Transport of crab larval patches in the coastal ocean

Abstract

We assessed the role of wind and buoyancy forcing on the transport of newly hatched blue crab larvae Callinectes sapidus near the mouth of Delaware Bay, USA. Eight patches of larvae were tagged with satellite-tracked drifters and followed for periods of 1 to 11 d. We conducted daily plankton tows within a 3.2 km 2 area circumscribing each drifter. This allowed us to verify that each drifter remained within a patch. Trajectories of the patches were assessed in relation to physical data (salinity, temperature, winds, and river discharge) and compared to trajectories predicted by a 2-dimensional mathematical model. Analysis of CTD data indicated that 3 patches were initially tagged in the coastal plume emanating from Delaware Bay, while 5 patches were initially tagged in shelf water adjacent to the plume. Patches occurring in plume waters traveled farther down-shelf (southward) than those in adjacent shelf waters. Winds modified this along-shelf transport. Specifically, upwelling-favorable (northward) winds mixed larvae offshore and out of the coastal plume. Downwelling-favorable (southward) winds drove patches of larvae across the continental shelf and toward the coast. Trajectories were simulated well by the mathematical model. As river discharge diminished in the late summer, the effect of winds (relative to buoyancy forcing) on larval transport increased. This shift in the dominant forcing mechanism was demonstrated both by the trajectories of the tagged larval patches and by predictions from the mathematical model.