The salt marsh is a transition area from terrestrial to marine boundaries exposed to frequent inundations, which results in different zonation of the vegetation. This in turn affects the community structure of soil organisms and the structure of soil food webs. However, the distribution and position of soil mesofauna in the food web of salt marshes received little attention. To close this gap of knowledge we investigated the following questions: how is the salt marsh soil animal community structured and how are the trophic levels and species interactions organized? For this purpose, six plots per vegetation zone were sampled: upper salt marsh, lower salt marsh and pioneer zone on Spiekeroog, East Frisian Islands, Germany, in September 2014. Four samples were taken with a soil corer from 0 to 5 cm per plot, resulting in 72 samples. The soil fauna was extracted by heat and identified to the level of species. To identify the trophic position and the food source of the species, stable isotope (15N and 13C) signatures of the animals were determined.
The soil fauna consisted of 82 taxa including 50 mesofauna species. The mesofauna was dominated by Mesostigmata, Oribatida and Collembola. Density and diversity of soil animal species decreased significantly from the land to the sea and the community structure differing significantly between the three salt marsh zones. Meadow species dominated in the upper salt marsh, whereas few specialized species occurred in the pioneer zone. Based on the δ15N values, four trophic levels (I-IV) were identified: decomposers (I), omnivores (II-III), primary and secondary predators (III-IV). The structure of trophic levels shifted from complex food webs to more simple food webs along the land-sea transect. Due to changing food resource availability from C3/C4-plants in the upper and lower salt marsh to C4-plants/algae in the pioneer zone, soil animal species were significantly less depleted in δ13C in the pioneer zone. The results suggest that decomposer species feeding on autochthonous material such as plant detritus dominated in the upper salt marsh. In contrast, littoral species in the pioneer zone fed on marsh detritus and micro- and macroalgae suggesting that they heavily rely on allochthonous resources.