Abstract
Global change processes affect seasonal dynamics of salt marshes and thereby their plant and animal communities. However, these changes have been little investigated for microarthropod communities. We studied the effect of seasonality and changes in sea level on oribatid mites in the natural salt marsh and on artificial islands in the back-barrier environment of the island Spiekeroog (Wadden Sea, Germany). Three zones of the artificial islands were filled with transplanted sods from the lower salt marsh zone and thereby exposed to three different inundation frequencies. We hypothesized that oribatid mite communities will differ along the natural salt marsh vegetation zones [upper salt marsh (USM), lower salt marsh (LSM), pioneer zone (PZ)], which are influenced by different tidal regimes. Accordingly, total oribatid mite densities declined from the USM and LSM to the PZ. Similarly, oribatid mite species compositions changed along the salt marsh transect and also responded to variations in inundation frequency in LSM on artificial islands with typical species of the USM, LSM and PZ being Multioppia neglecta (USM), Hermannia pulchella (LSM), Zachvatkinibates quadrivertex (LSM, PZ) and Ameronothrus schneideri (LSM, PZ). Oribatid mite density in the salt marsh and on the artificial islands was at a maximum in winter and spring; this was due in part to high density of juveniles, pointing to two reproductive periods. We hypothesized that oribatid mite trophic structure changes due to variations in abiotic (e.g., tidal dynamics, temperature) and biotic conditions (e.g., resource availability). Stable isotope (15N, 13C) and neutral lipid fatty acid analyses indicated that oribatid mite species have different diets with e.g., Z. quadrivertex feeding on macroalgae and fungi, A. schneideri feeding on microalgae and bacteria, and Scheloribates laevigatus and M. neglecta feeding on dead organic matter, bacteria and fungi. Overall, the results indicate that oribatid mite species in salt marshes are affected by changes in environmental factors such as inundation intensity, with the effects being most pronounced in species with narrow trophic niches and limited niche plasticity. The results also indicate that oribatid mite communities of the LSM respond little to short-term (one year) changes in inundation frequency.
Highlights
Salt marshes form the boundary between terrestrial and marine realms and are characterized by harsh and dynamic environmental conditions
The East Frisian Islands show a characteristic zonation pattern: The upper salt marsh (USM) reaches from 35 cm above the MHWL up to the storm tide limit and is inundated 35–70 times a year and the salinity ranges from 5 to 20 ‰, the flora is dominated by Elymus athericus
This indicates that either the amount or quality of resources declines from the USM and LSM to pioneer zone (PZ) and / or that abiotic conditions deteriorate from higher to lower salt marsh zones
Summary
Salt marshes form the boundary between terrestrial and marine realms and are characterized by harsh and dynamic environmental conditions. They stretch from the mean high-water line to the highest water spring tide levels [1]. Climate scenarios predict a continuous sea level rise of about 1–2 m to the end of this century, with accompanying stronger climate extremes [4,5] This is likely to result in a decline of salt marshes and their associated animal communities, since vegetation may not respond fast enough to cope with rising sea levels and recover from climate extremes [6]. In Konigshafen, a tidal bay of the Island of Sylt (northern Wadden Sea, Germany) the mean height of the tide increased by approximately 20 cm from 1930 to 2008, with marked concomitant changes in zonation patterns and species composition of tidal flat animals [7]
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