Impact of North Brazil Current rings on local circulation and coral reef fish recruitment to Barbados, West Indies

Abstract

Early observations of the flow environment around the island of Barbados indicated frequent occurrence of strong current reversals associated with surface salinity fronts. Higher resolution spatial and temporal measurements of the flow regime in 1996 and 1997 provided a comprehensive view of the local surface circulation (0-100 m), revealing that external forcing by North Brazil Current (NBC) rings plays a dominant role in the near-field flow variability surrounding the island. NBC ring forcing had comparable effects on the velocity field during both years, indicating that the ring structure was retained while interacting with topography. In the present study, the interaction of NBC rings with coastal flow dynamics and the biological response of the system as measured by recruitment of coral reef fishes is examined. Our observations show that NBC rings can remain quite coherent as they pass the Tobago-Barbados ridge. Further, the flow direction and associated residence time in the vicinity of the island appear to vary depending on the orientation of the rings as they collide with the island. Concurrent biological samples revealed complex responses to the presence of rings in that during some of the events, larval fishes appeared to be rapidly advected away, resulting in a failure of larval settlement, whereas under other conditions larval retention was enhanced and was followed by a settlement pulse. Impingement by a ring did not alter the concentration of water column chlorophyll a (Chl a ), but it did influence the depth of the Chl a maximum. Simultaneous changes were observed in the vertical distribution of fish larvae. Larval fish encountering ring waters exhibited reduced growth rates and longer larval periods, both potentially reducing survival and, ultimately, recruitment success. Overall, results demonstrate that NBC rings interfere with the islandscale flow dynamics around Barbados and interject considerable variability in the local recruitment signal of coral reef fishes.