Seasonal macrophyte growth constrains extent, but improves quality, of cold‐water habitat in a spring‐fed river

Rooted aquatic macrophytes affect abiotic conditions in low‐gradient rivers by altering channel hydraulics, consuming biologically available nutrients, controlling sediment transport and deposition, and shading the water surface. Due to seasonal macrophyte growth and senescence, the magnitude of these effects may vary temporally. Seasonal changes in aquatic macrophyte biomass, channel roughness and flow velocity, were quantified and trends were related to spatiotemporal patterns in water temperature in a low‐gradient, spring‐fed river downstream from high‐volume, constant‐temperature groundwater springs. Between spring and summer, a nearly threefold increase in macrophyte biomass was positively correlated with channel roughness and inversely related to flow velocity. On average, flow velocity declined by 34% during the study period, and channel roughness increased 63% (from 0.064 to 0.104). During the spring and fall period, the location of a minimum water temperature variability “node” migrated upstream more than 4 km, whereas daily maximum water temperature cooled by 2–3°C. Water temperature modelling shows that the longitudinal extent of cold‐water habitat was shortened due to increased channel roughness independent of seasonal surface water diversions. These results suggest that macrophyte growth mediates spatiotemporal patterns of water temperature, constraining available cold‐water habitat while simultaneously improving its quality. Understanding complex spatial and temporal dynamics between macrophyte growth and water temperature is critical to developing regulatory standards reflective of naturally occurring variability and has important implications for the management and conservation of cold‐water biota.