The Dynamics of Wave-Driven Reef Pass Jets and Coral Population Recovery on an Island
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Ocean circulation is integral to reef ecosystem health and resilience; waves and currents replenish nutrients, transport coral and fish larvae between populations, and moderate temperatures. On many reefs, wave-driven or tidal flow through reef passes is the dominant mechanism of exchange with the open ocean. The fate of the jet as it flows onto the forereef slope, however, is understudied. In this study, numerical simulations of wave-driven flow on an idealized coral island with periodically spaced reef passes were conducted and examined across a range of planetary rotation and bottom friction conditions. For higher latitude, lower friction cases, adjacent jets interacted due to the deflection of the jet by planetary rotation and weak attenuation by friction - a previously undescribed phenomenon. The physics of reef pass jet deflection were then examined using a novel reformulation of the barotropic vorticity balance in terms of flow curvature. The resultant curvature gradient equation was used to interpret a series of idealized numerical modeling experiments of a barotropic outflow jet onto a slope in shallow water using a depth-averaged circulation model (ROMS). The trajectory of the jet was explained by the curvature dynamics along its center streamline, with topography, planetary rotation, and bottom friction strongly influencing the course. Articulating the key processes of nearfield jet deflection in terms of curvature distilled the complex 2D phenomenon into a tractable 1D initial value problem, exhibiting predictive skill and clarifying the poorly understood dynamics of a nonlinear feature. A third element of this work examined population dynamics on reefs. Larval flux, as determined by demography and hydrodynamics, is integral to the recovery of a subpopulation after a disturbance, but few studies have connected annual time-scale coral population dynamics to larval recruitment.Here, a model for coral population growth was developed that synthesized metapopulation theory with classical population dynamics. It was validated against a case study of remarkable coral recovery after near extirpation. Model simulations suggest that a combination of steady background larvae source and favorable local growth conditions explain the recovery of coral on this reef, implying that the succeeding coral colonies are likely endogenic.
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