The lateral transport of zooplankton explains trophic and taxonomic similarities over the zonal gradient of central Chile

The highly productive upwelling zone in the Southeast Pacific provides inputs of C and N to the oceanic deep-water system via lateral transport, although the quality of this organic matter, the organisms being transported, and the controlling mechanisms are unclear. Here, we assessed whether the taxonomic and trophic structure of zooplankton over more oceanic offshore waters differ from that of zooplankton located in the upwelling zone, along with the characterization of the oceanographic processes and variables associated with the lateral transport. For this, epipelagic, mesopelagic, and bathypelagic zooplankton along with oceanographic variables and particulate organic matter (POM) were analyzed at the upwelling zone and adjacent oceanic stations off Central Chile (27–33°S), some of which were located over the Atacama trench, during September 2016. The community structure of size-fractioned zooplankton (<200 μm, 200–500 μm, 500–1000 μm, 1000–2000 μm, and >2000 μm size classes) was assessed by an automated method, along with the analysis of the isotopic niche computed from their δ¹⁵N and δ¹³C. The isotope signatures of POM indicated that diatoms contributed mostly to production of new organic C, being the main food source for small-sized zooplankton in coastal waters. The zooplankton community structure differed between the upwelling and oceanic areas, but their isotopic signature showed a large overlap (78%). Satellite-derived geostrophic flow and the depth of the mixed layer appeared as the main factors explaining the homogeneity in the isotopic and biogeochemical signatures of zooplankton between the upwelling zone and offshore waters. Cross-shelf advection is thus suggested as a key process promoting zooplankton export to the deep-water ecosystem, including the ultra-deep Atacama Trench.