Vegetation is increasingly recognized for its importance in affecting water flow and thereby shaping terrestrial, tidal and fluvial landscapes. However, most current approaches overlook the interactive bio-physical feedbacks between vegetation and hydrodynamics. The implications of these interactions for water flow regulation are hence largely unknown. We developed a spatially-explicit mathematical model on the feedback interaction between aquatic vegetation growth and flow redistribution in streams. Model predictions were compared to field observations on seasonal variations in macrophyte cover and discharge from two chalk streams over two annual growth cycles, involving water level and flow velocity measurements within and around submerged vegetation dominated by water crowfoot (Ranunculus spp.).
The model predicts that feedback interactions between vegetation growth and flow redistribution control flow velocities and water levels in streams, in the face of varying water discharge. Data from natural chalk streams supported these predictions, showing that submerged macrophytes adjust their vegetation cover in response to changing discharge, and as such they maintain mean water depths and flow velocities relatively constant over time, both within and between vegetation, despite variations in discharge. Our results highlight that the interplay of vegetation growth and hydrodynamics leads to spatial separation of the stream into densely vegetated, low-flow zones and low-density, high-flow zones. This self-organization process effectively decouples the relation between discharge and both water levels and flow velocity, ensuring the delivery of two important ecosystem services: buffering of water level changes for the stream community, and efficient flow conveyance in fast-flowing channels throughout the annual growth cycle. Moreover, the self-organization process increases the resistance of stream ecosystems by enabling aquatic vegetation to effectively adapt to changing discharge regimes, due to e.g. climate change or land use change in river catchments. Our results reveal an important link between the self-organization processes that are characteristic of undisturbed stream ecosystems and the ecosystem services these streams provide.