| Summary: | We investigated the biomass concentration and natural abundance of nitrogen and carbon stable isotopes (δ15N and δ13C) of size-fractionated zooplankton collected on two cruises to the Amazon River Plume (ARP) region. Mean animal biomass concentration was 3.34 mg/m2 during the day and 2.68 mg/m2 on Cruise EN614. The mean biomass on Cruise EN640 was 7.22 mg/m2 during the day and 9.60 mg/m2 at night. . Sampling stations were classified into habitat types based on biologically relevant physical and chemical parameters: the young plume core (YPC), old plume core (OPC), outer plume margin (OPM), western plume margin (WPM), and oceanic seawater (OSW). In general, zooplankton δ15N varied markedly and significantly among habitats for both cruises, with generally higher values in the YPC, OPC, WPM, and OSW habitats relative to the OPM habitat. Zooplankton δ15N values reflect the sources of nitrogen supporting biological production, and trophic processing within the food web. Suspended particles collected from these habitats showed lower δ15N values than the zooplankton, but we did not find a consistent increase in animal δ15N with size. Vertical migration led to diel shifts in zooplankton δ15N at the surface, with generally higher δ15N values at night than during the day during both cruises. Zooplankton δ13C values varied less than their δ15N, with no significant differences among habitats or with animal size. M.S.
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