Dynamic Transitions in Fish Community Composition From the Amazon River Plume to Offshore Reefs

Abstract

Aim To investigate the spatial and temporal dynamics of fish assemblages across the Northern Brazilian Shelf, a vast and complex ecosystem influenced by the Amazon River plume, we aimed to identify fish assemblage types, understand transitions between them from the river mouth to offshore reefs, and examine seasonal changes related to plume dynamics. Location Northern Brazilian Shelf (Western Atlantic Ocean), from the Amazon River mouth to the Great Amazon Reef System (GARS). Taxon Actinopterygii and Elasmobranchii. Methods We applied a presence‐absence Latent Dirichlet Allocation (LDA) model to fish occurrence records. This Bayesian mixed‐membership probabilistic approach inferred latent assemblages by analysing co‐occurring species and quantifying community types at the same site in varying proportions, suitable for characterising transitions. We then mapped these assemblages' proportional contributions and analysed their spatial distribution relative to seasonal changes in river discharge and freshwater plume influence. Results Four fish assemblages were identified with a clear coast‐to‐offshore zonation: (1) freshwater‐tolerant plume assemblage near the river mouth; (2) an estuarine–mud–plain assemblage on the inner shelf; (3) a mixed‐sand corridor assemblage in mid‐shelf; and (4) a reef‐associated assemblage on outer‐shelf GARS. The first three assemblages showed a seasonal reorganisation. During the wet season, strong plume influence led to spatially segregated assemblages. Conversely, the dry season's reduced plume resulted in increased community overlap, centred on the mid‐shelf assemblage. The reef assemblage (4) remained spatially stable across seasons. Main Conclusions The Amazon River plume's seasonal dynamics primarily drive fish community structure and connectivity across the Northern Brazilian Shelf. This study uncovers how assemblages transition from being shaped by environmental filtering (exhibiting strong zonation during peak plume) to a more overlapping distribution as plume influence weakens. These findings underscore the need for dynamic, seasonally adjusted management to conserve biodiversity and ecosystem function in this significant, and increasingly threatened, estuarine–marine transition zone.

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