DOI: 10.5061/dryad.dfn2z3587
title: Light and water quality observations from neutrally buoyant drifters in rivers

description: 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.  
description: 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
 m<sup>3</sup>s<sup>-1</sup>), but drifter
 deployments on the Neuse River occurred over a wider range of flow
 conditions (24-365 m<sup>3</sup>s<sup>-1</sup>).
 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 z<sub>eff </sub>and
 z<sub>fun </sub>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).
description: 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 drifter; type=surface or subsurface drifter; hr=hour of the day;
 date=date; time=time in local time zone; channel=channel (1-3) of the UMR.
 This only applies to UMR data; depth=depth from bottom of drifter to water
 surface in meters estimated from pressure; depth.top=corrected depth at
 the top of the drifter in meters; lux=light in lux (lumens/m2); doy=doy of
 year; doy.name=doy of year + drifter name for unique identifier; river=UMR
 or NR; deg\_from\_vert=degrees tilted from vertical axes;
 deg\_from\_vert\_est=missing data filled in the median deg\_from\_vert;
 lux\_tilt=cosine corrected lux for sensor tilt; tilt\_flag=flag data that
 is tilting too much to reasonably correct; lux\_tilt\_rm=new column for
 lux\_tilt setting flagged data to NA; error\_flag=flag bad data due to
 trapping in wood jams, loss of neutral buoyancy, etc.; diff.time=time
 between measurements in minutes; solar.dt=solar date time; zenith=solar
 zenith angle given the date-time and location; solar.time=time of day in
 solar time; sunrise=time of sunrise; sunset=time of sunset; start.dt =
 date when drifter deployment started diff = time difference between
 consecutive observations (minutes) Additional columns in
 NR\_drifter\_depth.csv; UMR\_drifter\_depth.csv: pressure = pressure
 measured by internal sensor in hydrosphere drifter (mbar) DO.p = percent
 saturation of dissolved oxygen; DO.mg = Dissolved Oxygen (mg/L). Note this
 data field has not been quality controlled and should be treated with
 caution; lat = latitude in decimal degrees of drifter location taken using
 field GPS and radio tracker; long= longitude in decimal degrees of drifter
 location taken using field GPS and radio tracker; time.n = arbitrary time
 counter for each data point (row) for each drifter deployment csum =
 cumulative sum of lux values over each drifter deployment river.depth =
 overall water depth of river channel at discrete points in thalweg
 measured in the field