The trophic structure of a Wadden Sea fish community and its feeding interactions with alien species.

The Wadden Sea is an important nursery area for commercial fish species of the North Sea and habitat for many small fish species. It is a very variable habitat with high variations in salinity and temperature. The Wadden Sea ecosystem has been modified by the utilization of humans since millineries. Species composition and food web of the fish of the Wadden Sea are influenced by destruction and alterations of habitats, fisheries, introduced species and climate change. The species distribution shows marked differences in species occurrence between inter- and subtidal areas and between the 4 different seasons of a year. Pomatoschistus microps was the most abundant fish species in intertidal areas and Clupea harengus and Ammodytes tobianus were abundant in subtidal areas. Variation was also discovered in the occurrence of feeding guilds found by diet overlap calculations. Only calanoid copepod eaters and Crangon crangon eaters were present year round. Fish of the Sylt-Romo Bight show a broad spectrum of prey and opportunistic feeding behavior. Convergence towards abundant prey organisms in summer and divergence of feeding in winter was observed. An increase in the abundance of zooplanktivorous fish compared to previous investigations was found. Stable isotope analysis of carbon and nitrogen revealed a lower mean trophic position for zooplanktivorous fish than for fish feeding on zoobenthos. Merlangius merlangus was found at the highest trophic position (3.84) and Limand limanda at the lowest (2.94). The overall low trophic position of the fish of the Sylt-Romo Bight pointed to short food chains and therefore to an immature ecological state of the bight. Immature systems are characterized by a high degree of unutilized resources and are therefore more susceptible to introductions of new species. The influence of these species on native fish was shown in the example of the herring (Clupea harengus) and 2 new species in the food web, the sea walnut (Mnemiopsis leidyi) and the sand smelt (Atherina presbyter). The sea walnut was introduced into the North Sea most likely between the year 2000 and 2006. It first occurred in samples in early summer and reached its highest abundance in fall (10.3 ± 8.2 inidviduals m-3). Herring and sea walnut showed temporarily high diet overlap which was related to the high amount of ingested calanoid copepods by both species. High amounts of bivalve larvae were found in the diet of the sea walnut in August, which could not be found in the guts of herring. In an experiment it was demonstrated that in herring intraspecific competition was significantly higher than competition from the sea walnut. Nevertheless, competition between both species could occur if their feeding resources should be limited. The distributional range of the sand smelt was possibly extended to the North due to increasing sea water temperatures induced by climate change. Only recently increasing abundance of this fish species in the Sylt-Romo Bight is observed. The results of our investigation show only a temporarily high diet overlap due to ingestion of nauplius and cypris larvae of both species. The sand smelt shows positive selection of these meroplankter and avoidance of calanoid copepods, which are the main diet of herring. There is still place for new species in the food web of the Sylt-Romo Bight. Its biology determines how a new species will integrate into the food web. The sea walnut poses a potential threat to native zooplanktivores due to overlapping diets and its high fertility and abundance. This potential increases should the competition pressure from native fish be reduced. The sand smelt uses mainly different habitats than native zooplanktivorous fish and could therefore integrate into the fish food web.