Littoral flow rates within and around submersed macrophyte communities

SUMMARY The magnitude and range of water flow rates were measured within and adjacent to plant beds at different depths and among different dominant submersed plant species in the littoral zones of two lakes with contrasting morphometry. There was very little variability in within‐bed flow rates, either for locations within or among beds. However, when significant differences occurred in within‐bed flow rates, the higher rates occurred predominately near the bottom of the Scirpus subterminalis bed where the plant surface area to water volume ratio was lowest. Factors such as bed depth and dominant species had little effect on within‐bed flow rate variance. Flows external to the plant beds were dissipated within 10–15 cm of the outer plant‐bed boundary even under severe external flow‐rate conditions (flow rate ∼ 30cms −1 ). The mean within‐bed flow rate was 0.07cms −1 and individual experiment means ranged from 0.03 to 0.46cms −1 . These flow rates resulted in estimates of laminar flow boundary layer thickness, 1 mm from the leading edge of the leaf, ranging from 9.1 to 2.3mm. These estimates are much larger than submersed macrophyte leaf thicknesses themselves (<1 mm).