Importance of microhabitat characteristics in the macrobenthos microdistribution of a large river reach

1. The importance of microhabitat characteristics for macroinvertebrate microdistribution was studied within a reach of the main channel of the Upper Rhône River (France). 9 locations (3 on the left hand side, 3 across the middle, and 3 on the right hand side) were sampled twice on two dates (as duplicates in June and July 1982). From 19 variables, including substratum and hydraulic parameters, 14 were selected to describe the physical habitat of each location. 47 macroinvertebrate species were collected by means of dredging with a modified Rallier du Baty dredge. The habitat-fauna co-structure was studied using co-inertia analysis. 2. The river reach was transversally asymmetrical : shallow on one side (up to 2.80 m) with coarse sediments rich in periphyton, and much deeper on the other side (up to 4.80 m) with a gravel bottom rich in organic particles. In the middle of the channel these conditions were intermediate, but the current was stronger. Velocity and the other hydraulic parameters varied independently of the substratum characteristics. This independence is a consequence of old man-made impoundments (inherited structures) and perhaps of present paving of the substratum also. 3. The macroinvertebrate fauna was closely linked to this transverse asymmetry of the reach, with the highest richness and abundance to the shallow side of the channel and the lowest species richness in the rapid central part. The most significant faunistic differences were linked i) to water depth, acting on both hydraulic parameters and periphyton (by light transmission), ii) to the inherited old bottom substratum, and iii) to a lesser extent, to the present current velocity