Organic enrichment simplifies marine benthic food web structure

Abstract

Food web structure is a key determinant for community functioning and stability. Still, the architecture of trophic interactions is often ignored in assessments of stressor- or disturbance-induced changes in ecological communities. We established anthropogenic structuring of trophic networks by revisiting the Pearson-Rosenberg paradigm on changing benthic communities along gradients of organic enrichment. Using monitoring data from two sites with point-source organic loading, acknowledging associated changes in, e.g., water chemistry, we studied network topology of soft-sediment macroinvertebrate communities. We found that organic enrichment led to a progressive simplification of food web structure. The simplification manifested both horizontally and vertically in the web, and networks at impacted sites consisted of fewer nodes, fewer links, and had lower linkage density than more pristine communities. Contrary to our expectation, organic enrichment did not change the directed connectance of the networks, nor affect proportions of top-level species and carnivorous taxa in the assemblages. There were, however, changes in the proportions of basal-level and intermediate-level nodes, which together with decreasing mean trophic height further reveal a disintegration of the trophic network. We suggest the Pearson-Rosenberg model is expanded to include a progressive simplification of benthic food webs. Adding a dimension of interactions (in this case feeding links) to existing biodiversity assessments enhances our understanding of the consequences of environmental degradation.