High-Frequency Monitoring System
We monitored high-frequency light absorbance at multiple depths in FCR
using a s::can Spectrolyser UV-Visible spectrophotometer (s::can
Messtechnik GmbH, Vienna, Austria). This spectrophotometer was coupled
with a multiplexor pumping system (’MUX’ from MultiplexÔ, LLC; for
technical details on the multiplexor pumping system and the sensor
setup, refer to Birgand et al. 2016 and Figures S1-S3). The MUX pumps
water samples from multiple depths into a flow-through cuvette where the
UV-vis absorbance spectra of the sample are measured by the
spectrophotometer. The system used in our study collected measurements
of light absorbance every 2.5 nm wavelengths from 200 nm to 732.5 nm
(optical path length of 10 mm) approximately at an hourly time step for
six monitoring depths in the reservoir.
The MUX system was used to collect high-frequency data during two time
periods: reservoir turnover (“Turnover Deployment”) and the initiation
of HOx operation ( “Oxygen On Deployment”). The Turnover Deployment
captured the natural oxygenation and mixing processes that occurred
during reservoir turnover and lasted from 16 October to 9 November 2020.
In this study, fall turnover was defined as the first time when the
temperature differential between 0.1 m and 9 m depths in the reservoir
was <1 oC after summer stratification
(following McClure et al. 2018), which occurred on 2 November 2020.
During this time period the HOx system was on, so the hypolimnion was
oxic before turnover, but the reservoir was thermally stratified. The
Oxygen On Deployment was conducted between 26 May and 21 June 2021;
during that time the HOx system was initiated on 11 June 2021 at 11:00
EDT. This deployment captured the engineered oxygenation and mixing
processes resulting from the initiation of HOx operation. The reservoir
was thermally-stratified and the hypolimnion was anoxic (DO <
1 mg/L) prior to HOx operation and thus, while HOx operation added
oxygen to the hypolimnion, we observed a limited increase in DO
concentrations due to high chemical oxygen demand. The HOx system
induced internal mixing within the hypolimnion, but the overall thermal
stratification of the reservoir was not affected.
We took multiple steps to limit the influence of fouling of the internal
components of the MUX system, due to precipitation of Fe in contact with
oxygen in the measuring cuvette. Between each pump cycle, deionized
water was flushed through the system. At the end of each pump sequence
(one sample from each depth), dilute hydrochloric acid (5%) was
automatically pumped through the system and allowed to sit in the
flow-through cuvette for approximately 2 minutes. We also collected a
reference measurement in air at the end of each cycle, which was useful
in determining the extent of fouling. Despite these efforts, some
fouling was still evident during certain time periods (see Figures
S4-5). Fouling was most pronounced in the lower wavelengths (200-250 nm;
see Figures S6-7) and therefore we removed values for wavelengths less
than 250 nm before fitting PLSR models.