Seasonal succession of tropical community structure, abundance, and biomass of five zooplankton taxa in the central Mexican Pacific

The seasonal succession of species community structure, abundance and carbon biomass of five zooplankton taxa (Copepoda, Euphausiacea, Chaetognatha, Amphipoda and fish larvae) and their variability during different oceanographic conditions was investigated on the continental shelf of the central Mexican Pacific (19°N, 105°W) during a monthly time-series (1996–1998). These zooplanktonic taxa included 291 species, with maximum species richness during El Niño 1997–98, when 54 tropical oceanic species invaded the continental shelf. This typical offshore community was advected onshore by the anomalous intrusion of Subsurface Equatorial water mass which caused + 7 °C temperature anomalies. Nineteen species comprised the bulk of the abundance (91%) and biomass (87%) of the five zooplankton taxa. Multivariate statistical analysis revealed four species assemblages associated with distinct oceanographic conditions: (1) mixed water column period (upwelling; February–June, non-El Niño period) characterized by high abundance and carbon biomass of omnivorous zooplankters (copepods Subeucalanus subcrassus, Centropages furcatus, Canthocalanus pauper, Temora discaudata, and Subeucalanus subtenuis, euphausiid Euphausia distinguenda, and fish larvae Bregmaceros bathymaster); (2) stratified water column period (July–January, non-El Niño period) with low-moderate abundance and biomass of the predominant species of the first group plus the euphausiid E. lamelligera, the chaetognaths Flaccisagitta enflata and Zonosagitta bedoti, and the tropical amphipod Hyperioides sibaginis; and (3 and 4) two highly diverse assemblages associated with El Niño-stratified conditions and post El Niño (July 1997–December 1998). These last two assemblages were distinguished by a high numerical dominance of carnivores (F. enflata, Z. bedoti, and H. sibaginis) and a pronounced decline of abundances of omnivore copepods, euphausiids, and fish larvae. Both the seasonal upwelling dynamics and the interannual-scale El Niño events shaped the tropical zooplankton community structure succession of this neritic ecosystem. The local community structure modulates the function and productivity of this ecosystem, which sustains regional fisheries and ecotouristic activities. More frequent, intense or even longer El Niño events may significantly decrease zooplanktonic carbon biomass and trophic transfer through the neritic epipelagic food web, resulting in impoverishment of the shelf biota with negative economic implications.