Description: Marine ecosystems are exposed to increasing human pressures and climatic change worldwide. It has therefore become essential to describe ecosystem statuses with respect to multinational protection schemes, often necessitating long-term monitoring programmes. Changes in the food-web structure, which can be monitored via stable isotope measurements, represent an important descriptor of the status of marine ecosystems. We investigated long-term changes (29 years) in isotopic values (δ13C and δ15N) in four indicative organisms at different trophic levels in the southern North and Baltic Seas: bladderwrack (Fucus vesiculosus), blue mussel (Mytilus ssp.), eelpout (Zoarces viviparus), and herring gull (Larus argentatus). Time series analyses using generalised additive models revealed largely consistent declines in δ13C and δ15N throughout all trophic levels of the coastal food web at all study sites, indicating a clear change in these coastal regions from 1988 to 2016. There were no clear long-term patterns in egg biometrics for herring gulls, except for a consistent increase in eggshell thickness. The declines in stable isotope values were in line with the results of previous long-term studies of single higher-trophic-level species, which suggested that the noted changes were mainly caused by altered foraging patterns of the studied species. The current results demonstrate that declines in δ13Cand δ15N have occurred throughout the whole food web, not just in particular species. We discuss the possible reasons for the decrease in stable isotope values, including decreasing eutrophication and an increase in terrestrial carbon sources

Description: The non-breeding distribution of common scoter (Melanitta nigra), which occur in high densities offshore from the eastern Wadden Sea, has been well documented in recent decades, but factors influencing these patterns remain poorly known. We investigated the prey choice of 88 common scoter using stomach contents of beached birds collected over a period of 12 years. We then used benthos data and hydrodynamic parameters to model the June–September distribution patterns during the flightless period of common scoter based on transect count data off the north-eastern German Wadden Sea. Benthic fauna biomass in areas used by common scoter was higher than in areas where the species was absent. American razor clam (Ensis leei) invaded the Wadden Sea during the end of the 1970s and was the most common prey, both in benthos samples from below feeding common scoter and in stomach samples. The relatively high flesh to shell ratio of the American razor clam makes it the most attractive available prey item for common scoter. The abundance of American razor clams explained the distribution of scoter to a high degree, although the best model fit included negative effects of water depth and positive effects of bed shear stress intensity on scoter abundance. Our data show that the neobiotic American razor clams have become an important part of the diet of these sea ducks, whereas the cut trough shell (Spisula subtruncata) was of substantially lesser importance than expected from historical data. Our study shows the ability of a top avian predator to respond to novel developments in bivalve prey availability over periods of a few decades.

Description: Germany is an EU member state and is thus obliged to monitor and protect its marine and coastal areas in the context of international treaties (e.g. EU Marine Strategy Framework Directive, EU Natura2000 directives, Trilateral Monitoring and Assessment Programme, OSPAR) to achieve the good environmental status (GES). Suitable indicators thus need to be developed to assess the status of relevant ecosystems. Intertidal bivalve beds belong to the most productive habitats in the shallow coastal waters of the Wadden Sea but are highly dynamic and difficult to monitor. Extensive benthos and sediment sampling are required to study their development and distribution. We therefore evaluated the suitability of herring gulls (Larus argentatus) as a bioindicator species for monitoring these habitats. As flexible and opportunistic top-predators, herring gulls prefer to forage on intertidal flats and feed on bivalves (Cerastoderma edule, Mytilus edulis, Ensis leei) and crustaceans, with intertidal habitats near the breeding colony preferred during the incubation period. We used state of the art tracking devices to analyse habitat selection by herring gulls, and assessed how this was driven by biotic and abiotic factors. We then designed a habitat model to predict the occurrence of intertidal bivalve beds for directed monitoring, based on GPS logger data from herring gulls and hydrodynamic data. Both methods showed high correlations of herring gull foraging patterns with bivalve biomass and inundation time, depending on the distance from the breeding colony. The movement patterns of herring gulls clearly mirrored intertidal bivalve distributions. These results showed that combining GPS-logger data for top-predators, such as birds, with environmental data has great potential for monitoring and assessing the GES. We propose that herring gulls may be used as an indicator to locate bivalve beds in the Wadden Sea and to detect changes in the food web.

