Seasonal Variations of the Zooplankton Community in the Western Gulf of Mexico: is there an Influence of the Warm Eddy Jumbo?

Abstract

In the Gulf of Mexico, the Loop Current sporadically sheds warm anticyclonic eddies that travel into the gulf and whose influence on the zooplankton community of the western region is not known. This research examined the zooplankton community dynamics in the western Gulf of Mexico during three seasons: July 2010 (summer), January 2011 (winter), and October-November 2012 (fall), and the possible effect of the warm eddy called Jumbo, released from the Loop Current in the middle of 2012 and that approached the western side of the gulf at the end of the year. We hypothesized shifts in the composition and/or biomass of the zooplankton fauna collected during the fall period due to the transport of organisms from elsewhere or because of a rapid response of zooplankton to warmer environmental conditions. This could result in a greater similarity of the fall season to the summer rather than to the winter. Zooplankton samples were taken onboard the oceanographic vessel Justo Sierra and a total of 82 oceanographic stations were sampled with a Bongo net; at each sampling station, temperature and salinity were measured with a CTD profiler. Both environmental and zooplankton data were treated through a Principal Coordinate Analysis (PCO) to explore their relationship. Fourteen zooplankton groups were recognized in all three sampling periods, with seasonal variations in biomass. The PCO showed that July was characterized by high-temperature values (~27°C), low chlorophyll concentration (< 1 mg/m³), the dominance of copepods, chaetognaths, and luciferids, as well as high biomass values of crustacean larvae (decapods stomatopods), signaling this season as the reproductive period. January was characterized by higher chlorophyll concentration (1-1.3 mg/m³), lower temperatures (18-22°C), and a high biomass of amphipods, ostracods, and jellyfishes; October-November registered similar environmental conditions to July, but the PCO and the associated distance among centroids indicated that the zooplankton community structure was more similar to January. The occurrence of the 14 groups in all the seasons, reveals no shifts in the composition in the study area. Besides, the similarity of the fall to the winter in the zooplankton structure discarded the hypothesis. Our results suggest that the zooplankton community follows its natural seasonal dynamics and shows high resilience to eventual hydrographic phenomena, such as anticyclonic eddies.