Zooplankton Taxonomic and Trophic Community Structure Across Biogeochemical Regions in the Eastern South Pacific

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 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.

Environmental Gradients and Spatial Patterns of Calanoid Copepods in the Southeast Pacific

“…Figure 1 illustrates our study area and the sites from which zooplankton samples have been obtained and used in this study and information of the samples sources and data are summarized in Supplementary Table S1. González et al (2019) distinguished four ecological zones in this region, as based on the surface chlorophyll-a (Chla-a) concentration, from the highly productive upwelling area toward the central south Pacific gyre. This study found significant changes in zooplankton community structure among these zones, defined as: the CUP-Z, the coastal transition zone (CTZ), the oligotrophic zone (OLIGO-Z) and the ultra-oligotrophic zone (U-OLIGO-Z) which are illustrated in Figure 1.…”

“…Biodiversity and species distribution of zooplankton are known to be affected by changes in oceanographic conditions, such as temperature, oxygenation, salinity and stratification, as it has been demonstrated in other regions of the world (Beaugrand et al, 2002;Beaugrand, 2003;Richardson and Schoeman, 2004;Peterson et al, 2006;Aronés et al, 2009;Gewin, 2010;Seibel, 2011), revealing the strong dependence of these organisms on oceanographic variables due to their limited migration capacity and so reflecting the strong effects of hydrographic patterns on their distribution and that of their prey (McClain and Barry, 2010). In this context, the Southeast Pacific (SEP) shows a great heterogeneity in hydrographic conditions from coastal to open sea gradient evidenced by a progressive increase of depth of the mixing layer, dissolved oxygen, temperature and surface salinity (Palma and Silva, 2006;González et al, 2019). Also, within the same gradient it is possible to observe drastic changes in biological productivity, which is high in the coastal zone (Humboldt Current) and extremely low in the ultraoligotrophic central gyre (Bonnet et al, 2008;Morel et al, 2010;Kletou and Hall-Spencer, 2012;Mann and Lazier, 2013).…”

“…For instance, along coastal and oceanic areas of the SEP five ecoregions have been suggested: Central Peru, Humboldtian, Central Chile, Araucanian, Juan Fernandez, and Desventuradas (Spalding et al, 2007). Although, a recent work based on ecological and biogeochemical division of zooplankton, proposed the existence of four regions over the zonal gradient, predefined in terms of surface chlorophyll-a as a proxy for productivity (González et al, 2019). Same work suggested that changes in the community structure of zooplankton in the SEP could be linked to the factors or processes that modulate the availability of nutrients at lower trophic levels, such as N-limitation and N-fixation (González et al, 2019).…”

“…Although, a recent work based on ecological and biogeochemical division of zooplankton, proposed the existence of four regions over the zonal gradient, predefined in terms of surface chlorophyll-a as a proxy for productivity (González et al, 2019). Same work suggested that changes in the community structure of zooplankton in the SEP could be linked to the factors or processes that modulate the availability of nutrients at lower trophic levels, such as N-limitation and N-fixation (González et al, 2019). Nevertheless, it has now become clear that a changing temperature of the ocean is the major driver for altered diversity and distribution patterns of marine organisms (Antão et al, 2020).…”

Comparison of invertebrate diversity in lake waters and their resting eggs in sediments, as revealed by high-throughput sequencing (HTS)

“…Zooplankton and other aquatic invertebrates play an essential role in the ecology of aquatic ecosystems (González et al, 2018). Some invertebrates are sensitive to changes in water quality and therefore are indicators of water quality (Berraho et al, 2019).…”