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

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, dispersal-enhancing, 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 assemblages 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 (R, environmental data; L, species incidence data; Q, species trait matrix) analysis revealed the predominance of sedentary 'Scolecida' and 'Sabellida'-like 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, resulted 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.