Spatial patterns of mesozooplankton around the Iberian Peninsula

Abstract

Zooplankton community structure has traditionally been studied using taxon-based methods, which relate species abundance and diversity to environmental variability. Although size-based approaches provide a functional perspective on ecosystem structure, their application across contrasting regions and extended vertical gradients remains limited. Here, we provide the first integrated assessment of mesozooplankton taxonomic and size-based structure by investigating the geographical, vertical, and diel distribution of biomass, abundance, diversity, and normalized biomass size spectra (NBSS) down to 700 m in five zones (Balearic Islands, Alboran Sea, Cadiz, Lisbon, and Galicia) during autumn 2020. Marked abundance and biomass differences were observed around the Iberian Peninsula, with significant spatial and depth-related variability but no consistent diel variations. NBSS slopes – used as an indicator of zooplankton community structure – were consistently flatter than −1, suggesting relatively high trophic transfer efficiency. We identified a marked shift in the NBSS slopes and size-diversity at the epipelagic – upper-mesopelagic boundary layers (∼300–500 m), with the magnitude of this transition varying based on regional productivity. In low-productivity areas (Balears and Cadiz), the transition toward larger size organisms occurred at greater depths, whereas in high-productivity regions (Lisbon and Galicia), the transition occurred shallower, hosting larger mesozooplankton closer to the surface. Finally, size diversity decreased from the southwestern Mediterranean to the North Atlantic, while abundance and biomass peaked off Lisbon, the most productive region. By linking vertical size-structure transitions to productivity gradients across a major biogeographic boundary, this study underscores the value of integrating size-based metrics into regional ecosystem assessments.