Fine scale zooplankton distribution across the North Balearic Front during late spring

Abstract. Observations, models and theory have suggested that ocean fronts are ecological hotspots, generally associated with higher diversity and biomass across many trophic levels. Nutrient injections are often associated with higher chlorophyll concentrations at fronts, but the response of the zooplankton community is largely unknown. The present study investigates mesozooplankton stocks and composition during late spring, northeast of Menorca along two north-south transects that crossed the North Balearic Front (NBF) separating central water of the Northwestern Mediterranean Sea (NWMS) gyre from peripheral waters originating from the Algerian basin. During the BioSWOT-Med campaign, samples were collected using vertical triple net tows at three depths (100 m, 200 m, 400 m) with 200 μm and 500 μm mesh nets, processed with ZooScan, and the organisms assigned to eight taxonomic groups. Zooplankton distributions were analysed for the surface (0–100 m), intermediate (100–200 m), and deeper (200–400 m) layers. The results showed no significant biomass increase at the front across all vertical layers. The NBF seems to act more like a boundary between communities rather than a pronounced area of active or passive zooplankton accumulation. Analysis of stratified vertical distributions of zooplankton highlighted distinct taxonomic compositions in the surface, intermediate, and deeper layers, and a progressive homogenization of community structure with depth, reflecting a weaker impact of hydrological processes on deeper communities. The front’s clearest impact was within the upper 100 meters, where the taxonomic composition showed differences between the front and the adjacent water masses, with a decrease in Eu-malacostraca and Foraminifera, while Cnidaria increased sharply. In the 100–200 m layer, the front also influenced community composition, although to a lesser extent, with a marked increase in Foraminifera and a strong decline in Cnidaria. Moreover, the northern water mass and the front were dominated by large copepods, while the southern water mass exhibited higher diversity of zooplankton and a community of smaller-sized copepods. The results of this study highlight the complexity of processes shaping planktonic communities over time and space in the NBF zone and its adjacent waters. These processes include zooplankton stock reduction in the transitional post-bloom period, marked effect of diel variation linked to vertical migrations, and potentially the impact of storm-related mixing in the surface layer that can disrupt established ecological patterns.