Aquatic plants and algae are significant contributors to the supply of dissolved oxygen (DO) in aquatic environments. Dissolved oxygen refers to the free oxygen molecules present in water, which are necessary for the survival of most aquatic life, including fish and invertebrates. Adequate DO levels are a primary indicator of water quality, as low concentrations can stress or kill aquatic organisms. This process of oxygen production is influenced by a balance of biological mechanisms and environmental factors.
The Science of Oxygen Release
Aquatic plants, including submerged vascular plants and microscopic phytoplankton, generate oxygen through photosynthesis. This process uses light energy to convert carbon dioxide and water into glucose, a sugar used for energy and growth, and oxygen.
This oxygen is released as a byproduct of splitting water molecules during the light-dependent reactions of photosynthesis. Unlike terrestrial plants, the oxygen produced by submerged aquatic plants does not immediately enter the atmosphere but is released directly into the surrounding water column. This release significantly enhances the concentration of dissolved oxygen in the immediate environment.
Under conditions of high light intensity and sufficient carbon availability, the rate of oxygen production can exceed the water’s capacity to hold the gas. This surplus oxygen can sometimes be observed visually as small bubbles forming and rising from the leaves of the plants, a phenomenon known as “pearling”. The oxygen made available by aquatic plants is a resource for all aerobic organisms sharing the ecosystem.
The Counterbalance Why Plants Also Consume Oxygen
While plants are oxygen producers during the day, they must perform cellular respiration continuously to break down the glucose they create and release energy for growth and maintenance. This process consumes oxygen and releases carbon dioxide.
During daylight hours, the rate of photosynthetic oxygen production typically far exceeds the rate of respiratory consumption, resulting in a net surplus of dissolved oxygen. This balance changes dramatically after sunset when photosynthesis stops due to the lack of light.
Throughout the night, the plants, along with fish, bacteria, and other organisms, continue to respire, drawing dissolved oxygen from the water. In aquatic systems with dense vegetation, this nighttime consumption can lead to a significant drop in DO levels, with the lowest concentrations often occurring just before dawn. This diurnal fluctuation highlights that the simple presence of plants does not guarantee high oxygen levels.
Environmental Limits on Oxygen Production
Several external environmental factors determine the actual net amount of oxygen an aquatic system gains from plants. Light availability is a limiting factor because photosynthesis cannot occur in the dark. In deep water or water with high turbidity, light penetration is reduced, which significantly limits the depth and health of submerged plant life and their ability to produce oxygen.
Water temperature also plays a dual role in limiting oxygen delivery. First, warmer water naturally holds less dissolved oxygen because the solubility of gases decreases as temperature increases.
Second, higher temperatures increase the metabolic rates of plants and all other organisms in the water, which in turn increases their rate of oxygen consumption through respiration. The combination of reduced oxygen-holding capacity and increased consumption demand means plants must work harder to maintain a net positive DO balance in warmer conditions.
Nutrient availability, particularly nitrogen and phosphorus, is necessary for plant growth and photosynthetic capacity. However, excessive nutrients can trigger algal blooms. When these blooms die and decompose, the process consumes vast amounts of dissolved oxygen, potentially leading to hypoxic conditions.