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Ökologischer Vergleich von zwei Sandstränden mit unterschiedlicher Morphodynamik in der Nordsee = Ecological comparison of two sandy shores with different wave energy and morphodynamics in the North Sea
Menn, I. (2002). Ökologischer Vergleich von zwei Sandstränden mit unterschiedlicher Morphodynamik in der Nordsee = Ecological comparison of two sandy shores with different wave energy and morphodynamics in the North Sea. Ber. Polarforsch. Meeresforsch. 417: 1-170
In: Berichte zur Polar- und Meeresforschung = Reports on Polar and Marine Research. Alfred-Wegener-Institut für Polar- und Meeresforschung: Bremerhaven. ISSN 1618-3193, more
Peer reviewed article  

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Keywords
    Beaches; Coastal morphology; Ecology; Wave energy; ANE, North Sea [Marine Regions]; Marine

Author  Top 
  • Menn, I.

Abstract
    Exposed sandy beaches are physically harsh and highly dynamic environments. Waves, tides and sediment type form a largely physically controlled habitat for the sandy beach organisms. The present study is an ecological comparison of two sandy shores differing in wave energy and morphodynamics. The study sites were located in the eastern North Sea on the German island of Sylt and the neighbouring Danish island of Rømø. The Sylt shore is coarse grained, steep profiled, and receives high wave energy. Beach nourishment is used to counterbalance erosion. lt is a higbly dynamic, intermediate beach type. In contrast, the Rømø shore is fine grained, flat profiled, accreting and with low wave energy. lt represents a relatively stable, dissipative beach type. The primary objective of this thesis are the effects of wave energy and sandy shore morphology on the biotic community and its food supply. The studied community components include the meio- and macroinfauna, aquatic epibenthos, and shorebirds. A simultaneous and detailed analysis of the first two provides insights on the ecological distinction between meio- and macrofauna in the beach system including their temporal variability and spatial zonation across the shore. In addition, the ecological consequences of beach nourishment on the infauna were studied. Overall meiofaunal abundances (average of inter- and subtidal) were similar on both shores, but the taxonomic composition differed. On the intermediate shore with high wave energy, meiofauna was composed of roughly equal proportions of several major taxa, while there was a strong numerical dominance of nematodes on the dissipative shore with low wave energy. Meiofaunal zonation on the intermediate shore showed higher abundances in the intertidal than in the subtidal, while meiofauna was more evenly abundant across the entire dissipative shore. Species density and diversity of interstitial plathelminths and polychaetes were higher at Sylt than at Rømø. Both numbers peaked at mean low water and in the shallow subtidal at Sylt, while the maximum occurred at the mid shore at Rømø. In contrast to meiofauna, overall macrofaunal abundance, species density, and diversity were all lower on the intermediate than on the dissipative shore. Polychaetes were the most abundant taxon on both shores, followed by amphipods and isopods at Sylt, and by bivalves and amphipods at Rømø. Zonation revealed higher macrofaunal abundance and species density in the subtidal than in the intertidal at Sylt and a even distribution across the Rømø shore. Temporal variability of meiofaunal abundances was higher on the intermediate shore,while this was the case with macrofauna on the dissipative shore. In both faunal components highly mobile species were more prominent at Sylt than at Rømø. Corresponding to the impoverished macrofauna on the intermediate shore, abundances of epibenthic predators, such as shrimps, crabs, fishes, and shorebirds were also lower at Sylt than at Rømø. Additionally, the concentrations of Chlorophyll a and particulate organic carbon and the C/N ratio in the sediment were all lower on the intermediate than on the dissipative shore. Nutrient concentrations in interstitial and surf waters indicate oxic nutrient regeneration at Sylt, while at Rømø also anoxic mineralisation occurs. In conclusion, high wave energy on the eroding, dynamic intermediate shore limited the macrofauna, particularly at the steep intertidal slope, while it had weak effects on the meiofauna. The latter is abundant and of high evenness between major taxa. Further, it is mainly composed of agile taxa, which are able to quickly exploit the fresh organic material pathing through the beach under high oxic conditions. Low wave energy across the accreting, dissipative shore favoured rnacrofauna, and nematodes adapted to low oxygen conditions. Organic materials may intermittently accumulate on this shore and zoomass builds up to support abundant visitors from higher trophic levels. This comparison of two sandy beaches differing in wave energy offers first ideas on the consequences of increasing wave energy and erosion on exposed sandy shores. Such increases are expected as a corollary of global warming and the concomitant sea level rise. This is likely to provoke an increasing use of beach nourishment to combat erosion, creating highly dynamic intermediate shores away from morphodynamic equilibrium. The Sylt shore with its described ecosystem may be an example for this future beach state. A "small food web" takes over on such shores, particularly in the cold-temperate region of the North Sea, while the "large food web" will be impoverished. Disturbances by nourishment operations had minor impacts on the infauna of this dynamic system. Thus, from an ecological perspective, beach nourishments may be regarded as an acceptable method of coastal defence. However, no larger operations than the ones studied (350,000 m³/2 km) and intervals between those disturbances of at least two years at a given site are recommended.

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