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Frequent environmental changes and abiotic gradients of the Wadden Sea require appropriate adaptations of the local organisms and make it suitable for investigations on functional structure of macrozoobenthic communities from marine to terrestrial boundaries. To investigate community patterns and food use of the macrozoobenthos, a transect of 11 stations was sampled for species number, abundance and stable isotope values (δ13C and δ15N) of macrozoobenthos and for stable isotope values of potential food resources. The transect was located in the back-barrier system of the island of Spiekeroog (southern North Sea, Germany). Our results show that surface and subsurface deposit feeders, such as Peringia ulvae and different oligochaete species, dominated the community, which was poor in species, while species present at the transect stations reached high abundance. The only exception was the upper salt marsh with low abundances but higher species richness because of the presence of specialized semi-terrestrial and terrestrial taxa. The macrozoobenthos relied predominantly on marine resources irrespective of the locality in the intertidal zone, although δ13C values of the consumers decreased from − 14.1 ± 1.6‰ (tidal flats) to − 21.5 ± 2.4‰ (salt marsh). However, the ubiquitous polychaete Hediste diversicolor showed a δ15N enrichment of 2.8‰ (an increase of about one trophic level) from bare sediments to the first vegetated transect station, presumably due to switching from suspension or deposit feeding to predation on smaller invertebrates. Hence, we conclude that changes in feeding mode represent an important mechanism of adaptation to different Wadden Sea habitats.

Tidal flats are subject to dynamic processes, characterized by sedimentation and erosion, which become more important in times of climate change. To investigate impacts of elevation levels on ecosystem functioning, we focused on macrozoobenthos as an essential ecosystem component that links trophic levels and determines sediment structure. Therefore, three elevation levels of six experimental islands (each 2 × 6 m) and six natural reference sites (nearby tidal flats) were sampled for species richness, abundance and biomass in the back-barrier system of the island of Spiekeroog (German Wadden Sea, southern North Sea) from 2014 to 2016. Reference sites had constant biodiversity and abundance over time that were higher compared to the three elevations of the experimental islands. At the lowest elevation, the community became more diverse over time, including marine and terrestrial taxa. At the middle elevation, community shifted from a marine species inventory over an opportunist-dominated assemblage to a less diverse community dominated by Enchytraeidae (Oligochaeta). The community of the highest elevation mostly remained constant, resembling the second, opportunistic community from the middle elevation that was dominated by the snail Peringia ulvae (Pennant, 1777) and the oligochaetes Enchytraeidae spp. and Tubificoides benedii (d'Udekem, 1855). Correspondingly, food web structure was more complex and comprised more trophic interactions at the lowest elevation compared to the other elevations mainly inhabited by deposit feeders. Results demonstrate negative impacts of increased sediment elevation on macrofaunal biodiversity and functional diversity, originating from reduced inundation and vegetation establishment.