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System-wide Monitoring Program

Dissolved Oxygen (DO)

Aerobic aquatic organisms such as zooplankton, invertebrates and fish require sufficient levels of dissolved oxygen (DO) to survive. The amount of dissolved oxygen in the water is a factor in determining the species and abundance of organisms that can live in an estuary.

Oxygen is supplied to estuarine waters through two natural processes: a) diffusion of atmospheric oxygen into the water and b) photosynthesis by phytoplankton and aquatic macrophytes (seaweeds and seagrasses), which results in production of oxygen. Mixing of surface waters by wind and waves increases the rate of absorption of atmospheric oxygen into the water.

As mentioned earlier, dissolved oxygen levels are influenced by temperature and salinity. The solubility of oxygen, or its ability to dissolve in water, decreases with increasing temperature and salinity. Dissolved oxygen levels in an estuary vary seasonally, with the lowest levels occurring during the late summer months when temperatures climb to their highest levels of the year.

Oxygen is removed from the water by aerobic respiration and bacterial decomposition. Respiration is a process in which animals and plants take up oxygen from the water and produce carbon dioxide. Respiration occurs all the time, while photosynthetic production of oxygen by plants occurs only during daylight hours. As a result, dissolved oxygen levels in an estuary may vary widely because of differences in the amount of oxygen produced by plants.

Bacteria, fungi and other organisms affect DO levels in an estuary because they consume oxygen while breaking down organic matter produced in the estuary or delivered from the uplands by streamflow and runoff. These decomposers consume oxygen in the process of gaining energy through the breaking of chemical bonds in organic matter.

Oxygen depletion may occur in an estuary when many plants die and decompose, or when runoff or poorly treated wastewater containing large amounts of organic matter enters the estuary. In some estuaries, large nutrient inputs, normally from sewage inputs, stimulate phytoplankton blooms. When these organisms die, their bodies fall to the bottom of the estuary and begin to decompose. The decomposition process depletes the surrounding water of oxygen and, in severe cases, may lead to anoxic (very low oxygen) conditions that kill bottom-dwelling organisms. Shallow, well mixed estuaries are less susceptible to this phenomenon because wave action and circulation patterns can easily supply the deeper waters with oxygen.