Dissolved oxygen (DO) refers to the amount of oxygen gas present in water, which is necessary for the survival of aquatic organisms. Water temperature measures the thermal energy within the water. In aquatic environments, these two parameters are linked by an inverse relationship: as water temperature increases, the amount of dissolved oxygen generally decreases. This connection plays a significant role in determining the health and capacity of aquatic ecosystems to support life.
The Core Relationship: Temperature’s Impact on Dissolved Oxygen
The inverse relationship between water temperature and dissolved oxygen is a direct consequence of gas solubility in liquids. As water warms, the kinetic energy of its molecules increases, causing them to move more rapidly. This increased molecular motion makes it more challenging for oxygen gas molecules to remain trapped within the water’s structure, leading them to escape into the atmosphere. Warmer water cannot hold as much dissolved oxygen as colder water.
This property is seen across aquatic environments. Cold water bodies like polar oceans and mountain streams contain higher concentrations of dissolved oxygen. In contrast, warmer environments, such as tropical lakes or stagnant ponds, have lower DO levels. The solubility of oxygen in freshwater, for example, decreases from approximately 14.6 mg/L at 0°C to about 8.24 mg/L at 38°C (100°F), a significant reduction as temperature rises. This principle underscores why temperature is a primary determinant of dissolved oxygen concentrations in water.
Implications for Aquatic Life and Ecosystems
Temperature-driven changes in dissolved oxygen have significant consequences for aquatic environments. Aquatic organisms, including fish, invertebrates, and microorganisms, depend on sufficient dissolved oxygen for respiration and survival. When water temperatures rise, and DO levels fall, these organisms experience stress, which can lead to reduced growth, impaired reproduction, and even death. For example, many fish species begin to show distress at DO levels below 5.0 mg/L, and concentrations below 3 mg/L are considered too low to support most fish.
Widespread low dissolved oxygen, known as hypoxia (less than 2-3 mg/L of oxygen) or anoxia (near zero oxygen), can create “dead zones” in lakes and oceans, impacting food webs. These zones alter species composition, reduce biodiversity, and can lead to mass mortality. Over the past four decades, oxygen levels in the deep waters of temperate lakes have dropped by almost 19%, a decline occurring 3-9 times faster than in the oceans. Higher temperatures also directly increase the metabolic rates of cold-blooded aquatic organisms, simultaneously raising their oxygen demand while the available oxygen supply diminishes.
Additional Factors Influencing Dissolved Oxygen Levels
While temperature is a major factor, several other variables influence dissolved oxygen levels in aquatic systems. Atmospheric pressure plays a role, with higher pressure allowing more gas to dissolve, meaning water at lower elevations holds more DO than at higher altitudes due to greater barometric pressure. Salinity also affects DO; increased salinity reduces oxygen solubility because salt ions compete with oxygen molecules for space among water molecules. For instance, seawater holds approximately 20% less oxygen than freshwater at the same temperature and altitude.
Biological activities within the water body impact DO concentrations. Photosynthesis by aquatic plants and algae releases oxygen into the water, increasing DO levels. Conversely, respiration by all aquatic organisms, including plants at night, and the decomposition of organic matter by bacteria consume dissolved oxygen, leading to lower levels, especially in water bodies with abundant decaying material. Water movement, such as turbulence from rapids or waves, enhances oxygen exchange, increasing dissolved oxygen. Conversely, organic pollution, from sources like sewage or agricultural runoff, can lead to increased microbial decomposition, which consumes oxygen, depleting DO levels.