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

Abstract

Summary The finding that most copepod species in the central andnorthern Gulf of California do not exhibit DVM (con-centrating in the upper 75 m) is relevant in a regionaltropho-dynamics perspective as it results in similar day–night copepod availability for their epipelagic planktivor-ous predators. The absence of copepod DVM, but notother planktonic organisms, such as euphausiids, seemsto be regulated by the combination of vertical and sea-sonal environmental conditions. Copepod distributionand abundance was positively associated with tempera-ture, depth of the thermocline, chl-a, zooplanktonbiomass and concentration of dissolved oxygen. Largeconnectivity rates are known to exist along the easternside of the Gulf and relatively high rates of retentionalong the western side (Marinone, 2012; Soria et al.,2012), causing a coast-ocean-coast ecosystem that con-trasts with cross-shelf-oceanic conditions that prevail incoastal upwelling regions (Peterson et al., 1979; Peterson,1998). All factors combined seem to favor copepodspecies that live under mesotrophic conditions in theupper 50 m and are transported across poor and richtrophic habitats in relativelyshort periods within their lifespan, as suggested by recent three-dimensional models ofparticulates tracked in surface waters. Although most ofthe copepod assemblage is of tropical zoogeographic af-finity, we found shifts in winter–summer relative abun-dance (!19% of all species) in dominant species havingdifferent body sizes. However, seasonal changes incopepod assemblages do not seem to significantly modifythe day–night copepod biomass stock for predators inthe epipelagic ecosystem of the Gulf. It is important tostudychanges in sizewithin the zooplankton assemblagesto measure biomass and vital rates (molt, growth and re-production) of at least the numerically-dominant speciesto estimate secondary production to conceptually scalethe actual productivity of the Gulf of California(Go´mez-Gutie´rrez et al., 2012; Palomares-Garci´a et al.,2013). Until this study, there were no data on which tobase such an approach in this species-diverse and eu-trophic ecosystem.