There’s more than one way to shade a river: Contrasting influence of canyon orientation and water clarity on aquatic invertebrate densities
Light availability fundamentally impacts the structure and function of all ecosystems. Uniquely in aquatic ecosystems, however, light availability on the benthic layer, or bed surface, can be controlled by two factors: by light insolation or the amount of light available at the water surface, and by turbidity or water clarity affecting light subsurface penetration to the bed. In this study, we utilized the 21-km reach of the Little Colorado River, a spring-fed, canyon-bound, travertine system in northern Arizona, USA, as a case study of the contrasting effects of water surface insolation (as affected by canyon orientation) and water clarity (as affected by travertine deposition) on aquatic macroinvertebrate communities. Using sticky traps, light traps, and benthic nets, we sampled along the entirety of the Little Colorado River’s perennial course, from its tightly canyon-bound but clear-water spring source to its more open but cloudy-water confluence with the main stem Colorado River in Grand Canyon.
Contrasting somewhat with expectations, macroinvertebrate densities did not follow predictable patterns with respect to hydraulic geometry and the River Continuum Concept (i.e., opening of the channel and less shading downstream). Instead, densities decreased gradually from the upstream spring source to ~9 km downstream, where they decreased abruptly, increased again over the next ~2 km, then decreased gradually over the remaining 9 km to the river mouth. The gradual trends overlap well with longitudinal patterns of water clarity in the Little Colorado River, whereby gradually-increasing levels of travertine deposition transform the river visually from crystal clear at its spring source to a thick, milky blue at its mouth. The abrupt drop in macroinvertebrate densities 9 km downstream from the spring source coincided with a bend in the canyon’s general orientation from North-South to East-West, with concomitant effects on insolation reaching the water surface. Thus, macroinvertebrate densities in the Little Colorado River do appear to be controlled by light availability, but uniquely as dictated by the two contrasting and occasionally interacting effects of water clarity and canyon orientation.