Strong spatial partitioning of mesozooplankton in a dynamic frontal zone shapes trophic transfer efficiency

Abstract Frontal zones are often areas of increased biological productivity and have hydrographic gradients on short spatial scales. The small scale of gradients poses a challenge in identifying differences in zooplankton community structure and trophic transfer efficiency through the pelagic food web. In this study, a towed in situ imaging system targeting meso‐ and macrozooplankton was deployed in a tidal mixing front in the North Sea along transects (with lengths of 5–34 km) covering the transition from the shallow Dogger Bank to deeper shelf waters. Environmental data, such as temperature and chlorophyll a (Chl a ), were used to distinguish the dominant hydrographic regimes, and trophic transfer efficiency was calculated for each regime from planktonic size spectra. Zooplankton showed strong and significant partitioning among the hydrographic regimes. The warm oligotrophic surface layer was dominated by appendicularia and benthic meroplankton (mainly echinoderm larvae) that occurred in dense aggregations. Within regimes of high Chl a , we found significant differences in food web composition and efficiency. The subsurface chlorophyll maxima were narrow zones 5–10 m below the thermocline that supported the highest zooplankton biovolume and trophic transfer efficiency with high copepod abundance (3.1 ). By contrast, enhanced Chl a regions above the bottom at the edge of the bank were dominated by centimeter‐long diatom chains, and had low zooplankton abundances and reduced trophic transfer efficiency. The strong spatial partitioning of zooplankton communities and differences in trophic transfer efficiency among small‐scale hydrographic regimes underscores the importance of high resolution, meter‐scale sampling.