Nitrogen loadings affect trophic structure in stream food webs on the Tibetan Plateau, China

Anthropogenic activities, such as agricultural and industrial development, have increased nutrient inputs into waterways, which affect trophic interactions and the flow of energy through food webs in the aquatic ecosystems. However, the responses of food web structure and function to specific anthropogenic stressors in the alpine stream systems remain unclear. Here, we studied the stream food webs in the Lhasa River on the Tibetan Plateau, China. We measured the isotopic ratios (δ13C and δ15N) of macroinvertebrate and fish functional feeding groups (FFGs) and their basal resources in the streams. Dietary contributions of basal resources to consumers and food web metrics including trophic length, diversity, and redundancy were used to quantify changes in stream food webs in response to anthropogenic disturbance. Dietary analysis showed that allochthonous resources contributed more than autochthonous resources to macroinvertebrate primary consumers regardless of the disturbance intensity in the adjacent land areas. Anthropogenic activities increased the δ15N values in epilithic algae and isotopic variation in basal resources and fish but reduced the trophic length and redundancy (i.e., fewer species or taxon at each trophic level) in food webs. Additionally, the total nitrogen concentration in waters was the most important environmental variable affecting trophic diversity and redundancy. Therefore, the reduction of nitrogen inputs into streams is critical for sustainable river management and biodiversity conservation in the streams on the Tibetan Plateau.