Light and water quality observations from neutrally buoyant drifters in rivers

Vertical motion is an important driver of sunlight exposure in aquatic environments, shaping the growth and fate of materials and organisms. We derive a simple model accounting for turbulent depth fluctuations of particles to predict the depth that contributes the most sunlight exposure (effective depth) as well as the single depth that, if measured at one place over time, produces the same total sunlight exposure as a moving particle (functional depth). Field measurements of light and depth in rivers using neutrally buoyant drifters and buoys validate our model. Effective depth varied from 0.1-1.5 m below the water surface and was ~30% of the overall water depth on average. Functional depth varied from 0.67-2.3 m and was ~50% of the overall water depth on average. Functional and effective depth are physically based concepts incorporating turbulent motion, spatial variability, and water clarity offering new approaches to characterize light exposure in aquatic environments.

Methods
Drifters (HydroSphere, Planktos Instruments, LLC, Morehead City, North Carolina, USA) were ballasted so that the light sensors (HOBO Pendant, Onset, Massachusetts, USA) faced up. Lux, depth, and tilt angle were logged every 0.5 to 2 minutes and light was cosine corrected with the tilt angle. Neutral buoyancy was achieved with mass adjustments prior to deployments based on measured salinity and temperature. Nine deployment-days were collected in the Neuse River where 1-2 drifters were released and recorded data over distances of 5.3-43 kilometers lasting 5-35.5 hours. Sixteen deployment-days were collected in the Upper Mississippi River and limited to a maximum duration of 8 hours due to downstream dams, covering a distance of 1.2 to 9.4 kilometers. Drifter deployments on the Mississippi River occurred over a narrow range of flow conditions (821-1665 m3s-1), but drifter deployments on the Neuse River occurred over a wider range of flow conditions (24-365 m3s-1). The buoys were anchored to the bed but able to rise and fall with the water surface so that light was always measured at the same depth relative to the water surface.

We evaluated zeff and zfun using modeled sensitivity analysis and field measurements of light and depth using neutrally buoyant drifters (HydroSphere, Planktos Instruments, LLC, Morehead City, North Carolina, USA) and moored buoys. Data were collected on the Upper Mississippi River in Wisconsin, and the Neuse River in North Carolina between 2014 and 2016. Light was measured as illuminance, the light visible to humans in units of lux, as a proxy for more biologically relevant spectra such as UV or PAR. The drifters are 0.4 m diameter spheres responding only to turbulent eddies with characteristic length scales ≥ 0.4 m (Rutherford 1994; D'Asaro et al. 1996), and are not a perfect analog for small particles or solutes. In rivers, maximum eddy size is typically limited by depth (Rutherford 1994; Nadaoka and Yagi 1998; Jirka 2001) which was 1–10 m in our study reaches. Therefore, the largest turbulent eddies were 2–20 times greater than the drifter size suggesting our drifters can respond to multiple scales of turbulence. Our previous work showed drifters were an effective proxy for light exposure of moving particles. For more drifter details, see supplementary information (Figure S2), Ensign et al. (2017), and Gardner et al. (2019).