Testing the interaction between leaf litter diversity and salmon carcasses on in-stream leaf decomposition: An experimental manipulation

Marine derived nutrients are a vital source of energy to freshwater ecosystems in the Pacific Northwest. Another important allochthonous source for low order streams is leaf litter from riparian vegetation. Both of these energy inputs provide resources which stimulate fungal biomass and invertebrate communities creating more biodiversity and biomass in streams. We hypothesized that salmon carcasses deposited in streams would influence leaf decomposition through changes in macroinvertebrate assemblages. We also hypothesized that diversity among leaf litter species would influence leaf decomposition. This experiment included seven different diversity treatments attached either next to a salmon or to a no-salmon control. Diversity treatments included Alnus rubra (red alder), Acer macrophyllum (big-leaf maple) and Populus balsamifera (black cottonwood) in single species litter bags, three treatments of two species bags, and one treatment including all three species. Thirty replicates of each treatment were placed in the stream for three specified harvest dates (n = 5). The experiment was placed in a low order stream with annual chum salmon (Oncorhynchus keta) runs in the South Puget Sound, WA. Initial results show that salmon presence mildly decelerates leaf litter decomposition. Additionally, 30% of the variation in decomposition is influenced by litter diversity. As a single species, A. macrophyllum decomposed slowest. The fastest treatment was the A. rubra and P. balsamifera mixture, while the other two species mixtures which included A. macrophyllum, showed significantly slower decomposition rates. This indicates that A. macrophyllum may have a decelerating influence on leaf litter decomposition rates.