Euphausiids (Crustacea, Euphausiacea) and summer mesoscale features in the Gulf of Mexico

Seventeen euphausiid species were identified from 97 zooplankton samples collected in the southern part of the Gulf of Mexico during July 1988. About 75% of adult individuals belonged to three species: Stylocheiron carinatum (28.3%), S. suhmi (25.5%) and Euphausia tenera (20.1%). Seaward of the continental shelf, between 61 and 443 adults 1000 m⁻³ of water filtered were collected in the upper 200 m at night, compared with generally less than 20–94 adults 1000m⁻³ during daytime. Calyptopis larvae occurred in 78 tows, at mean density of 1346 1000 m⁻³. Cluster analysis using Bray-Curtis ordination indicated there were day-night and neritic-oceanic differences in the euphausiid community. A separate ordination of deepwater night collections indicated there was no difference between the euphausiid numerical abundance at stations in the Bay of Campeche (area of divergence), compared with those made offshore within Lazy Eddy, a warm-core eddy (area of convergence) which had been shed from the Loop Current the previous Fall 1987. However, although euphausiid species richness was similar in both areas, the two most common species of Stylocheiron dominated the numerical abundance in Lazy Eddy, whereas species of Euphausia were co-dominant with those of Stylocheiron in the Bay of Campeche. These two genera were also co-dominant in the leading edge of a second warm-core eddy (Murphy) that had separated from the Loop Current in April 1988.