Effects of aquatic macrophytes on organic matter deposition, resuspension and phosphorus entrainment in a lowland river

摘要

1. Although macrophytes play a key role in the structure and functioning of lowland rivers, most of the basic plant, hydrodynamic and sediment-water interactions have only been described qualitatively. We therefore studied quantitatively, the seasonal dynamics of matter deposition and mobilisation inside and outside (free path) a representative patch of arrowhead, Sagittaria sagittifolia, in the lowland River Spree, NE Germany, in August 2006. Our in situ study combined resuspension experiments, a hydrodynamically calibrated erosion chamber and concurrent measurements of the prevailing flow characteristics and bed load. 2. Increasing entrainment rates (E) of particles (ESPM) and total P (ETP), with increments of shear velocity (U*) from 0.53 to 2.42 cm sp#, were significantly higher inside the plant patch than outside. Indeed, ESPM and ETP at the lowest U* were 8- and 12-fold higher inside than outside the patch, reflecting the resuspension potential of the upper nutrient-enriched layer and the extent of pulsed P inputs even at small increases in U*. 3. Vertical distribution of velocity (u) revealed a flow pattern of a mixing layer inside the S. sagittifolia patch, and that of a boundary layer in the free path. The highest gradient of u in the mixing layer was located in the water column at about 0.5 m depth, whereas the highest gradient of u for the boundary layer was found near the riverbed. The maximum of U* (1.65 cm sp#) was only 4 mm above the sediment. 4. A plant mosaic provides a low-energetic environment promoting extensive particle trapping and the accumulation of a fine-grained, nutrient-enriched sediment, and forming a large resuspension potential. Consequently, after plant decay and the concomitant increase of U* this material is preferentially entrained at higher rates. Hence, the key role of submerged macrophytes in lowland rivers is more directly related to modifying the dynamic equilibria between vegetation trapping and resuspension, than to the retention of nutrients, particularly P, and the reduction of P loads downstream to other waters.