Seasonal upwelling and depth-driven gradients foster functional overdispersion in Southwestern Atlantic annelid assemblages

Samuel Delgado Mendes, Paulo Cesar de Paiva, Rodolfo Leandro Nascimento 

Marine communities on continental shelves form through a combination of environmental filtering,
biotic interaction, and dispersal-based processes. These shelves present depth-related environmental
gradients from nearshore and seasonal upwelling systems, which periodically supply cold, dispersalenhancing,
nutrient-rich waters, providing an ideal setting to explore spatiotemporal trait-based
assembly patterns and underlying mechanisms. Although eastern boundary upwelling ecosystems are
well-studied, the role of upwelling in southwestern Atlantic benthic assembly remains less
understood, especially from a functional perspective. Here, we integrated the morphological, size,
reproductive, and behavioral traits of annelids with environmental variables. We found that deeper,
low-dynamic, coarser, poorly sorted sediments with moderate nutrient amounts and quality on the
outer shelf maintained overdispersed annelid assemblages. Nevertheless, particularly in the Austral
summer, cold bottom water from South Atlantic Central Water (SACW) on the northern São Paulo
inner shelf promoted functional overdispersion, a pattern generally assumed to occur over
evolutionary timescales. Immediately after SACW retraction, we observed trait clustering, typical of
environmental filtering likely by intensifying hydrodynamics and lower-quality organic inputs
expected for the area. RLQ analysis revealed the predominance of sedentarian Scolecida and
Sabellida traits on the inner shelf, whereas outer shelf traits were broader but dominated by errant,
larger, mobile carnivorous annelids. However, high environmental heterogeneity, likely modulated by
spatial contingency results in random assemblages on the inner shelf overall. Our study highlights
how long-established spatial gradients and short-term events can temporarily alter community
assembly processes, underscoring the need for more nuanced temporal and functional approaches to
marine biodiversity assessments and conservation strategies in upwelling-impacted shelf systems.