Experimental flume study on Potamogeton natans and Ranunculus fluitans macrophytes: impact of hydrodynamics on 15N-ammonium uptake rates

By means of incubations in a flume tank using 15N enriched ammonium label we investigate the concomitant effects of (i) different morphologies and specific surfaces, (ii) bulk water flow successively set at 0.3 and 0.1 m s-1 and (iii) three patch configurations on the rates of 15N-ammonium uptake rates for two macrophytes species: Potamogeton natans and Ranunculus fluitans. Because recent studies have shown that the rate of ammonium uptake by seagrass specimens depends on hydrodynamic factors such as shear stress and turbulence -which notably influence the rate at which ammonium is delivered to the surface of the leaves- these incubations were performed together with high resolution flow velocity measurements. Results of our experiments show higher 15N-ammonium uptake rates at the edges of patches in some of the incubations involving the two species. Currents and turbulence which are supposed to be the highest at the edges of the patches can explain these patterns. R. fluitans showing the highest specific surface of the two species is also the most efficient at obtaining 15N-ammonium from the water column in all experimental conditions. Results also show enhanced 15N-ammonium uptake rates with increased bulk water flow for both species. This result which is consistent with that of previous studies on seagrass communities is explained by turbulent energy and stresses factors that increase both advection of nutrients through the community and rates of diffusion at the surfaces of the specimens as flow velocity increases. Finally, flume configuration only affects uptake rates for R. fluitans at lower bulk flow velocity (0.1 m s-1). Higher 15N-ammonium uptake rates are observed for this species in the configuration when the flume is half filled compare to the configuration when the flume fully filled with both species. No significant difference is observed between the two configurations for P. natans. Our findings provide an improved insight in the concomitant effects of factors and processes occurring in streaming rivers on the nitrogen uptake by the two studied macrophytes. This is essential for incorporating the role of macrophytes in the scope of integrated water management and for furthering knowledge of the structure and functioning