbreaks of this starfish on reefs.
Two hypotheses regarding the origin of post-larval Sillaginodes punctata that recruit to nursery areas in South Australia were assessed. These were that post-larvae originate either hundreds of kilometres to the west and are delivered by weather-determined ocean currents or they originate from numerous spawning grounds located only
The dispersal of eggs and sperm of crown-of-thorns starfish, Acanthaster planci (L.), was measured in the field using an array of collectors up to 10 m downstream of a spawning starfish. Hydrodynamic measurements, gamete dispersal numerical models, and the gamete cloud dispersal measurements for the first time quantified the relationship between hydrodynamic conditions and the dispersion of eggs and sperm in the field. In general, gamete concentrations fell rapidly and logarithmically with distance from the spawning starfish; egg concentrations at 3 m were 1% of those near the starfish. Simplified dispersal models showed a good correspondence with these field data, and confirmed the observation that eggs rose higher in the water column and spread more laterally at low current speeds over the short spatial scales being considered. Fertilization rates, predicted from laboratory measurements of fertilization success and the gamete concentrations measured in the field, were estimated to be 90-100% within 1 m and 70-100% at 10 m. These results are explained by high success rates of fertilization (fertilizing capacity) at the measured dilutions, and were similar to fertilization rates previously measured by others for crown-of-thorns starfish in the field. Although the eggs were observed to spread upwards into the water column due to turbulence, laboratory measurements of sinking rates showed eggs to be very slightly negatively buoyant (median fall velocity of 0.072 mm·s-1), whereas sperm were neutrally buoyant. A significant fraction of eggs also entered the seabed near the starfish; the proportion decreased with increasing current strength. This process may provide a mechanism for enhanced fertilization of these gametes and/or a mechanism for self-recruitment to a given reef population.
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