NUTRIENT TRANSFER BETWEEN PARAFLUVIAL INTERSTITIAL WATER AND RIVER WATER - INFLUENCE OF GRAVEL BAR HETEROGENEITY

1. Nutrient concentrations in interstitial water springs at the downst ream ends of two gravel bars in the River Rhone were measured in order to estimate the importance of nutrient inputs [nitrate, dissolved org anic carbon (DOG), biodegradable and refractory fractions of DOG] from the parafluvial zone (saturated sediments adjacent to the wetted chan nel, i.e. interstitial habitats of gravel bars) to the river as well a s seasonal variations in these inputs. 2. Compared with surface water, parafluvial water had lower concentrations of DOC (especially the ref ractory fraction) and generally higher concentrations of nitrate. Thes e differences were at their lowest during winter. 3. The influence of gravel bar geomorphological heterogeneity (grain size and spatial dist ribution of fine sediments) on parafluvial inputs was studied in terms of nutrient content and biofilm characteristics along transects acros s the gravel bars. 4. A gravel bar located in a stable area of the riv er had low transversal heterogeneity and acted as a sink for DOC and a s a source of nitrate. The low fine sediment content and the high oxyg enation of interstitial water must have enabled aerobic processes such as mineralization of organic matter and nitrification to occur. 5. in contrast, gravel bars located in a degradation zone of the river had a spatially variable structure, acting as a sink for DOC and nitrate b ecause localized accumulation of fine sediment and ensuing hypoxic con ditions induced anaerobic processes such as denitrification. 6. This s tudy highlights the important influence of geomorphological heterogene ity in gravel bars for nutrient transformations as well as nutrient ex changes between parafluvial interstitial systems and the adjacent rive r over a seasonal cycle.