Diel dissolved oxygen patterns and aquatic life use assessment in freshwater streams near Austin, Texas

Background/Question/Methods

Dissolved oxygen (DO) is an indicator of stream health, and is used by regulatory authorities like the Texas Commission on Environmental Quality to identify stream segments that may have aquatic life impairments as part of the Clean Water Act Section 303(d) assessment process.  Once a water body has been identified as impaired, steps must be taken to remediate the impairment which may require significant investments of limited resources.  DO data may be used in place of benthic macroinvertebrate data for aquatic life use support determinations, and is easier to collect than direct biological samples which require significant post-processing for taxonomic identification.  Diel dissolved oxygen (DO) data from 318 deployments at 38 stream sites in Austin, Texas, were evaluated for spatial and temporal patterns and assessed using TCEQ aquatic life use DO criteria.  DO deployments were conducted across a range of disturbance conditions including some deployments following raw sewage spills into creeks.  Aquatic life use determinations from DO data were compared to aquatic life use designations derived from benthic macroinvertebrate rapid biological assessment sampling.

Results/Conclusions

Diel DO when evaluated by site averages indicates that routine monitoring streams in Austin, including those that are dominated by wastewater effluent, generally maintain high or excellent aquatic life use potential.  The lack of consistent, expected diel DO impacts in effluent-dominated streams or following domestic sewage overflows suggests DO may not be best indicator of degradation in Austin’s creeks.  There is little correlation between biological measures and diel DO, suggesting that traditional DO degradation of streams from excessive organic matter loading or eutrophication may not be advanced enough in Austin to significantly impair macrobenthic communities.  Diel DO measurements when assessed by TCEQ methods may yield a large proportion of identified aquatic life use impairments when no impairment is supported by direct monitoring of macrobenthos.  Careful consideration should be used when deciding on monitoring strategies for diel DO if only diel DO and not biological data are measured.