Effects of the Amazon River plume in benthic distribution across the Amazon-Guianas shelf

The Amazon–Guianas continental shelves form a highly dynamic tropical margin under the influence of the Amazon and Orinoco rivers. Regional circulation and river-derived nutrient inputs influence the delivery of particulate organic carbon (POC) to the continental shelf and may play an important role in structuring benthic ecosystems. Although POC production has previously been associated with river plume dynamics, its effects on shelf benthos remain poorly understood. Here, we investigated this relationship using a time-series analysis of Amazon River discharge at the Óbidos station, 21 years of MODIS-Aqua satellite-derived POC data (2003–2023), and benthic invertebrate occurrence records from the Ocean Biodiversity Information System (OBIS). We quantified plume-driven variability in POC and examined its relationship with regional benthic biogeographic patterns across the Amazon–Guianas shelf. We found persistently elevated POC concentrations north of the Amazon mouth, consistent with plume advection by the North Brazil Current, and identified four shelf sectors with distinct 21-year mean POC regimes (Mouth: 127 ± 37.7 mg m−3; North: 318 ± 88.9 mg m−3; Guianan: 174.8 ± 42.3 mg m−3; Orinoco: 270 ± 85.1 mg m−3). OBIS records comprised 4,541 occurrences across 12 phyla and were dominated by Arthropoda, Cnidaria, Echinodermata, and Mollusca, with clear regional differences in phylum composition. Bray–Curtis/UPGMA clustering separated the Mouth region from northern sectors, indicating a distinct assemblage at the phylum level. Machine-learning importance patterns and ordination of predictor space further suggested region-specific controls, with plume-related signals such as POC and salinity minima being more relevant near the Amazon Mouth and Guiana regions, whereas hydrographic constraints including salinity variability, temperature, and depth were more influential in the North and Orinoco sectors. Together, these results indicate a strong link between river-plume carbon dispersal and broad-scale benthic biogeographic organization across the Amazon–Guianas shelf.