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.
 , 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 t..., , This data set and code is associated with Gardner et al. 2019 Limnology and Oceanography and Gardner et al., 2023 Limnology and Oceanography Letters. In the data folder there are raw and cleaned drifter datasets from the Upper Mississippi River (UMR) in Wisconsin and Neuse River (NR) in North Carolina (NR\_drifter\_clean.csv; NR\_drifter\_raw.csv; UMR\_drifter\_clean.csv; UMR\_drifter\_raw.csv) as well as a corrected version of the drifter depth data (NR\_drifter\_depth.csv; UMR\_drifter\_depth.csv). Missing data is identified as NA and -888 in the case of lux data.

Columns in NR\_drifter\_clean.csv; NR\_drifter\_raw.csv; UMR\_drifter\_clean.csv; UMR\_drifter\_raw.csv:
dt=date time (UMR in CDT, NR in EST);
temp=temperature in C from Hydrosphere sensor;
accelerometer data=
ax,
ay,
az,
gx,
gy,
gz,
ax.c,
ay.c,
az.c;
distance=meters along a river interpolated from sparse GPS coordinates;
velocity=m/s calculated from the distance and time traveled in between coordinated;
name=the ID of the ...