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Functioning of Intertidal Ecosystems of the Wadden Sea Material exchange of the Sylt-Rømø Bight and its relation to habitat and species diversity

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Asmus, H. (2011): Functioning of Intertidal Ecosystems of the Wadden Sea Material exchange of the Sylt-Rømø Bight and its relation to habitat and species diversity , Habil thesis, Alfred Wegener Institute, University of Kiel.
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Abstract:

The Wadden Sea can be considered as a functionally diverse ecosystem. In the present thesis, various global system indices based on information theory describe the developmental and organizational state of the ecosystem. The wide range illustrates different states of organization in the various communities of the Sylt- Rømø Bight. Energy flow studies of the Wadden Sea were up to now restricted to pure sand and mud flats, and have not been combined with studies on material exchange processes. I could show that energy flow varies considerably among the different habitats and that there is varying trophic efficiency due to habitat type. This results in low trophic efficiency when considering the total intertidal area of a tidal basin, and is primarily caused by low use of microphytobenthos and an overexploitation of autochthonous planktonic primary production in the dominant sand flat communities. Material fluxes of 8 different habitats were measured using large in situ flumes (a special type of large scale field enclosure). Investigations on community metabolism and community production were performed and the results were synthesized by Ecological Network Analysis. The innovative potential of this work arises out of the combination of material flux studies, carried out with the novel in situ flume technology, and the Ecological Network Analysis (ENA) analysing the elemental flow and the food web from a holistic point of view. I consider the sink and source functions on a community basis. This approach allows estimation of the filtration potential of a community and can distinguish it from passive sedimentation. This demonstrates that, presently, sedimentation in the intertidal part of the Sylt-Rømø Bight is the more relevant process compared to the function as a biological filter. Material exchange and material cycles differ distinctly between the communities of the Sylt- Rømø Bight. The exchange of carbon varies both in magnitude and in the distribution pattern into different carbon forms, such as particulate, dissolved and living matter. Considering the different elements of the organic material such as carbon, nitrogen and phosphorus, there are also differences in flow characteristics and the prevalent form of material transport within one community. The present work shows that communities with a high allochthonous material input show a high internal material turnover. In communities exposed to higher currents and wave action such as sparse seagrass beds, sandy shoals and sandy beaches, I observe a higher percentage of “small” material cycles involving 2 compartments, compared to sheltered communities such as mud flats, dense seagrass beds and muddy sands, where 2 compartment cycles transport half or less than half the total amount of cycled material. This indicates both a quick cycling and a larger importance of a self-contained microbial loop in exposed communities. In view of the existence of strong links between many predators and few prey species (i.e. few alternative or parallel pathways), especially the mussel bed system can be considered to possess less stability when faced with external perturbations (indicated by a low redundancy value). Redundancy indices for other communities are relatively high, reflecting multiple parallel pathways and thus, more resistance to external perturbations. A further new insight from this analysis is how the amounts of the recycled elements carbon, nitrogen and phosphorus are distributed amongst the path length by which these elements are cycled. This new result was estimated for the total Sylt-Rømø Bight but will be of general importance for ecosystems: About 99% of carbon (C) is cycled over short path lengths involving 2–3 model compartments, and although path lengths of up to 9 were identified, the amount of carbon transportedover longer ones is miniscule. Nitrogen (N) shows a clear bimodal distribution. Phosphorus (P) on the other hand shows a peak of recycling at path lengths 3 to 5, over which about 81% of P is recycled. Both N and P appear to be cycled over longer path lengths than C. This may also underline the importance of higher trophic levels such as invertebrates, fish and birds for element cycles such as nitrogen and phosphorus, whereas in carbon cycling, clearly short cycles are important, and microbial components are dominant. Because the present network analysis is made with a high resolution down to species level, it can also be used to highlight the relationship between biodiversity and ecosystem functioning based on trophic system. I therefore found new quantitative evidence for the hypothesis of whether higher diversity is coupled with higher internal use of resources and with better cooperation within the system. This was tested by using simple correlations between certain system level indices and the diversity considered for the particular community . Since more members in a food web of a community are interacting (indicated by higher internal and food web connectivity), the resources may be used more efficiently without larger losses (indicated by Finn Cycling Index). Because biodiversity and the recycling potential are positively correlated, we conclude from these results a new theory that the more diverse a community is and the more niches and members occupying these niches (in terms of species) it provides, the less is the material loss from this community. Considering the export function of communities, there is a clear decreasing trend of relative redundancy with increasing export of carbon from the community, supporting the hypothesis that more parallel cycles in a community improve the use and processing of the food sources and diminish the export of unused food. The Sylt-Rømø Bight is thus one of the first marine intertidal systems where the food web structure has been described including the habitat constituents. We now understand better the spatial differences of functional resource use and resource partitioning in a tidal basin, and we know that the characteristics of the food web of the communities differ widely with respect to their dependence on benthic or pelagic sources. It is also the first time in a marine intertidal system that exchange processes have been related with food web structure, and that global system indices derived from network analysis were used for understanding the relation between diversity and functioning of benthic systems.

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