Leaf litter decomposition rates in freshwaters differ by ecosystem

Terrestrial leaf litter is a large contributor to the metabolism and secondary production of freshwaters. Decomposition rates of leaf litter in freshwaters are often used as a proxy for ecosystem function; however, there are many sources of variation in decomposition rates within and between freshwater ecosystems that still have not been tested. In particular, the variation in decomposition rates between different freshwater ecosystems has rarely been addressed. Here, we compared decomposition rates of red alder (Alnus rubra) leaf litter in streams, ponds, and lakes within a single forest, while controlling for water temperatures. Using coarse-mesh and fine-mesh bags we found that when accounting for degree days, decomposition rates were higher in streams than ponds, and twice as high in streams than lakes, for either mesh size. The overall densities of invertebrates per leaf pack or per gram of leaf litter were very similar between the three ecosystems. However, detritivores were six-fold or more abundant in leaf packs from streams than those from lakes or ponds. There were fewer specialized macroinvertebrate consumers of leaf litter in the lentic environments. Specialized shredders such as Plecoptera were absent from lentic sites, and their absence in terms of decomposition rates was not compensated for by litter decomposition by generalist taxa. While we did not test the specific mechanisms responsible, differences may be associated with the relative temporal and spatial variation in the abundance of this resource and lack of specialist consumers in lakes and ponds.