Leaf litter decomposition in boreal lakes: variable mass loss and nutrient release ratios across a geographic gradient

Here, we assess regional differences in decomposition rates of allochthonous plant detritus in the littoral zones of lake ecosystems. Specifically, we measured breakdown rates and elemental composition of aspen leaves (Populus tremuloides) over 60–70 days in 14 lakes from four lake regions located > 1000 km apart in Ontario, Canada. We found substantial differences in leaf breakdown among regions with much faster rates seen in more nutrient-rich and warmer lakes. While breakdown rates increased slightly with larger mesh size, which provided greater access by macroinvertebrates, these effects were negligible compared to those produced by regional differences in nutrients and temperature. We also found regional differences in detrital nutrient release, with variable N- or P-specific fluxes and their ratios, which indicates differential release of these nutrients back into the lake’s water column. Release ratios varied most in litterbags that showed the least mass loss, which indicates microbial uptake and release dynamics of N and P can uncouple under low nutrient conditions. Our results demonstrate that terrestrial leaf material and its associated nutrients may experience contrasting fates among lakes in the boreal landscapes with possible effects on lake nutrient cycles.