Elevation gradient affects the development of macrozoobenthic communities in the Wadden Sea: A field experiment with artificial islands

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.