Eutrophication modulates plant-litter diversity effects on litter decomposition in streams

Abstract

Freshwater ecosystems are severely impacted by changes in riparian vegetation and eutrophication, but their interactive effects on litter decomposition and associated biota remain poorly understood. We placed 5 leaf species in coarse-mesh bags alone or in mixtures and immersed them in 6 low-order streams along a eutrophication gradient. Fungal and invertebrate assemblages were mainly structured by stream eutrophication. The quality of leaf species also structured fungal assemblages, whereas the number of leaf species structured invertebrate assemblages. Effects of leaf diversity on decomposition were synergistic and increased with the number of leaf species. However, the synergistic diversity effects were found only in streams with lower nutrient levels, a result suggesting that oligotrophic streams depend more than eutrophic streams on the number of plant litter species. On the other hand, leaf species identity affected leaf-litter decomposition and fungal and invertebrate biomasses on leaves. Initial leaf N concentration and leaf-litter decomposition were positively linearly related, and this relationship became stronger as eutrophication increased. This result suggests that leaf-litter decomposition depends more on the quality than the number of plant litter species in eutrophic streams. Overall, our results highlight that eutrophication modulates leaf diversity effects on leaf-litter decomposition with potential implications for stream ecosystem management.