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Dissolved Oxygen Monitoring Program in Connecticut

The U.S. Geological Survey (USGS), in cooperation with the Connecticut Department of Energy and Environmental Protection (CT DEEP) and New England Interstate Water Pollution Control Commission (NEIWPCC), implemented a summer dissolved oxygen (DO) monitoring program from 2015 to 2018 to examine the effects of phosphorus loading in streams.

Continuous water-quality monitor at the Salmon River, Connecticut
Photograph showing a continuous water-quality monitor at the Salmon River near East Hampton, Connecticut. (Credit: Brittney Izbicki, U.S. Geological Survey. Public domain.)

The USGS monitored DO at sites in Connecticut from 2015 to 2018 and estimated daily gross primary productivity, ecosystem respiration, and a standardized coefficient rate for gas exchange for selected streams (Izbicki and Morrison, 2021). The variability of the annual total phosphorus and maximum daily DO concentrations were evaluated at the selected sites from year to year.

Streams identified as impaired by the CT DEEP, such as Naugatuck River at Beacon Falls (USGS station 01208500), Still River at Route 7 at Brookfield Center (USGS station 01201487), and Quinnipiac River at Wallingford (USGS station 01196500), had higher total phosphorus concentrations (greater than 0.10 milligram per liter [mg/L]) throughout the study period.

Those reference streams that were unimpaired had lower concentrations of total phosphorus (less than 0.10 mg/L). Maximum daily DO concentration ranged from the study ranged from 1.4 to 7.7 mg/L, reference sites typically had ranges of daily DO that were often less than 2 mg/L and most sites downstream from wastewater treatment plants having ranges greater than 2 mg/L. The Naugatuck River at Beacon Falls, Still River at Brookfield, and North Branch Park River at Hartford (USGS station 01191000) stations recorded the largest maximum daily DO ranges. The Still River at Brookfield station recorded a maximum daily DO concentration of 5.9 mg/L in 2016.

Daily fluctuations of DO concentrations are the result of changes in oxygen concentrations from primary productivity and respiration. Large diel cycles observed at some sites are a result of a productive ecosystem. Daily gross primary productivity and ecosystem respiration were summarized for 11 sites using the maximum likelihood estimation model of the streamMetabolizer package in the R statistical program.

The models indicated that most sites had an estimated negative net primary productivity, based on the daily estimates of gross primary productivity and ecosystem respiration, which indicates that the systems are heterotrophic and dominated by respiration. Stream metabolism, based on continuous dissolved oxygen data, can be used to evaluate the effect of nutrient contamination in streams.

Reference Cited

Izbicki, B., and Morrison, J., 2021, Use of dissolved oxygen monitoring to evaluate phosphorus loading in Connecticut streams, 2015–18: U.S. Geological Survey Scientific Investigations Report 2021–5024, 25 p.,

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