A Case for Using Litter Breakdown to Assess Functional Stream Integrity

Assessment of the condition of ecosystems is a critical prerequisite for alleviating effects of the multiple anthropogenic stresses imposed on them. For stream ecosystems, a multitude of approaches has been proposed for this purpose. However, they all rest on the assessment of structural attributes, even though it is generally recognized that adequate characterization of ecosystems requires information on both structure (pattern) and function (process). Therefore, we propose a complementary approach to stream assessment based on evaluating ecosystem-level processes. Leaf litter breakdown is a prime candidate to consider in this context. This is because of the pivotal role that allochthonous litter plays in streams, the demonstrated effects of anthropogenic perturbations on litter breakdown, and the relative ease of implementation. Leaf breakdown is governed by a variety of internal and external factors that complicate the partitioning of effects due to anthropogenic stress and natural variability (background noise), thus potentially limiting the sensitivity and robustness of litter breakdown assays. However, internal regulation factors can be controlled by standardizing assessment procedures, while variability due to external factors can be accounted for by stream classification and/or a comparative approach (e.g., downstream–upstream comparisons). Composite parameters such as ratios of breakdown rates in fine-mesh and coarse-mesh bags may further increase the power of litter breakdown assays. Analyses may also be extended to include both leaf-associated decomposer assemblages (i.e., structural measures) and processes (i.e., additional functional measures). Significant efforts are required for developing standard assessment schemes as refined as extant procedures based on structural stream attributes (e.g., structure of macroinvertebrate assemblages). These efforts are nevertheless worthwhile in view of the new dimension that is added to current assessment procedures when functional elements are incorporated.