Winter and summer vertical distribution of epipelagic copepods in the Gulf of California

Abstract

The epipelagic copepod assemblages and vertical distribution (Pleuromamma gracilis, Calanus pacificus, Rhincalanus nasutus and Scolecithrix danae) had their greatest weighted mean depth abundance (WMDA) in the 0–50-m layer, closely associated with the maximum concentration of chlorophyll a. During summer, small tropical species (Centropages furcatus, Clausocalanus furcatus and Canthocalanus pauper) had higher WMDA (25–50 m) below the thermocline. A canonical correspondence analysis showed that copepod vertical distribution and abundance was positively associated with vertical temperature structure during summer and with thermocline depth, percentage of oxygen saturation, zooplankton volume and concentration of chlorophyll-a during winter. We did not detect any evidence of daily vertical migration to depths of 0–200 m for 43 species; most species were concentrated in the mixed layer; seven had deep distribution 100–200 m, and two had seasonal vertical migration. This means that nektonic predators of epipelagic copepods prey on almost the same abundance of copepods in the 0–50 m range day or night and in environmentally contrasting seasons.