Seasonal aquatic macrophytes reduce water temperatures via a riverine canopy in a spring-fed stream

Abstract

Maximum water temperatures in streams throughout the western USA typically occur in late summer and early autumn, coinciding with low stream flow. However, in the spring-fed Big Springs Creek in northern California, where constant-temperature groundwater springs provide relatively stable stream flow throughout the year, peak water temperatures and maximum diurnal variability occur in spring. We attribute this anomaly to the riverine canopy provided by emergent aquatic macrophytes (e.g., Polygonum hydropiperoides and Nasturtium officinale), which mimics the shade function of a riparian canopy. Macrophyte biomass increased 264% between January and August 2011. This increase coincided with a 111% reduction in flow velocity and a 53% increase in stream depth. Solar radiation was reduced by an average of 88% in patches of macrophytes that covered~50% of the water surface during the summer. Decreased solar radiation reduced rates of stream heating, maximum temperatures, and temperature variability. We tested the riverine canopy hypothesis analytically based on a 2-dimensional hydrodynamic and water-temperature model. The model predicted that emergent aquatic macrophytes reduce maximum water temperatures by an average of 5.17C (p < 0.001) during late summer, when water temperatures in northern California streams typically increase. Our study shows the influence of a riverine canopy on naturally occurring temporal patterns of water temperature in a spring-fed stream. Our results could inform basin-scale management or regulatory strategies to address water-temperature conditions.

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

“…Without the shading effects of riparian vegetation, stable and nutrient‐rich streamflow in the low‐gradient (mean slope = 0.003), wide, and shallow (mean bankfull width to depth ratio = 84; Nichols et al, ) Big Springs Creek promotes extraordinary primary productivity, principally characterized by seasonal growth of native emergent and submerged rooted aquatic macrophytes (Willis et al, ). The macrophyte species assemblage in Big Springs Creek is dominated by emergent smartweed ( Polygonum hydropiperoides ) and watercress ( Nasturtium officinale ), as well as submerged northern watermilfoil ( Myriophyllum sibiricum ).…”

“…The narrower and deeper downstream reaches of the Shasta River (Nichols et al, ) also exhibit a complex macrophyte species assemblage dominated by submerged species including pondweed (Potamogeton pectinatus ) and white water buttercup (Ranunculus aquatilus ; NCRWQCB, ). Macrophyte communities in Big Springs Creek and the Shasta River exhibit pronounced growth and senescence cycles generally characterized by minimum macrophyte biomass in winter and early spring and maximum biomass in late summer and early fall (Willis et al, ).…”

“…Although velocity reductions associated with seasonal macrophyte growth did not affect the location of the water temperature variability antinode at the mouth of Big Springs Creek, the hydraulic and shading effects of progressively emerging macrophytes did influence seasonal patterns of water temperature magnitude and variability at the primary antinode location. As shown by Willis et al (2017), the effects of macrophyte growth on water temperature patterns in Big Springs Creek can be generally segregated into spring and summer temporal periods. During spring, much of the emergent macrophyte community has yet to emerge above the water surface.…”

“…More recently, seasonal macrophyte growth has been identified as an important habitat feature associated with juvenile salmonid rearing, due to its various and numerous effects on hydrology, temperature, and prey. For instance, Lusardi, Jeffres, and Moyle () found that macrophytes reduced water velocity and increase prey availability for foraging fishes when compared with other typical bedforms such as gravel, whereas Willis et al () found that macrophytes, via their canopy, contributed to shading and cooling of stream temperature with important implications for salmonid management. Despite these recent studies, the broader spatial dynamics and effects of macrophytes on hydrology and temperature at the landscape scale and the implications for cold‐water biota management, particularly in the face of a changing climate, are relatively unknown.…”

“…Macrophyte growth patterns affect hydraulic, hydrogeomorphic, and water temperature processes in numerous ways. For example, macrophyte growth typically increases channel resistance to flow (De Doncker et al, 2009;Green, 2005a), progressively reducing flow velocity (Champion & Tanner, 2000) and increasing flow depth per unit volume of discharge (Willis et al, 2017). Patches of aquatic macrophytes also create localized (Green, 2005b) and reach-scale (Nepf, 2012) zones of low velocities bounded by areas of high turbulence, which helps trap mobile bed sediments (Cotton, Wharton, Bass, Heppell, & Wotton, 2006).…”