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 and δ 13 C and δ 15 N for 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.

Global environmental changes are challenging the structure and functioning of ecosystems. However, a mechanistic understanding of how global environmental changes will affect ecosystems is still lacking. The complex and interacting biological and physical processes spanning vast temporal and spatial scales that constitute an ecosystem make this a formidable problem. A unifying framework based on ecological theory, that considers fundamental and realized niches, combined with metabolic, evolutionary, and climate change studies, is needed to provide the mechanistic understanding required to evaluate and forecast the future of marine communities, ecosystems, and their services. The Future of Marine Ecosystems The ocean absorbs most (93%) of the heat generated by greenhouse gas emissions, resulting in a predicted increase in the sea surface temperature of 1-10 C over the next 100 years [1]. The ocean also absorbs CO 2 released to the atmosphere from anthropogenic sources (currently 1/3 of this CO 2), resulting in a profound change in the carbonate chemistry and predicted increased acidity of seawater [1] to 100-150% above pre-industrial era values [1]. In addition to ocean warming and acidification, anthropogenic stressors are decreasing the concentration of dissolved oxygen and consequently expanding oxygen minimum zones [2] as well as potentially modifying large-scale oceanic circulation patterns [3]. These environmental changes might also impact fundamental community-structuring processes (i.e., selection, dispersal, drift, and speciation) [4], changing the relative importance of ecological processes for structuring of communities. Collectively, these changes will alter the structure and functioning of marine organisms and ecosystems and, consequently, the biogeochemical cycles of the ocean [5-8]. Generally recognized predictions regarding climate-induced changes on the composition and distribution of the marine biota include shifts in the species distribution from lower to higher latitudes, shifts from near-surface to deeper waters, shifts in annual phenology, declines in calcifying species, and increases in the abundance of warm-water species [1,9]. However, most models of the response of biological communities to climate change assume a fixed, genetically determined environmental niche for each species, and the migration of intact (i.e., nonadapting or nonevolving) populations, so that their distribution on our future planet is basically governed by the environmental conditions [10-12]. Yet, local populations may evolve, acclimate, and adapt to environmental changes. In fact, local adaptation is a recognized phenomenon in ecological studies on terrestrial systems [13,14]. In contrast to terrestrial systems where most (z96%) of the living biomass are plants, most of the biomass of the ocean (z70%) is microbial [15]. Since microbes have short generation times and large population sizes, it is possible that these engines of the Earth's biogeochemical cycles might be particularly capable of adapting to global envir...

Eurythenes S.I. Smith in Scudder, 1882 (Crustacea: Amphipoda) are prevalent scavengers of the benthopelagic community from bathyal to hadal depths. While a well-studied genus, molecular systematic studies have uncovered cryptic speciation and multiple undescribed lineages. Here, we apply an integrative taxonomic approach and describe the tenth species, Eurythenes atacamensis sp. nov., based on specimens from the 2018 Atacamex and RV Sonne SO261 Expeditions to the southern sector of the Peru-Chile Trench, the Atacama Trench (24–⁠21°S). Eurythenes atacamensis sp. nov. is a large species, max. observed length 83.2 mm, possesses diagnostic features, including a short gnathopod 1 palm and a chelate gnathopod 2 palm, and a distinct genetic lineage based on a 16S rRNA and COI phylogeny. This species is a dominant bait-attending fauna with an extensive bathymetric range, spanning from 4974 to 8081 m. The RV Sonne SO261 specimens were recovered along a 10-station transect from abyssal to hadal depths and further examined for demographic and bathymetric-related patterns. Ontogenetic vertical stratification was evident across the trench axis, with only juveniles present at abyssal depths (4974–6025 m). Total length-depth analysis revealed that the size of females was unrelated to depth, whereas juveniles followed a sigmoidal relationship with a step-up in size at depths >7200 m. Thus, these bathymetric trends suggest that juveniles and females employ differing ecological strategies in subduction trench environments. This study highlights that even dominant and ecologically important species are still being discovered within the abyssal and hadal environments. Continued systematic expeditions will lead to an improved understanding of the eco-evolutionary drivers of speciation in the world’s largest ecosystem.