Siphonophores (Cnidaria) and summer mesoscale features in the Gulf of Mexico

The species composition and numerical abundance of siphonophores were studied in the southern Gulf of Mexico from epipelagic zooplankton samples collected during July, 1988. Thirty-one species of Siphonophora were recorded at 97 stations, with Muggiaea kochi, Diphyes dispar, Bassia bassensis, and Eudoxoides spiralis the most abundant overall. The presence of two warm-core anticyclonic eddies, one cold-core cyclonic eddy, and upwelling on the Yucatan shelf and in the southern Bay of Campeche were expected to influence patterns of siphonophore distribution. A relatively low siphonophore abundance was anticipated to be found within the oligotrophic anticyclonic eddies, whereas high density was expected within the local divergence created by the cyclonic eddy, and in the upwelling areas. Differences in overall density and in species composition of siphonophores were recorded between the two anticyclones, which probably reflect the ages of the two warm-core eddies. Nighttime densities of siphonophores were similar throughout July 1988, but daytime densities were higher within the cyclone and within the younger of the two anticyclones than those recorded in the other areas or mesoscale features. Siphonophores were absent or scarce in the area of main upwelling off northeastern Yucatan where cold waters reach the surface. Nevertheless, cluster analysis using the Bray-Curtis Index yielded just three distinct assemblages, which were correlated not with the mesoscale circulation regime but rather with distance offshore.