What Is Dissolved Oxygen?
Dissolved Oxygen (DO) is the amount of oxygen dissolved in water. Adequate levels of DO are an important parameter for a water body's ability to support aquatic life. Just as humans need oxygen to breathe, aquatic animals need DO to "breathe" as it is absorbed through their gills. Oxygen enters the water by absorption directly from the atmosphere or by aquatic plant and algae photosynthesis. Oxygen is removed from the water through animal respiration and decomposition of organic matter. Low levels of DO can jeopardize the health of aquatic animals and cause other water quality impairments. The Department of Ecology has listed Lake Whatcom as an impaired waterbody for low levels of dissolved oxygen.
What Affects The Level Of Dissolved Oxygen?
The amount of DO in water depends on several factors, including temperature, organic matter, the amount of organisms using oxygen for respiration, and nutrients. The colder the water, the more oxygen can be dissolved in the water. Therefore, DO concentrations are usually higher in the winter than in the summer.
Organic wastes that enter a body of water include leaves, grass clippings, dead plants or animals, animal droppings, and sewage are decomposed by bacteria; these bacteria remove dissolved oxygen from the water when they breathe.
During photosynthesis, plants release oxygen into the water. During respiration, plants remove oxygen from the water. Bacteria and fungi use oxygen as they decompose dead organic matter in the stream. The type of organisms present (plant, bacteria, fungi) affect the DO concentration in a water body. If many plants are present, the water can be supersaturated with DO during the day, as photosynthesis occurs. Concentrations of oxygen can decrease significantly during the night, due to respiration. DO concentrations are usually highest in the late afternoon, because photosynthesis has been occurring all day.
How Do Nutrients Affect Dissolved Oxygen?
In Lake Whatcom, phosphorus is a nutrient that can lead to excessive algal growth and ultimately low levels of DO. Increased algal production can cloud the water and block sunlight from reaching photosynthetic organisms, such as aquatic grasses. These organisms cannot survive without sunlight and if they die off there is will be a decrease in food and shelter for other life in the lake. As the over abundance of algae dies and decomposes, DO is consumed at a greater rate. Less DO is then available to other aquatic life that depends on it for survival. The depletion of photosynthetic organisms also reduces the source of DO, leaving no replenishment as it depletes. Within the watershed there are many sources of phosphorus that contribute to the over abundance of the nutrient. Exposed soils, septic systems, detergents, fertilizers, and animal waste all contain phosphorus that enters Lake Whatcom and affect its health.
Eutrophication is the process by which a waterbody becomes enriched with nutrients. This is a slow, natural aging process of lakes. However, when human activities greatly accelerate this process in lakes it is known as cultural eutrophication.