Vertical assemblage of the holoplanktonic mollusks (Pteropoda and Pterotracheoidea: Carinaiidae, Pterotracheidae) in the Campeche Canyon, southern Gulf of Mexico, during a “Nortes” season

This study examines the vertical assemblages of the holoplanktonic mollusks (Pteropoda and Pterotracheoidea) in the Campeche Canyon, southern Gulf of Mexico, during a "Nortes" season (February 21-28, 2011) and explores their relationship with the hydrography and the geostrophic circulation pattern. High-resolution hydrographic data were acquired during a multidisciplinary research cruise of 48 hydrographic stations. Zooplankton samples were collected at 24 stations from four depths (10, 50, 100, and 200 m) using a multiple open/closed net system. The results revealed a deep thermocline at a depth of 90 m and a circulation pattern dominated by cyclonic and anticyclonic eddies that induced cold and warm cores. Thirty-three Pteropoda and three Pterotracheoidea species were identified, with the highest richness at a depth of 100 m (just below the thermocline). The highest organism densities were observed at a depth of 10 m. The diversity index (<i>H</i>') showed variations depending on the sampling depth, with the highest values (1.9 bits ind^-1) at 100 and 200 m, while at 10 m depth the lowest values (1.45 bits ind^-1) were observed. Multivariate analyses showed that dissolved oxygen, and temperature were the main environmental factors determining organism density.