INFLUENCE OF SUBMERGED MACROPHYTES ON SEDIMENT COMPOSITION AND NEAR-BED FLOW IN LOWLAND STREAMS

1. Submerged macrophytes have important physical and structural effect s on lowland streams. This study investigated the ability of submerged macrophytes to modify the near-bed now and to retain mineral and orga nic particles in patches of four common macrophytes in shallow Danish streams during mid-summer. 2. In dense patches of Callitriche cophocar pa and Elodea canadensis, where near-bed velocity was reduced, the sed iment surface was markedly raised and enriched with fine particles. In dense patches of Ranunculus peltatus, fine sediments were deposited a mong rooted shoots in the upstream part of the patches, while erosion and coarse sediments prevailed in the downstream part of the patches b ecause of the strong vortices that formed at the rear and moved up und er the trailing canopy. The open canopy of Sparganium emersum, with it s streamlined leaves, had little effect on flow and sediment. 3. Patte rns of sediment deposition and composition were closely related to the morphology and canopy structure of plant species and the presence of low velocity above the sediment among the rooted shoots. The mineral p articles retained probably originate from bed-load, and the enrichment with finer particles within the patches probably results mainly from size-selective processes during erosion and transport of particles rat her than during deposition. The mixed sediment composition within patc hes suggests that the flow-resistant shoots generate an environment co nducive to deposition of all transported particles. 4. Fine sediments within macrophyte beds contained high concentrations of organic matter , carbon, nitrogen and phosphorus. The wide scatter in the relationshi ps between mineral grain size and the content of organic matter and nu trients reflects the spatial and temporal complexity of erosion, trans port and sedimentation of mineral and organic particles. 5. Enrichment of sediment within macrophyte beds relative to the surrounding substr atum ranged from 780 g organic matter m(-2), 30 g N m(-2) and 25 g P m (-2) for the Bow-resistant dense canopies af Callitriche cophocarpa to 150 g organic matter m(-2) 6.6 g N m(-2) and 3.4 g P m(-2) for the op en canopies of Sparganium emersum. Retention of nutrient-rich particle s within the macrophyte beds is probably of limited importance for pla nt growth in most lowland European streams, because macrophyte growth is rarely nutrient limited.