Dissolved oxygen is critical to the river's ability to support aquatic life such as fish and macroinvertebrates (insects that depend on aquatic habitat for part or all of their lives). The concentration of dissolved oxygen can vary over the day. Aquatic plants and bacteria in the sediments remove dissolved oxygen from the water when they respire. Because plants respire mainly at night, the lowest dissolved oxygen concentrations of the day occur in the early morning. During the day, plants add oxygen to the water column through photosynthesis. Low dissolved oxygen concentrations (<5.0 mg/L) and large changes in the concentration of dissolved oxygen over the day (diurnal variation) are damaging to habitat. Very high (>150% saturation) concentrations are also harmful to fish. Under the Clean Water Act, all our mainstem rivers are classified as warm water fisheries. Additionally, the DO standard for the lower Sudbury River (below Saxonville dam) is "Warm water Fishery-Aquatic Life." This standard for DO is less stringent than for the other river segments, reflecting the influence of extensive wetlands and slow-moving water of the lower Sudbury.