Comparing field and laboratory breakdown rates of coarse particulate organic matter : Sediment dynamics mask the impacts of dissolved nutrients on CPOM mass loss in streams

Microbial decomposition of leaf litter and woody debris is stimulated by an increase in dissolved nutrients. Thus, breakdown rates of coarse particulate organic matter (CPOM) might be useful for assessing the impact of eutrophication on stream ecosystems. The objective of this study was to compare breakdown rates of leaf litter, toothpicks, and twigs in an upland stream (Ilm, Thuringia, Germany) and two of its tributaries to determine the best CPOM type for investigating moderate eutrophication effects on CPOM mass loss. A total of 160 fine-mesh bags (1 mm) that excluded invertebrate shredders were exposed at 4 sampling sites from December 2004 to May 2005. Simultaneously, CPOM was incubated in aquaria filled with ambient stream water from each of the 4 study sites under standardized conditions. In aquaria, toothpicks showed increasing breakdown rates in treatments with increasing dissolved nutrient concentrations, whereas no significant effects on breakdown rates of beech leaves and twigs were detected. In the field, physical fragmentation due to high velocities and sediment transport at 3 sites dominated by gravels accelerated leaf breakdown rates. In the sandy tributary, bags were covered by deposited sand that might have decreased leaf breakdown rates despite high nutrient concentrations. Mass loss of the woody debris (toothpicks and twigs) was affected by abrasion in the gravel streams and by burying in the sand stream, but to a lesser extent than leaf litter. Thus, physical fragmentation by abrasion and grinding of CPOM by moving sediment can mask the effect of nutrients on breakdown rates. Because of a fast breakdown rate and resistance to fragmentation, toothpicks could be used as a tool to assess water quality in streams with low to intermediate flow velocities.