Impact of the Congo River plume and tropical upwelling on Sardinella spawning in the southeastern Atlantic

The Congo River releases nutrient-rich freshwater into the southeastern Atlantic, forming a plume that fuels regional productivity sustaining small pelagic fish such as Sardinella ( Sardinella aurita and Sardinella maderensis ). However, the impact of the plume on Sardinella reproduction is still uncertain. A survey conducted in austral summer 2014, coinciding with the Congo River secondary seasonal discharge peak and tropical upwelling, provided evidence on how riverine runoff and upwelling jointly influence Sardinella spawning. Congo River discharge was prominent along the coast, with low-salinity, high-chlorophyll surface waters spreading north- and northwestward. Spawning was observed north of the Congo River mouth, with the peak confined to the plume region, as also corroborated by generalized additive models. Wind and ocean current patterns suggested a northwest drift of ichthyoplankton trapped within the plume, with offshore loss near southern Gabon. A Lagrangian drift simulation aligned with these observation pathways. Plume dynamics were absent along the Angolan coast, where the tropical upwelling seemed the sole driver of coastal productivity. No Sardinella eggs were observed there, but larvae were distributed, aligned with the upwelling front signatures. Our findings underscore the Congo River plume as a key spawning habitat for Sardinella during austral summer. Higher spawning activity within the plume likely suggests that its physical and biogeochemical conditions are more favourable than adjacent upwelling zones. The study highlights the need to consider riverine influences alongside upwelling dynamics in understanding and managing small pelagic fish populations in tropical marine ecosystems. • Congo plume drives Sardinella spawning in austral summer; upwelling minor. • Spawning peaks align with warm, low-salinity, high-chlorophyll plume waters. • Modeling confirms northwest drift of ichthyoplankton within the Congo plume. • Congo River input is key to managing tropical small pelagic fish stocks.