Dissolved oxygen is critical to a river’s ability to support aquatic life, such as fish and aquatic macroinvertebrates (insects that depend on aquatic habitats for part or all their lives). The concentration of dissolved oxygen can vary over the day. During the day, aquatic plants and algae add oxygen to the water column through photosynthesis. However, aquatic plants, algae, and the bacteria in the sediments remove dissolved oxygen from the water when they respire, both day and night. Because there is no photosynthesis at night, the lowest dissolved oxygen concentrations occur in the early morning. Low dissolved oxygen concentrations (<5.0 mg/L) and large changes in the concentration of dissolved oxygen over the day (diurnal variation) are stressful to aquatic life and degrade habitat. Very high concentrations (>150% saturation) also harm fish.
Dissolved oxygen scores are calculated based on comparison with Massachusetts water quality standards, EPA recommended criteria, EPA Ecoregion XIV background statistics, and published fish tolerances. Under the Clean Water Act, all our mainstem rivers are classified as Warm Water Fisheries, for which Massachusetts sets a standard minimum of 5 mg/L. However, the dissolved oxygen standard for the lower Sudbury River (below Saxonville dam) is “Warm Water Fishery-Aquatic Life,” with a lower standard limit of 3 mg/L, reflecting the influence of extensive wetlands and slow-moving water.