Beyond taxonomy: Functional diversity reveals Hidden impacts of urban wastewater on marine-coastal zooplankton

Zooplankton in tropical marine-coastal ecosystems are increasingly threatened by untreated wastewater discharges that degrade water quality and alter community structure. This study assessed these impacts in the coastal zone of Santa Marta (Colombia), comparing disturbed sectors influenced by the Manzanares River and a submarine outfall with undisturbed sectors in the Tayrona National Natural Park (Concha Bay, Neguanje Bay, and Isla Aguja) across dry and rainy seasons (March 2023-February 2024). Bimonthly sampling was conducted to characterize physicochemical parameters and their relationships with mesozooplankton taxonomic diversity and functional diversity, based on traits of body size, trophic group, feeding and spawning strategy. Untreated discharges disrupted natural physicochemical patterns, driving eutrophication marked by elevated turbidity, organic enrichment, and inorganic nutrients. It was observed that human disturbance canceled the natural seasonal pattern of diversity variation and promoted environmental filters associated with eutrophication of the system and limited the colonization of specialized and less abundant species of zooplankton communities. However, taxonomic diversity indices showed limited responses and functional metrics were more sensitive: relative functional richness declined significantly in disturbed sectors during the dry season, and functional distance revealed selective shifts in trait composition across disturbance states and seasons. Disturbances favored small-bodied, omnivorous, detritivorous, and bacterivorous taxa with ambush feeding and egg-sac reproduction, reflecting environmental filtering that homogenizes functional space and reduces resilience. In contrast, functional stability in the protected sectors highlights their role as essential reservoirs for preserving mesozooplankton integrity and providing baseline conditions for impact assessment. Our findings demonstrate that integrating taxonomic and functional approaches improves the detection of early ecological impacts and underscores the need for effective wastewater treatment to sustain biodiversity and ecosystem services in tropical coastal systems.