The flow of coarse particulate allochthonous and autochthonous organic matter in a side arm of the River Rhone
This study concerns a side arm of the River Rhone, 20 km upstream from the city of Lyons, southeastern France. The arm opens into the channel of the river downstream, but it is closed and fed by groundwater upstream. Terrestrial particulate organic matter enters the water through the ecotone of the banks. This flow and that of carbon, nitrogen and phosphorus were measured, using sediment traps and a grab, by the monthly sedimentation of allochthonous organic matter on the bottom, corrected for the time of exposure under the water. The input seemed to be somewhat greater than in the temperate woodland streams described in the literature. The greatest input of terrestrial leaves occurred in November, the greatest input of wood and other debris in April, presumably as a consequence of the flooding of the undergrowth. The arm was narrower upstream than downstream (respectively 10-25 and 60-80 m in width). Though the banks were equally and densely forested, the bottom sediment received more coarse particulate matter upstream, and this matter was equally distributed. In contrast, less organic matter was deposited along the banks downstream and still less in the middle of the arm, where partially, decomposed matter was more abundant. These differences may be explained by the ratio between the banks and the water area, by a certain transport during the floods, and by the faster processing of particulate matter closer to the channel. Hence there seems to be a decreasing gradient of particulate organic matter as one goes downstream along the arm. The heterotrophic communities in the arm were fed successively by the decay of autochthonous submerged macrophytes such as Potamogeton in summer and autumn, then by that of terrestrial leaves (mostly alder, poplar and willow with respectively fast, medium and slow processing velocities) and emergent macrophytes such as Phragmites, during winter and spring. A comparison of invertebrate biomass and community structure shows that shredders and collectors are more numerous in winter and spring, white terrestrial leaves are being processed.