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

Abstract

Summary 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 s−1, 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 s−1) 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.

Complex hydromorphology of meanders can support benthic invertebrate diversity in rivers

“…The magnitude of lateral sediment redistribution depends primarily on seasonal changes in discharge, as well as macrophyte biomass, density, and distribution. In lowland rivers, deposition may require relatively high macrophyte densities, which only occurs over a 2-3 week peak growth period during the summer (Kleeberg et al, 2010). As plants decay, the accumulated particulate matter will gradually be resuspended with increasing bed shear stresses during higher winter discharges.…”

“…As plants decay, the accumulated particulate matter will gradually be resuspended with increasing bed shear stresses during higher winter discharges. Even a small increase in stresses may lead to significant resuspension and entrainment of fine sediments into the water column (Kleeberg et al, 2010). Morphological changes induced by macrophytes during unvegetated periods of the year are therefore reversed.…”

Short term changes in pore water chemistry in river sediments during the early colonization by Vallisneria spiralis

“…The canopy of macrophytes can, however, favour the local reduction of water flow, enhance sedimentation rates and retain suspended particles, resulting in increased demand of electron acceptors within sediments (Sand-Jensen, 1998;Kleeberg et al, 2010). Moreover, live roots can exude labile organic compounds while decaying roots, stems and leaves are a source of both labile and refractory organic matter (Karjalainen et al, 2001;Lomstein et al, 2006).…”

Diversity and biomass of native macrophytes are negatively related to dominance of an invasive Poaceae in Brazilian sub-tropical streams

“…Macrophyte stands house a great diversity of organisms that rely on them for food (Casatti et al, 2003;Pelicice and Agostinho, 2006) or shelter and refuge (Agostinho et al, 2007). They also participate in biogeochemical (Urban et al, 2009) and hydrodynamic (Kleeberg et al, 2010) processes and determine many abiotic features of water, such as dissolved oxygen concentrations (Caraco et al, 2006), underwater light (Rodríguez et al, 2012) and turbidity (Horppila and Nurminen, 2003).…”