Abstract
Seasonal variability of the community structure of copepods in the Eastern tropical Pacific off central Mexico was studied during three distinct hydrodynamic periods in 2010 using statistical and multivariate analyses. The survey period included the second half of the 2009-2010 El Niño (January), the neutral transition period (May-June), and the first half of the 2010-2011 La Niña (October). Seventy-eight copepod species were identified; richness ranged from 11 to 47 species per station, with seasonal averages from 25 species in May to 35 species in January. Cluster analysis indicated that there were four principal groups present across the surveyed periods, defined by January (El Niño), October (La Niña), May offshore stations, and May upwelling stations (cyclonic eddy and coastal stations). There were no significant differences in abundance, but the January (El Niño) cluster was most diverse with 32 species, May offshore and October (La Niña) clusters each had 25 species, and the May upwelling was the least diverse cluster with 18 species. Mesoscale processes were strongest during May, which was the only period with a significant inshore-offshore gradient of species richness and diversity. Canonical correspondence analysis (CCA) revealed that variability was primarily driven by subsurface (75-200 m) ammonium, and surface (0-50 m) temperature, nitrates+nitrites, salinity and phosphorus. Copepodites and adults of the primarily herbivorous Eucalanidae dominated the stations of the upwelling cluster, while copepodites and adults of the carnivorous Euchaetidae dominated the January (El Niño) station cluster. The higher Chl a levels during the less productive (reduced upwelling) El Niño period were probably due to reduced grazing activities and increased ammonium availability through increased zooplankton metabolism. The horizontal distribution of copepods in the Eastern Tropical Pacific off Mexico appears to be principally defined by mesoscale eddy processes (offshore) and upwelling (coastal). These mesoscale processes were affected by El Niño - La Niña transitions, which subsequently disrupted the inshore-offshore gradient and in the case of El Niño likely caused reductions in copepod abundance across the entire region which persisted for the entire study period, and possibly longer.
Highlights
Copepods are an important planktonic group, and account for most of the total biomass and species diversity in pelagic marine ecosystems
In coastal and oceanic regions of the Eastern Tropical Pacific off Mexico (ETPM), they have been reported to comprise around 66-76 % of total zooplankton abundance (Franco-Gordo et al, 2015; PelayoMartínez et al, 2017)
These dynamic mesoscale structures (DMS) are usually caused by gravity, density gradients and wind forcing, and provoke a resuspension or sinking of particles and organisms, which generates patches of organisms according to hydrological conditions, reflected in the abundance of distinct zooplankton groups (Pelayo-Martínez et al, 2017)
Summary
Copepods are an important planktonic group, and account for most of the total biomass and species diversity in pelagic marine ecosystems. Distribution, feeding and reproduction of zooplankton are influenced by abiotic factors like bathymetry, thermocline depth, temperature, primary productivity and food availability These factors are in turn modulated by dynamic structures such as eddies (cyclonic and anticyclonic), upwelling events and filaments, which due to their 1-100 km size are referred to as dynamic mesoscale structures (DMS). Copepods form important prey for the most common fish larvae found in the ETP off Mexico (ETPM), Bregmaceros bathymaster (Siordia-Cermeño, Sánchez-Velasco, Sánchez-Ramirez, & FrancoGordo, 2006; Davies et al, 2015) This permits the establishment of complex food webs with a seasonal component and ecological importance for a region considered to be oligotrophic, but which supports a variety of artisanal fisheries (Rojo-Vázquez et al, 2008). The goal of this study was to provide the first analysis of seasonal changes in the copepod community structure of the ETPM through the El Niño-La Niña transition of 2010, and to examine the relationship between the distribution of copepods in the ETPM and environmental variables
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