Abstract
Biochemical conditions and taxonomic composition of size-fractioned mesozooplankton were studied after a cruise conducted in September 2015 between the Chilean coast (70°W) and Easter Island (110°W) within the central south Pacific gyre. Taxonomy was assessed with an automated method based on image analysis and biochemical conditions assessed by analyses of C and N contents and stable isotope composition. Based on surface Chlorophyll-a levels, four regions were distinguished across the zonal gradient: eutrophic (Chilean upwelling zone), mesotrophic (Coastal Transition Zone), oligotrophic (open ocean water) and ultraoligotrophic (central south Pacific gyre). The zones had marked differences in temperature, oxygen, salinity and Chlorophyll-a, and they also exhibited significant differences in zooplankton composition, C/N ratios and13C and 15Nfor all size fractions of zooplankton. Variability in the sources of C and N, linked to biogeochemical processes, such as new production and denitrification in the upwelling zone, potential diazotrophy, highly regenerated C and N and extreme oligotrophy (N-deficiency) in oceanic areas, are suggested as the key drivers of these differences. Our findings also suggest a strong coupling between taxonomic and size zooplankton-diversity and the sources of nutrients that fuel phytoplankton, the major food source for zooplankton. Although multiple factors and processes can modulate C and N and their isotopes composition of zooplankton biomass, our study shows that changes in community structure are linked to different biogeochemical regions across the zonal gradient, providing the basis for ecological zonation associated with nutrient utilization at lower trophic levels.
Highlights
A substantial amount of research has been conducted on the functioning and structure of the eastern boundary upwelling system (EBUS) off Chile (e.g., Cury et al, 2000; Daneri et al, 2000; Hormazabal et al, 2001), including the mesotrophic coastal transition zone (CTZ) (Morales et al, 1999, 2010)
In spite of the strong variability in oceanographic conditions observed over the zonal gradient of the eastern South Pacific, only two biogeochemical provinces have been suggested for the whole area which are characterized by globally unique and distinguishable biogeochemical processes, corresponding to the provinces of the Humboldt current coast and South Pacific gyre (Oliver and Irwin, 2008; Longhurst, 2010; Reygondeau et al, 2013)
There were five stations sampled at nighttime conditions Once onboard, samples were collected in buckets and depending on the amount of zooplankton they were split in two fractions; one fraction was immediately preserved in 10% formalin and the second fraction was frozen at −20◦C for C and N measurements
Summary
The eastern South Pacific region is characterized by a strong coastal-offshore oceanographic gradient from the highly productive (eutrophic) coastal upwelling zone toward the oligotrophic and even ultraoligotrophic region in the central south Pacific gyre (Raimbault and Garcia, 2008; Von Dassow and Collado-Fabbri, 2014; Moutin et al, 2017). In spite of the strong variability in oceanographic conditions observed over the zonal gradient of the eastern South Pacific, only two biogeochemical provinces have been suggested for the whole area which are characterized by globally unique and distinguishable biogeochemical processes, corresponding to the provinces of the Humboldt current coast and South Pacific gyre (Oliver and Irwin, 2008; Longhurst, 2010; Reygondeau et al, 2013) Within these provinces it is possible to distinguish ecoregions, being defined on the basis of species composition and ecological dynamics. The interaction between the physical/biogeochemical gradient and the zooplankton community structure is unknown for this large region covering the area from the coastal upwelling zone up to the central Pacific gyre Understanding such interaction can provide insights on the ecological and evolutionary processes controlling diversity and adaptability of plankton to a changing ocean, both in space and time. We aimed at elucidating how changing oceanographic conditions across the coastal-offshore gradient and different biogeochemistry processes determining the sources of C and N affecting the food resource of zooplankton, can influence their biochemical conditions and community structure
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