Circulation dynamics and larval transport mechanisms in the Florida Big Bend

The Florida Big Bend region in the northeastern Gulf of Mexico contains both spawning sites and nursery habitats for a variety of economically valuable marine species. One species, the gag grouper ( Mycteroperca microlepis), relies on the shelf circulation to distribute larvae from shelf-break spawning grounds to coastal seagrass nurseries each spring. Therefore, identifying the dominant circulation features and physical mechanisms that contribute to cross-shore transport during the springtime may provide valuable insight into the variation of the abundance of this reef fish. The physical mechanisms by which cross-shelf movement is possible, and the pathways by which materials may be transported onshore are examined. More specifically, variable wind stress and the conservation of potential vorticity are investigated for their role in setting the net across-shelf transport, using a very high horizontal resolution (800—900 m) numerical ocean model. The simulations demonstrate that the mean springtime shelf circulation is set by the rectification of flow during northwesterly or southeasterly directed wind stress, and significant cross-shelf flow may be generated during winds from the northwest. The springtime flow is mostly barotropic and tends to conserve potential vorticity over time scales shorter than about 12 hrs. For longer time scales, the nonconservation of potential vorticity enables movement of particles inshore. Particle advection experiments demonstrate that a primary pathway exists south of St. George Island by which particles are able to reach the nearshore environment, and that preferred release locations for particles to successfully arrive inshore coincide with known gag spawning aggregation sites. The results provide, for the first time, a description of the mechanisms by which onshore transport is possible from gag spawning sites at the shelf break to seagrass nurseries at the coast.