Macrophyte cover and standing crop in the River Scorff and its tributaries(Brittany, northwestern France): scale, patterns and process

In order to quantify macrophyte cover and standing crop, studies were perfo rmed in a main river and two tributaries in Brittany. Species and total pla nt cover were assessed using a pin-point method over a period of nearly thr ee years. Spring and summer standing crops in the smaller tributary were co llected along 1-m-long stretches, while, in the main river and the larger t ributary, summer mean biomass was obtained from quadrats chosen from previo us sampling on average macrophyte cover. Seasonal changes were found to be dependent on available light. They were more important in the tributary tha n in the main river. In the tributary, the species seasonal succession was: Ranunculus penicillatus, Oenanthe crocata, Apium nodiflorum. In the river, the following succession was observed: Bryophytes, R. penicillatus, Callit riche spp. (C. hamulata, C. platycarpa and C. obtusangula). In sunny stretc hes, minimum and maximum values of macrophyte cover were 17% and 89% of the stream bed, and 37% and 95% of the river bed, respectively. In the stream, mean summer standing crops were 28 g of dry weight per m(2) (gDW/m(2)) in shaded and 58 gDW/m(2) in sunny stretches. In the main river, the average b iomasses (with reduced standard deviation) were 96 and 187 gDW/m(2) for sha ded and sunny stretches, respectively. These results are explained with reg ard to plant life strategies and species morphology; the consequences for f ish habitat are pointed out.