Mesoscale Circulation and the Surface Distribution of Copepods near the South Florida Keys

Near-surface zooplankton samples were collected during an interdisciplinary study of biological responses to oceanographic circulation south of Looe Reef in the southern Straits of Florida (SSF) in late May 1994. Surface currents in the sampling region were monitored during the study with the University of Miami's high frequency (HF) Ocean Surface Current Radar (OSCR). HF-radar was highly effective in resolving submesoscale (<30 km) circulation patterns that would have been difficult to discern using conventional current meters and shipboard surveys, particularly in this coastal region where local circulation patterns undergo rapid temporal transitions. The present study focused on the surface abundance and distribution of copepod nauplii and copepodites. These groups generally remain in the near-surface layer, tend to behave as passive drifting particles and in the case of nauplii, are good indicators of recent egg production. Copepod nauplii and juveniles were found to be most abundant in cyclonic submesoscale eddies (SMEs; 10-12 km) that traversed the frontal-mixing zone between the Florida Current and nearshore coastal water during the field study. The region of greatest zooplankton abundance was also associated with elevated pigment fluorescence. These data suggest that the SMEs in the SSF may enhance biological productivity, probably through a combination of accumulation and retention of zooplankton and upwelling of nutrients. General distribution patterns of numerically dominant copepod taxa were determined and linked with water of Florida Current origin or nearshore coastal water. Frontal mixing zones in coastal regions have often been associated with enhanced secondary production resulting from convergence and the aggregation of phytoplankton and zooplankton (e.g., Floodgate et al., 1981; Denman and Powell, 1984; Mackas et al., 1985; Yamamoto and Nishizawa, 1986; Franks, 1992; Govoni and Grimes, 1992; Bjorkstedt and Roughgarden, 1997). Previous studies have revealed increased abundance of copepod nauplii in areas of frontal mixing associated with upwelling off the coasts of Peru (Boyd and Smith, 1983), Somalia (Smith, 1982; 1992), Oregon (Peterson et al., 1979) and California (Smith et al., 1986; Smith and Lane, 1991). Eddy circulation associated with frontal systems has also been shown to influence phytoplankton distribution and growth (Pingree, 1978; Pingree et al., 1979; Hayward and Mantyla, 1990; Longhurst, 1993). Similarly, reproductive success of some zooplankton species may depend on transport mechanisms that entrain adults and late stage juveniles in current systems or eddies that maintain them in, or periodically return them to, favorable food environments. For example, onshore transport through subsurface flow or eddy circulation in coastal upwelling regions has been hypothesized to play a key role in the reproductive success of Calanus marshallae off the Oregon coast (Peterson et al., 1979) and of Eucalanus californicus off the California coast (Smith and Lane, 1991). Similar mechanisms may be responsible for the success of Calanoides carinatus in the upwelling regions near the coasts of Somalia and Oman (Smith, 1992; 1995). Meroplankton and ichthyoplankton have also been hypothesized to utilize nearshore circulation to enhance recruitment suc-