A numerical study of larval retention in the southern straits of Florida

The physical processes governing the distribution and subsequent recruitment of planktonic larvae in Southeast Florida are poorly understood. At present, it is not known if larvae recruiting to this region were spawned locally or from distant upstream sources. Obtaining this knowledge is an expensive and time consuming process, but necessary if we are to manage our resources wisely. Towards this end, I have employed mathematical models to examine the hypothesis that larvae (eggs) spawned along the outer reefs in the Florida Keys can be retained by local hydrodynamic processes. The specific objectives were to: (1) develop and parameterize models of larval swimming; (2) determine whether random swimming is important to larval dispersion; (3) develop a simulation model that enables the determination of settlement probabilities for various hydrodynamic and behavioral hypotheses; and (4) assess the effects of various physical and biological variables on the probability that a larva will settle in a suitable habitat.The simulation results indicated that the relative importance of dispersion and mortality to the loss of locally-spawned larvae depends greatly on current velocity, turbulence intensity, and the degree of cross-shore inhomogeneity in the mortality field. Natural levels of variation in either the mortality field or flow regime can account for the large fluctuations in recruitment observed in the Florida Keys. Of the flow types examined, a recirculating gyre proved by far the most effective retention mechanism--it was the only flow type returning large numbers of larvae to within 20 km of their birth site. Settling rates tended to increase with an onshore Ekman drift unless the mortality rate was much higher over the reef than elsewhere