Functional and Trophic Structuring of Micronektonic Fishes in the Northeast Atlantic.

In pelagic ecosystems, sharp vertical gradients in resources and environmental conditions structure species adaptations, interactions and community assembly. Yet, the ecological mechanisms underlying these patterns remain poorly resolved. To investigate the functional and trophic structure of micronektonic fishes, specimens were collected across the epipelagic (0-200 m) and mesopelagic layers (200-1300 m) using a midwater trawl. A functional analysis of 85 species, characterized by 10 categorical traits, was conducted to define functional groups, identify key ecological trade-offs among them and characterize the functional structure across depth strata. Stable carbon and nitrogen isotope values of fish tissues provided complementary insights into trophic structure, linking functional diversity with patterns of resource partitioning along the vertical gradient. We identified four functional groups whose trade-offs reflect metabolic adaptations primarily shaped by diet. These adaptations enable species to cope with pressure and light gradients while enhancing resource acquisition and predator avoidance, particularly in upper layers. Communities showed a clear vertical transition from migratory, surface-linked zooplanktivores in upper layers to resident, deeper-dwelling assemblages with more diverse feeding guilds associated with larger body sizes and increased reliance on alternative carbon pathways. Consistent with ecological competition theory, deeper layers exhibited high functional and taxonomic diversity under food limitation, whereas food-rich upper layers showed lower functional diversity and greater ecological similarity. Overall, vertical partitioning and functional specialization jointly structure mesopelagic communities, shaping trophic pathways, resilience and contributions to carbon cycling in pelagic ecosystems.