181
6 Oceanography of the Planktonic Stages of Aggregation Spawning Reef Fishes
topographic eddy (site a), a second within an alongshore coastal boundary layer
immediately next to the reef (site b), and one where water is entrained but not released
into the current that fl ows around the eastern end of Pandora Reef (c). At the fi rst site,
if the current impinging on the reef is suffi ciently strong or if the reef suffi ciently
small or asymmetric, the eddy may have suffi cient vorticity to be released from reef
attachment, and the eddy and larvae will drift to the south, downstream, in a “von
Karman vortex street” (also Fig. 5 in Hamner and Hauri 1981 ) . For the second site,
the larvae will drift westward past the end of the reef, merge smoothly into the fl ood
tide far-fi eld current, and then drift south. The shear-zone between the main current
and the island-entrained eddy is impenetrable to passive particles, so larvae will not be
entrained into the downstream reef-generated eddy fi eld (Wolanski 2001 ) but drift
free of topographic infl uence. The concentration of larvae will decrease slowly and
smoothly according to normal dispersal and natural mortality, and they will advect
without interruption until the far-fi eld current changes direction or velocity. At the
third site (c), the eddy has upwelling at the center but no net horizontal advection, so
the larval cloud will remain within the eddy relatively close to the reef as long as the
current fl owing past the eastern end of the reef does not change direction or velocity.
During ebb tide, eggs spawned into an eddy entrained on the north side of the
western headland (site d in Fig. 6.9 ) will rotate clockwise and the cloud of larvae
will remain close to the reef and the larvae may become more concentrated on eddygenerated fronts. Later, however, when the tide turns this eddy will shed downstream but this time it will be a free-eddy. If fi sh spawn on the south side of the
island near the western headland (site e) during ebb tide, their eggs will be released
into an alongshore boundary current with no vorticity, they will fi rst advect west and
then north. Consequently, larvae spawned on a fl ood or an ebb tide at any one of
three different locations on the reef, all near headlands, can end up with a variety of
different patterns of advection, dispersal, and vorticity.
At Bowden Reef, Kingsford et al. ( 1991 ) investigated meroplankton and larval
fi shes in slicks of tidal, topographically eddy-induced fronts and also in Langmuir
circulation convergent fronts (Fig. 6.10 ), modelling fl ow around the reef mathematically. They sampled for meroplankton on surface slicks using a small plankton
purse-seine, and found that presettlement reef-fi shes and zooplankton were exceptionally abundant in frontal slicks. The waters around Bowden Reef, on the Great
Barrier Reef shelf, are vertically well mixed (Wolanski 2001 ) and fl ows around
these reefs are barotropic. It is interesting that the tidal fronts northwest and west of
Bowden Reef are suffi ciently robust that slicks of 1–2 km length retain their integrity
and oscillate in position over several or more tidal cycles.
6.4 Topographic Meso-scale Eddies: Oceanic Islands and Atolls
Shallow water wakes for the Great Barrier Reef are ubiquitous in the vertically wellmixed, barotropic waters over the continental shelf (Hamner and Hauri 1981 ;
Wolanski et al. 1984 ; Wolanski 2001 ) , but benthic friction generates a secondary
6 Oceanography of the Planktonic Stages of Aggregation Spawning Reef Fishes
topographic eddy (site a), a second within an alongshore coastal boundary layer
immediately next to the reef (site b), and one where water is entrained but not released
into the current that fl ows around the eastern end of Pandora Reef (c). At the fi rst site,
if the current impinging on the reef is suffi ciently strong or if the reef suffi ciently
small or asymmetric, the eddy may have suffi cient vorticity to be released from reef
attachment, and the eddy and larvae will drift to the south, downstream, in a “von
Karman vortex street” (also Fig. 5 in Hamner and Hauri 1981 ) . For the second site,
the larvae will drift westward past the end of the reef, merge smoothly into the fl ood
tide far-fi eld current, and then drift south. The shear-zone between the main current
and the island-entrained eddy is impenetrable to passive particles, so larvae will not be
entrained into the downstream reef-generated eddy fi eld (Wolanski 2001 ) but drift
free of topographic infl uence. The concentration of larvae will decrease slowly and
smoothly according to normal dispersal and natural mortality, and they will advect
without interruption until the far-fi eld current changes direction or velocity. At the
third site (c), the eddy has upwelling at the center but no net horizontal advection, so
the larval cloud will remain within the eddy relatively close to the reef as long as the
current fl owing past the eastern end of the reef does not change direction or velocity.
During ebb tide, eggs spawned into an eddy entrained on the north side of the
western headland (site d in Fig. 6.9 ) will rotate clockwise and the cloud of larvae
will remain close to the reef and the larvae may become more concentrated on eddygenerated fronts. Later, however, when the tide turns this eddy will shed downstream but this time it will be a free-eddy. If fi sh spawn on the south side of the
island near the western headland (site e) during ebb tide, their eggs will be released
into an alongshore boundary current with no vorticity, they will fi rst advect west and
then north. Consequently, larvae spawned on a fl ood or an ebb tide at any one of
three different locations on the reef, all near headlands, can end up with a variety of
different patterns of advection, dispersal, and vorticity.
At Bowden Reef, Kingsford et al. ( 1991 ) investigated meroplankton and larval
fi shes in slicks of tidal, topographically eddy-induced fronts and also in Langmuir
circulation convergent fronts (Fig. 6.10 ), modelling fl ow around the reef mathematically. They sampled for meroplankton on surface slicks using a small plankton
purse-seine, and found that presettlement reef-fi shes and zooplankton were exceptionally abundant in frontal slicks. The waters around Bowden Reef, on the Great
Barrier Reef shelf, are vertically well mixed (Wolanski 2001 ) and fl ows around
these reefs are barotropic. It is interesting that the tidal fronts northwest and west of
Bowden Reef are suffi ciently robust that slicks of 1–2 km length retain their integrity
and oscillate in position over several or more tidal cycles.
6.4 Topographic Meso-scale Eddies: Oceanic Islands and Atolls
Shallow water wakes for the Great Barrier Reef are ubiquitous in the vertically wellmixed, barotropic waters over the continental shelf (Hamner and Hauri 1981 ;
Wolanski et al. 1984 ; Wolanski 2001 ) , but benthic friction generates a secondary