Dispersion and recruitment of crab larvae in the chesapeake bay plume: Physical and biological controls

As part of the Microbial Exchanges and Coupling in Coastal Atlantic Systems (MECCAS) Project, crab larvae were collected in the shelf waters off Chesa peake Bay in June and August 1985 and April 1986. We conducted hydrographic (temperature, salinity, nutrients) and biological (chlorophyll, copepods) mapping in conjunction viith Eulerian and Lagrangian time studies of the ve rtical distribution of crab larvae in the Chesapeake Bay plume. These abund ance estimates are used with current meter records and drifter trajectories to infer mechanisms of larval crab dispersion to the shelf waters and recr uitment back into Chesapeake Bay. The highest numbers of crab larvae were u sually associated with the Chesapeake Bay plume, suggesting that it was the dominant source of crab larvae to shelf waters. Patches of crab larvae als o were found in the higher salinity shelf waters,:and possibly were remnant s of previous plume discharge events. The distribution of crab larvae in th e shelf waters changed on 1-2 d time scales as a consequence of both variat ions in the discharge rate of the Chesapeake Bay plume and local wind-drive n currents. Downwelling;favorable winds (NW) intensified the coastal jet an d confined the plume and crab larvae along the coast. In April during a dow nwelling event (when northwesterly winds predominated), crab zoeae were tra nsported southward along the coast at speeds that at times exceeded 168 km d(-1). During June and August the upwelling-favorable Hinds (S, SW) opposed the anticyclonic turn of the plume and, via Ekman circulation, forced the plume and crab larvae to spread seaward. Plume velocities during these cond itions generally were less,than 48 km d(-1). The recruitment of crab larvae to Chesapeake Bay is facilitated in late summer by the dominance of southe rly winds, which can reverse the southward flow of shelf waters. Periodic d ownwelling-favorable winds can result in surface waters and crab larvae mov ing toward the entrance of Chesapeake Bay. Approximately 27% of the larval crabs spend at least part of the day in bottom waters, which have a residua l drift toward the bay mouth. There appears to be a variety of physical tra nsport mechanisms that can enhance the recruitment of crab larvae into Ches apeake Bay.