Impact of a fluviatil channel by a former gravel-pit, on the longitudinal distribution of benthic macroinvertebrates
The main objective of this study was to determine the influence of a former gravel-pit of the Allier River, which was introduced into the stream in 1989, on the longitudinal distribution of its macroinvertebrates. Three successive riffles, one situated upstream and two others downstream of the gravel-pit, were studied for 900 m. Data on grain-size, biotopes and hydraulic variables were recorded for two successive seasons. Grain-size in the riffles showed a significant decrease in the median size of the particles, from 28 mm upstream to 20 mm downstream of the gravel-pit, which traps the coarser sediments. With respect to the macroinvertebrates, the observed decrease in species richness and density from upstream to downstream (Ephemeroptera, Plecoptera and Trichoptera were particularly sensitive) was less important for the third riffle. The taxonomic differences between the upstream and downstream capture zones, observed in the spring, were less marked in the summer. From riffle 1 to riffle 2 and for both seasons, we noted a transition from a mesotrophic - ß-mesosaprobic status to an oligotrophic - oligosaprobic status, the third riffle being intermediate. The gravel pit, therefore, acted as a decantation basin. Regardless of season, the macroinvertebrates collected upstream were essentially deposit feeders, filterers and predators, whereas downstream they were essentially scrapers and shredders. This break in the river continuum faded with downstream distance, revealing the resilient nature of these organisms.