Meiofauna and macrofauna decouple in two benthic habitats of the Baltic Sea

We explored meiofauna and macrofauna in shallow muddy and sandy habitats of two spatially separated marine protected areas, Fehmarn Belt (FB) and Odra Bank (OB), in the German Baltic Sea. Diversity, density, and biomass of both size compartments were quantified alongside sediment characteristics, hydrographic conditions, and bottom-trawling pressure as potential community drivers. Pronounced regional contrasts emerged. Meiofauna was markedly more abundant and diverse in FB (4395 ind./10 cm²; Shannon diversity H' = 0.91 ± 0.09) than in OB (851 ind./10 cm²; H' = 0.45 ± 0.06). The number of major meiofauna taxa in FB was 16, in OB 18. Macrofauna species richness was 85 in FB (H' = 1.41 ± 0.11) and 31 in OB (H' = 1.02 ± 0.03). However, macrofauna abundance was markedly higher in sandy OB (14607 ind./m²) compared to the FB (1263 ind./m²). Across regions, macrofaunal abundance correlated negatively with meiofaunal density (ρ = -0.72, p = 0.0015), indicating regional-scale decoupling between benthic size compartments. No significant correlations were detected within regions, suggesting scale-dependent dynamics. Macrofauna community variation was strongly explained by combined abiotic and biotic predictors, whereas meiofauna structure in FB remained largely unexplained by available environmental, trawling, and macrofaunal variables. In OB, macrofaunal composition (notably polychaetes and amphipods) and sediment characteristics contributed moderately to meiofaunal structuring. Overall, our results demonstrate marked regional decoupling of benthic size compartments along salinity and habitat gradients, and scale-dependent processes. High macrofaunal diversity and abundance do not necessarily predict corresponding meiofaunal patterns. This decoupling should be accounted for when assessing benthic ecosystem responses to environmental and anthropogenic changes.