Wind-generated transport of fictitious passive larvae into shallow tidal estuaries

Four 14-day field experiments were conducted to determine the densities of postlarval white shrimp and blue crab megelopae moving into a tidal inlet along the south-eastern United States. Certain winds directions were statistically associated with peaks in abundance, a result which motivated us to use a high resolution finite-element numerical model to simulate passive larval transport under a variety of wind directions into the inlet. The passive particles were initially distributed uniformly in a zone of the continental shelf which extended 20 km offshore and 20 km alongshore in either direction. Each simulation was conducted for five tidal cycles (2.5 days) under constant wind stress. These simulations indicated that larvae are withdrawn from the continental shelf into the inlet from a narrow zone parallel to the shoreline but extending less that 5 km offshore. The withdrawal zone changed to one directly offshore of the inlet mouth only for a wind direction that pointed directly toward the inlet mouth. Under downwelling-favourable winds, particles originating in the surface accumulate along the downwind boundary and drift shoreward with time thus causing a 'pooling' of larvae along the coast. This scenario is repeated with less efficiency for upwelling-favourable winds with particles originating near bottom. The 'pooling' process occurs over the scale of the particle domain. A second and smaller scale is indicated by the relatively few particles which are withdrawn into the inlet as they pass inside the 7-m isobath (within 5 km of the coast). Those that do pass become available for inlets farther downstream.