Beach morphology and food web structure: comparison of an eroding and an accreting sandy shore in the North Sea

Abstract.

Food web components and inorganic nutrients were studied on two sandy shores of the adjacent barrier islands of Sylt and Rømø in the North Sea, differing in morphodynamics. Implications of high and low wave energy on the food web structure were assessed. The Sylt shore represents a dynamic intermediate beach type, while the Rømø shore is morphologically stable and dissipative. On the steep-profiled, coarse-grained Sylt shore, strong hydrodynamics resulted in erosion and high fluxes of organic material through the beach, but prevented any storage of food sources. In contrast, the flat-profiled, fine-grained Rømø shore, with low wave energy and accretion, accumulated organic carbon from surf waters. At Sylt, oxic nutrient regeneration prevailed, while anoxic mineralization was more important at Rømø. Macrofauna on the Sylt shore was impoverished compared with the community at Rømø. Correspondingly, abundances of epibenthic predators such as shrimps, crabs, fish, and shorebirds were also lower at Sylt. Meiofauna was abundant on both shores, but differed in taxonomic composition. Several major taxa were represented in fairly equal proportions of individual numbers on the well-oxygenated Sylt shore, while nematodes strongly dominated the assemblage at Rømø. Thus, on cold-temperate, highly dynamic intermediate shores with high wave energy and subject to erosion, the "small food web" dominates. Organisms are agile and quickly exploit fresh organic material. Larger organisms and nematodes abound under stable, dissipative and accreting shore conditions, where some food materials may accumulate and zoomass builds up to support numerous visitors from higher trophic levels.