A perspective on leaf litter breakdown in streams

Leaf litter breakdown, a critical ecosystem level process in streams and other aquatic environments, has been conceptualized using models borrowed from terrestrial systems. We argue that current views of the process in freshwaters need to be conceptually improved. Specifically, we think the idea that breakdown proceeds in three distinct temporal stages (leaching, conditioning, fragmentation) has been overemphasized. Leaching, the massive loss of soluble leaf components within 24 h after immersion, is generally considered to constitute a well-defined first stage. Recent evidence suggests, however, that the initial solute losses are largely an effect of the unnatural drying procedures to which experimental leaves are normally subjected. Fresh leaf litter does lose solutes when immersed, but gradually throughout the breakdown process rather than instantly upon wetting. Conditioning, the second breakdown stage, describes the enhancement of leaf palatability for detritivores by microbial colonization, and is thus ultimately targeted towards a group of organisms (which contribute to litter degradation) rather than addressing the breakdown process per se. Furthermore, conditioning implies a key role for detritivorous invertebrates and underrates the established direct degradative activity of microbial decomposers. If, thus, leaching and conditioning are not generally useful operators to describe portions of the litter breakdown process in freshwaters, the traditional concept, which emphasises leaching, conditioning and fragmentation as three sequential stages, loses much of its appeal. Consequently, we propose a nea conceptual model, in which the coincidence and interplay of various subprocesses of litter breakdown is more strongly recognized. In this model, me propose to view the process in terms of the products of litter breakdown-as a complement to the usual perspective which focuses on litter mass loss. Six primary breakdown products are considered: bacterial, fungal and shredder biomass; dissolved organic matter; fine-particulate organic matter; and inorganic mineralization products such as CO2, NH4+ and PO43-. We present a scheme illustrating the hypothesized formation of these products throughout breakdown. However, to improve understanding of the process, application of the proposed conceptual framework in experimental work is necessary.