Horizontal distribution of mesozooplankton biomass and size spectra along the Amazon River plume continuum during the mesoscale North Brazil current retroflection

Abstract The Amazon River plume (ARP) and the North Brazil Current Retroflection (NBCR) influence the complex dynamics of the Western Tropical Atlantic (WTA), increasing biological productivity. This study estimated the spatial mesozooplankton distribution, community composition, horizontal carbon transport and normalized biovolume size spectra (NBSS) in different WTA current sectors, focusing on the ARP influence. To this end, oblique zooplankton bongo samples (300 μm) were obtained in October 2012 and analyzed using a ZooScan. Environmental conditions and current sectors were defined by in situ Conductivity, Temperature, Depth (CTD) sensor and Acoustic Doppler Current Profiler (ADCP) measurements. Zooplankton abundance and biovolume were highest in ARP-influenced areas, particularly near the shelf break and within the NBCR region. The ARP influence led to steeper NBSS slopes (more small-sized and fewer large-sized organisms). Horizontal carbon transport among all sectors was highest in the NBCR (93.54 ± 173.57 ktC/d), 22 times larger than the values recorded at the Amazonian continental shelf, indicating substantial transport from coastal to oceanic regions. No differences in trophic efficiency were observed across the WTA, attributed to the ARP impact on large organism abundance and community composition. This study highlights the crucial role of the ARP-NBCR system in seasonal zooplankton productivity enhancement and substantial coastal–oceanic carbon transport in the WTA.