The trophic position of dead autochthonous organic material and its treatment in trophic analyses

Abstract

1. The importance of the recycling of organic matter for the overall carbon and nutrient flow in a food web, e.g., by the microbial loop has been recognized for pelagic and other ecosystems during the last decade. In contrast, analyses of the trophic food web structure conducted, e.g., by network analysis based on mass-balanced flow diagrams (i.e., computation of, e.g., trophic positions and transfer efficiencies, organismal composition of trophic levels) which greatly contribute to our understanding of the flow and cycling of matter in food webs, have not yet responded adequately to this fact by developing coherent techniques with which dead organic matter and its consumers could be considered in the models. 2. At present, dead organic matter (measured in units of carbon or nutrients) is either allocated to a fixed trophic position (between zero and one), or the trophic position of dead autochthonous material depends on the trophic position of the organisms which released it. This causes partially ambiguous and inconsistent interpretations of key measures like trophic transfer efficiences and trophic positions and greatly hampers cross-system comparisons. 3. The present paper describes and compares four different definitions of the trophic position of dead autochthonous organic material which have either been newly invented or already used. Their impact on the resulting trophic positions of individual groups is illustrated using a food web model from the pelagic zone of Lake Constance. The present analysis evaluates the partially far reaching consequences of the definition chosen, and suggests to allocate all dead organic material to the 'zeroth' trophic level irrespectively of its origin (allochthonous or autochthonous), chemical composition and the commodity used to quantify the food web model (e.g., units of carbon or nutrients). By this means trophic positions and trophic transfer efficiencies get a clear and consistent ecological interpretation, while inconsistencies between analyses conducted in units of carbon or nutrients and some operational problems can be overcome and cross-system comparisons and empirical verification are facilitated.