Macrozoobenthic response to fishery: trophic interactions in highly dynamic coastal ecosystems

Abstract

This thesis aims at achieving deeper insights into the ecological functioning of German Bight benthos, i.e. ecosystem trophic properties that might explain resistance of the system in the light of decades of frequent bottom trawling, and recovery patterns of the benthic system during 14 months of trawling cessation.(i) Stable isotope ratios served as an indicator for species trophic elevation, i.e. the relative distance of a species to the primary food source within the food web hierarchy. Community trophic complexity and species trophic niche width were determined by means of interspecific and intraspecific variability of trophic elevation. Trophic significance, i.e. a metabolism scaled abundance, served as an indicator of trophic flow through particular populations.(ii) A new two-dimensional approach to trophic complexity (horizontal and vertical diversity) was developed. It was applied to the German Bight ecosystem, but concurrently provides a robust and standardised tool for the comparison of species and ecosystem trophic properties.(iii) The macrozoobenthic response to trawling cessation was investigated over the course of 14 months by two approaches: firstly, multivariate community analyses using numerical measures which follow changes at different system levels: the community level, the level of compartments (e.g. feeding guilds) and the species level; secondly community and species trophic properties, i.e. trophic elevation and trophic significance. For the first time, this approach identified significant changes due to trawling cessation on a short time-scale within a highly dynamic coastal ecosystem.The synthesis of all results provides a detailed picture of trophic functioning of the German Bight benthos: (a) Most species are trophic generalists, i.e. they are connected by many and mainly 'weak' trophic links. (b) Trophic redundancy is comparatively high, i.e. the loss of one species may be compensated functionally by one or more other species. (c) In terms of size distribution, the system is characterised by small-sized organisms with corresponding high turnover rates. These particular food-web properties correspond to high system resistance, i.e. the current state can be maintained despite the severe impact of continuing bottom trawling.