Fine-Scale Patterns of Water Velocity within Macrophyte Patches in Streams

Abstract

Rooted macrophytes in temperate lowland streams are often distributed in monospecific patches which control flow, carbon fluxes, and the abundance of invertebrates and fish. Small high-resolution hot-wire probes provided detailed measures of flow velocities within and around macrophyte patches of four plant species of contrasting morphologies in Danish streams. Flow velocity declined rapidly at the surface of the plant patches and species with large leaf area on bushy shoots (e.g. Callitriche cophocarpa and Elodea canadensis) reduced the flow more than species with streamlined, strap-formed leaves (e.g. Sparganium emersum). Variable flow-resistance resulted in flow velocities at 2 cm above the sediment which were 11-fold lower inside C. cophocarpa patches than upstream of the patches, whereas no significant differences in near-bed velocities were found inside and outside the more open patches of S. emersum. The reduced velocity within flow-resistant patches remains sufficiently fast (i.e. >1 cm s -1 ) to prevent carbon depletion and oxygen accumulation and should be optimal to photosynthesis and plant growth. The deflected flow is accelerated around the patches and contributes to form a mosaic of highly variable plant cover, flow and substrate conditions. These relations have important implications for flow resistance, areal expansion of patches and spatial variability of sediment and invertebrate composition in streams.