1 Department of Bioscience - Aquatic Biology, Department of Bioscience, Science and Technology, Aarhus University2 Department of Biological Sciences, Plant Biology, Faculty of Science, Aarhus University, Aarhus University3 Department of Bioscience - Lake Ecology, Department of Bioscience, Science and Technology, Aarhus University4 Department of Bioscience - Stream and Wetland Ecology, Department of Bioscience, Science and Technology, Aarhus University5 Departamento de Ecología & Evolución, CURE-Facultad de Ciencias, Maldonado, Universidad de6 Department of Bioscience - Aquatic Biology, Department of Bioscience, Science and Technology, Aarhus University7 Department of Bioscience - Stream and Wetland Ecology, Department of Bioscience, Science and Technology, Aarhus University8 Department of Bioscience - Lake Ecology, Department of Bioscience, Science and Technology, Aarhus University
Relationships between land use and stream consumer δ15N values are well established and, as a consequence, different primary consumers have been suggested as suitable system-specific baseline indicators in isotope studies of trophic structure. These baselines are often based on bulked data including several taxonomic groups and feeding types due to difficulties of finding ubiquitous taxonomic groups. We identified taxonomic groups which correlated well with land use in Danish lowland streams, and several of those are widely distributed in running waters in the Northern hemisphere. Particularly Baetidae, Simuliidae and Gammarus pulex appeared to be good indicators of land use. Scrapers (Baetidae), however, generally had higher δ15N than filtrators (Simuliidae) in catchments with a high level of anthropogenic activities, but lower δ15N in nature-dominated catchments. The trophic position of two common fish species (three-spined stickleback, Gasterosteus aculeatus, and brown trout Salmo trutta) differed markedly depending on the baseline chosen. The estimated trophic position was lowest when based on Baetidae and highest when using Simuliidae. The trophic position of Gasterosteus aculeatus was independent of land use (proxy used=%nature) when based on Gammarus pulex and Simuliidae, and the trophic position of Salmo trutta was independent of land use when based on Simuliidae only. The trophic position estimates based on Baetidae and mean primary consumers correlated with %nature in the catchment and had a slope deviating from zero for both fish species, despite gut content analysis revealed no such trophic level dependency of land use. This suggests that Baetidae are not good baseline indicators of trophic position, perhaps because their main food item (periphyton) may include nitrogen-fixing cyanobacteria, being potentially more important in less anthropogenic affected streams, resulting in lower scraper (e.g. Baetidae) δ15N values at such sites. We also found that within-site variation in δ15N was highest for Gammarus pulex, then Baetidae and the lowest for Simuliidae. Filtrators (e.g. Simuliidae) are thus apparently the best indicator of land uses, not least when the gradient of land use is large. Our results strongly support the use of these organisms as baseline in stable isotope studies of trophic webs.