Description: The Wadden Sea at the western coasts of Germany, Denmark and the Netherlands is one of the globally most important foraging areas for breeding and migrating birds which act at the same time as indicators for the ecological condition of the ecosystem. However, little is known about how the intense predation pressure of birds influences the Wadden Sea food web. The aim of the interdisciplinary project STopP (From Sediment to Top Predator) is to determine the food web structure in different Wadden Sea habitat types in terms of the interaction between the basis of the food web and birds as top predators. Studied habitat types included the most important foraging areas of birds; mussel banks, cockle beds, sand flats, mud flats, seagrass meadows and beds of the immigrant razor clam Ensis directus a recently preferred prey item of several bird species. Data were analysed using the Ecological Network Analysis (ENA) that reflects trophic structures within the systems and reveals direct and indirect relations between the lower and the upper trophic levels. Preliminary results show that bird predation increases the complexity of the food web due to an increase in connections and a higher total system throughput. On the other hand the predation has also a destabilizing effect due to a high demand of system’s carbon stocks and increased exports out of the tidal system. In addition, analyses show considerable indirect dependencies of birds to lower trophic levels such as sediment POC and phytoplankton. Future scenarios modelled with ENA shall show how changes within the lower trophic levels would affect foraging birds due to anthropogenic or natural impacts. Further analysis will focus on the importance of special habitat types for different bird species and the influence of changes in the biomass of key species for the whole ecosystem food web.

Description: The Wadden Sea at the western coasts of Germany, Denmark and the Netherlands is one of the globally most important foraging areas for breeding and migrating birds acting as indicators for the ecological condition of the ecosystem. However, little is known about how the intense predation pressure of birds influences the Wadden Sea food web. The aim of the interdisciplinary project STopP is to determine the food web structure in terms of the interaction between the basis of the food web and birds as top predators. Studied habitat types included the most important foraging areas of birds; mussel banks, cockle beds, sand flats, mud flats, seagrass meadows and beds of the immigrant razor clam Ensis directus a recently preferred prey item of several bird species. Data were analysed using the Ecological Network Analysis (ENA) that reflects trophic structures within the systems and reveals direct and indirect relations between the lower and the upper trophic levels. Preliminary results show that bird predation increases the complexity of the food web due to an increase in connections and a higher total system throughput. On the other hand the predation has also a destabilizing effect due to a high demand of system’s carbon stocks and increased exports out of the tidal system. Furthermore, analyses show considerable indirect dependencies of birds to lower trophic levels such as sediment POC and phytoplankton. Future scenarios modelled with ENA shall show how changes within the lower trophic levels would affect foraging birds due to anthropogenic or natural impacts.

Description: The Wadden Sea is one of the most important stop-over sites for 10 to 12 Million migrating birds per year. But little is known about the influence of birds on the entire ecosystem. We conducted Ecological Network Analysis (ENA) in an important resting site in the Wadden Sea to determine the influence of birds on the food web. A large variety of different bird species uses the area for foraging and is included directly or indirectly in most of the pathways. Birds induce a negative impact on their prey items with a positive feedback reaction to the preys’ competitors and food sources. There is also a strong negative impact among the bird compartments probably due to competition between the birds. Changes in the biomass of the birds revealed alterations in the food web structure. With a decline in the avian biomass the system showed a decrease in connectivity and diversity of flows but an increase in recycling. Changes in the bird population could affect the complexity and functioning of the entire ecosystem. Thus, the use of such holistic approaches would facilitate undertaking management measures.

Description: The Wadden Sea is one of the most important foraging areas for breeding and migrating birds. However, little is known about the preferred feeding places of birds and how the intense predation pressure influences the food web. Within the project STopP the trophodynamic structure of different Wadden Sea habitats is determined to get an idea how sediment characteristics and hydrodynamics modify habitats that serve as food sources for birds.

Description: The Wadden Sea is one of the most important foraging areas for breeding and migrating birds. However, little is known about the preferred feeding places of birds and how the intense predation pressure influences the food web. The aim of the STopP project is to determine the structure of the Wadden Sea food web and how sediment characteristics and hydrodynamics modify habitats that deal as food sources for birds.