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Ref. No. [UMCES] CBL 2016-015
ACT VS16-06
4
15week deployment and generated 9859 observations based on its 15 minute sampling interval for
a data completion result of 100%.
The measured DO range from our 142 discrete reference
samples was 10.249 to 14.007 compared to the full dynamic range of 9.01 to 14.03 mg/L observed
by the EXO2. The average and standard deviation of the measurement difference between the
EXO2 and reference samples over the total deployment was 0.37 ±0.10 mg/L with a total range of
-0.06 to 0.56 mg/L. A drift rate in accuracy, estimated by linear regression (r
2
=0.85) of the
difference across time, was 0.009 mg/L/d. This rate would include any biofouling effects as well
as any electronic or calibration drift. A linear regression of the instrument versus reference
measurements over the first month (r
2
= 0.98; p<0.0001) produced a slope of 0.747 and intercept of
3.64 resulting from a rapid change in response accuracy during the first few weeks.
At Chesapeake Biological Lab, the field test was conducted over 78 days with a mean
temperature and salinity of 25.6
o
C and 10.9. The measured DO range from our 142 discrete
reference samples was 4.37 – 10.86 mg/L compared to the broader dynamic range of 3.11 – 14.85
mg/L reported by the EXO2 over its 6027 accepted observations. The sonde had two brief periods
of interruption on July 8th and July 12th, and then quit operating permanently on July 31
st
for a
data completion result of 83% of expected. The average and standard deviation of the
measurement difference between the EXO2 and reference samples over the total deployment was
0.15 ±0.22 mg/L with a total range of -0.69 to 0.79 mg/L. There was no significant trend in
accuracy over time (slope = 0.0007 mg/L/d; r
2
= 0.002) over the deployment.
At Kaneohe Bay, HI the field test was conducted over 121 days with a mean temperature
and salinity of 25.8
and 33.4
o
C. The EXO2 operated successfully throughout the deployment and
generated 5653 observations conducted continuously at 30 minute intervals for a data completion
result of 100%.
The measured DO range from our 142 discrete reference samples was 3.63 – 9.85
mg/L compared to the broader dynamic range of 1.97 – 10.50 mg/L observed by the EXO2. The
average and standard deviation of the measurement difference between the EXO2 and reference
samples over the total deployment was -0.10 ±0.23 mg/L with a total range of -1.39 to 1.08 mg/L.
There was no significant drift in instrument offset (slope = 0.00017 mg/L/d; r
2
= 0.0008)
throughout the deployment period.
Overall, the EXO2 response showed good linearity across all three salinity ranges including
fresh, brackish, and oceanic water, covering an ambient DO range of 4 – 14 mg/L. There was
however a slightly higher offset for the cold freshwater test in Houghton, MI, and the overall
variability was slightly higher for the oceanic test in Kaneohe Bay.
The EXO2 was evaluated in a profiling field test in the Great Lakes at two separate
locations in order to experience transitions from surface waters into both normoxic and hypoxic
hypolimnion. In Muskegon Lake, the temperature ranged from 21.0
o
C at the surface to 13.5
o
C in
the hypolimnion, with corresponding DO concentrations of 7.8 and 2.8 mg/L, respectively. In
Lake Michigan, the temperature ranged from 21.0
o
C at the surface to 4.1
o
C in the hypolimnion,
with corresponding DO concentrations of 8.6 and 12.6 mg/L, respectively. Two profiling trials
were conducted at each location. The first trial involved equilibrating test instruments at the
surface (3m) for ten minutes and then collecting three Niskin bottle samples at one minute
intervals. Following the third sample, the rosette was quickly profiled into the hypolimnion where
samples were collected immediately upon arrival and then each minute for the next 6 minutes. The
second trial was performed in the reverse direction. Note for Muskegon Lake cast 1 was aborted
due to bottle misfires and repeated as cast 3. In Muskegon Lake, the EXO2 exhibited a negative
bias in the colder, low DO hypolimnion and a positive bias in the warm, high DO surface. Sensor