This sensor is at the surface, so we expect the oxygen levels to be fairly high due to wind and wave action, which mixes the oxygen into the water column. The amount of oxygen that water can hold depends on both the water temperature and salinity. The summer water might actually be close to saturation, while the winter water might seem to be unusually low. Saturation is measured relative to what we would expect for a body of still water that has reached equilibrium with the air. The temperature of the water in degrees Centigrade is shown here.
Temperature changes from season to season, and day to day with the tides.
Temperature also changes as you go from the surface to the bottom of the water column. In the summer, the water at the surface can get very warm, while the water at the bottom remains cold. When that happens, the water column is stratified. When the difference between the warm water and cool water is large, the boundary between them, or interface, acts to inhibit mixing of oxygen and nutrients to the bottom waters, causing a depletion of oxygen in the bottom waters, or hypoxia. When the temperature of the water is nearly the same from top to bottom, it indicates that the water has been well-mixed, this is the condition we see most frequently late fall through early spring in most of the Sound.