The Wadden Sea in northwest Germany is known for its high secondary production and strong environmental gradients. To investigate the potential adaptations this demanding environment requires, we focused on the macrozoobenthic community, which has an important impact on the structure and functioning of intertidal areas because it links primary producers with higher trophic levels. In September 2014, 11 stations were sampled along a land-sea transect at the back-barrier system of the East Frisian Island of Spiekeroog (German North Sea) to investigate the community structure of the macrozoobenthos from the bare tidal flats to the upper salt marshes with respect to species richness and abundance. Additionally, δ13C and δ15N stable isotope signatures of the macrozoobenthos and potential food sources (marine detritus/microphytobenthos, macroalgae, vascular plants, terrestrial detritus, soil organic matter, and meiofauna) were measured using a coupled system of an elemental analyzer and a mass spectrometer to investigate changes in the use of carbon sources and in the trophic structure of the macrozoobenthic community along marine-terrestrial boundaries.
Multivariate community analyses showed similarities of >40 % across the whole transect except for the upper salt marsh, which had irregular inundations and a (semi-)terrestrial community. The macrozoobenthos relied predominantly on marine resources irrespective of the locality in the intertidal zone. Soil organic matter and terrestrial resources only became important in the upper salt marsh. There were no significant changes in the overall food-web structure with increasing importance of terrestrial conditions and resources. However, δ15N values of a representative species (the omnivorous polychaete Hediste diversicolor) did increase significantly from 13.5 ‰ at the tidal flat up to 16.3 ‰ at higher intertidal stations, whereas δ13C values showed a significant, continuous decline from lowest tidal flats to the upper salt marsh (from ca. -12.0 to -23.0 ‰). The δ15N values indicate an increase of one trophic position in moving toward the higher stations, possibly because of a switch in feeding mode from suspension feeding at the longer inundated tidal flats to predation on other invertebrates in the salt marsh. This is supported by δ13C values, which correlated with those of oligochaetes. Hence, we conclude that changes in feeding appear to represent an important mechanism of adaptation to different Wadden Sea habitats.