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

The epipelagic copepod assemblages and vertical distribution (<200 m) of 52 species were surveyed in January (winter) and July (summer) 2007 in the central and northern region of the Gulf of California. During January, the water column was well mixed and cold (16-18°C) in the upper 100 m. During July, a pronounced seasonal thermocline (22-28°C) in the upper 50 m was present. Copepod diversity was slightly higher in summer, associated with the northward expansion of the Tropical Eastern Pacific (TEP) copepod community. During winter, large copepod species (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.