A study of crab larvae dispersal on the Western Iberian Shelf: Physical processes

A high resolution nested model of the Western Iberia shelf is developed and compared with field observations. We use an embedded particle tracking submodel that includes advection, diffusion, and diel vertical migration to simulate crab larvae dispersal. A set of experiments with varying oceanographic conditions and Lagrangian model parameters is analysed. It is found that major dispersal patterns that were observed in nature are reproduced in the model. The floats drift within meridionally elongated patches parallel to the coast usually inshore of the 100 m isobath. This distribution is controlled by local wind-driven circulation, buoyancy input from adjacent rivers, local topography, and vertical diel movements. River plumes are critical factors for retention patterns as they influence the near surface stratification, surface Ekman layer and innershelf processes. Weakly stratified coastal waters associated with low river run-off generate an enlarged innershelf whose dynamics tend to trap the floats and force them to drift along-wind. We find that the response to downwelling-favourable winds is always more dramatic than upwelling-favourable ones and this asymmetry results in a net Lagrangian transport directed poleward. Analyses of float distributions show that a large part of the floats ( 50%) is actually retained in the shelf zone near the emission points. A dispersal distance (Ld) of about 60 km is estimated which is mostly representative of along-shore dispersion. Some consequences of our results to local population dynamics are finally discussed.