Plant Litter Decomposition in Intermittent Rivers and Ephemeral Streams

International audienceIntermittent rivers and ephemeral streams (hereafter IRES) are waterways that temporarily cease to flow and/or dry up. They represent half the length of the global river network and are expanding in time and space in response to global change. The hydrological regimes of IRES are characterized by alternating flowing, non-flowing and dry phases, which translate to varying importance of in-stream litter accumulation, processing and downstream transport. Decomposition agents, processes and rates dramatically change among these hydrological phases, leading to decomposition dynamics that differ markedly from perennial rivers and streams. As a result, IRES have a specific “biogeochemical heartbeat” characterized by high temporal and spatial variability of leaf decomposition, and so they can be idealized as pulsed bioreactors. The ecological effects of flow cessation and drying are sometimes visible far beyond rewetting, generating “legacy effects” that become apparent even during later flowing phases. Rewetting events can represent “hot moments” of litter decomposition due to the intense biological and physical activities, generating pulses of transport and decomposition. Upscaling the abundant reach-scale knowledge to larger river-network scales is probably one of the most challenging but timely paths for future research