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

Abstract

In order to quantify macrophyte cover and standing crop, studies were performed in a main river and two tributaries in Brittany. Species and total plant cover were assessed using a pin-point method over a period of nearly three years. Spring and summer standing crops in the smaller tributary were collected along 1-m-long stretches, while, in the main river and the larger tributary, summer mean biomass was obtained from quadrats chosen from previous sampling on average macrophyte cover. Seasonal changes were found to be dependent on available light. They were more important in the tributary than 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, Callitriche spp. (C. hamulata, C. platycarpa and C. obtusangula). In sunny stretches, 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 m² (gDW/m²) in shaded and 58 gDW/m² in sunny stretches. In the main river, the average biomasses (with reduced standard deviation) were 96 and 187 gDW/m² for shaded and sunny stretches, respectively. These results are explained with regard to plant life strategies and species morphology; the consequences for fish habitat are pointed out.