Thresholds (mg/L)
Separate thresholds were calculated for dissolved oxygen measured in units of percent saturation and dissolved oxygen measured in units of mg per L. This page describes the thresholds for mg per L. Click here for the thresholds for percent saturation.
QC test thresholds for dissolved oxygen (mg/L) were based on historical Coastal Monitoring Program data. Preliminary quality control (QC) was applied to the historical data. Obvious outliers, suspected biofouling, and freshwater stations were omitted.
To date, dissolved oxygen (mg/L) has only been measured in two counties1 in adjacent waterbodies2. A single set of thresholds was determined for these counties.
The dissolved oxygen (mg/L) data has not been corrected for salinity. The threshold were calculated from and applied to the uncorrected data.
Gross Range Test
Sensor Thresholds
The sensor thresholds were determined based on the manual for the HOBO DO (Table 1).
Table 1: Dissolved oxygen (mg/L) sensor thresholds for the Gross Range Test.
User Thresholds
The dissolved oxygen (mg/L) observations are relatively normally distributed (Figure 1), and so the mean and standard deviation were used to determine \(user_{min}\) and \(user_{max}\). The statistics and threshold values are shown in Table 2.
Table 2: Gross Range Test statistics and user thresholds for dissolved oxygen (mg/L).
Climatological Test
Figure 2 shows the monthly mean and standard deviation of dissolved oxygen (mg/L) data. The observations are normally distributed within each month (Figure 3), and so the mean and standard deviation were used to calculate the seasonal thresholds (Table 3).
Table 2: Climatology statistics and thresholds for dissolved oxygen (mg/L).
Spike Test
The distribution of the spike value is skewed right (Figure 4), and so several upper quartile values (90th, 95th, and 99.7th quartile) were evaluated to use as the \(spike_{low}\).
There were relatively few large single-value spikes in the dissolved oxygen (mg/L) data, and so the 99.7th quartile was selected to avoid false positives. \(spike_{high}\) was defined as 3 * \(spike_{low}\) to identify especially egregious spike values.
Table 4: Spike thresholds for dissolved oxygen (mg/L).
Rolling Standard Deviation Test
The distribution of rolling standard deviation is skewed right (Figure 5), and so several upper quartile values were evaluated to use as the \(rolling\_sd\_max\).
The 90th, 95th, and 99.7th quartile values were each applied to the the raw data (no preliminary QC) and the results inspected. The 90th quartile was determined to be too stringent, as it generated false positives, while The 99.7th quartile was determined to be too lenient, as it resulted in false negatives. The 95th quartile identified 23 % of the raw data (no preliminary QC) as Suspect/Of Interest and was considered the most useful threshold.
As discussed for dissolved oxygen (percent saturation), this is a very high percent of observations to flag as Suspect/Of Interest. However, it was anticipated that a large proportion of the dissolved oxygen observations would be flagged by this test because there were substantial biofouling signals identified during the preliminary QC process. CMAR is currently assessing options to reduce biofouling on dissolved oxygen sensors.
There is no Fail flag for the Rolling Standard Deviation Test because of the natural variability of the Water Quality variables. However, CMAR recommends that the dissolved oxygen observations flagged as Suspect/Of Interest should be considered Suspect, and filtered out of most analyses.
Table 5: Rolling standard deviation threshold for dissolved oxygen (mg/L).