Oxygenated Water and Plant Growth

Oxygen in Water

Oxygen concentrations in the air you breathe remain fairly constant. Aquatic animals live in a somewhat more challenging environment. The solubility of oxygen in water changes with temperature, so oxygen levels fluctuate throughout the day. Aquatic plants release oxygen as a byproduct of photosynthesis, so they help boost the dissolved oxygen content of the water. This is only true up to a point, because over time, excess oxygen diffuses out into the atmosphere. Nonetheless, over shorter timescales, aquatic plants increase local oxygen concentrations.

Just like animals, however, aquatic plants burn up oxygen as part of their metabolism. During the daytime, the amount of oxygen they release outweighs what they consume. No photosynthesis takes place at night, and after dark, aquatic plants are actually reducing oxygen levels. Cloudy weather also decreases the rate of photosynthesis, and on overcast days, aquatic plants become net-oxygen consumers. Overall, fully submerged aquatic plants ultimately generate 5 times more oxygen than they consume -- but in oxygen-poor environments, nighttime and cloudy weather consumption can temporarily drive oxygen content down to low levels that are dangerous for fish.

Aquatic plants need ways to get oxygen to thick tissues like roots, where the oxygen concentration is low. Many freshwater aquatic plants feature tissues called aerenchyma, which have large air spaces between cells. Oxygen from photosynthesis diffuses into this tissue more rapidly than it does into the surrounding water -- so rapidly, in fact, that oxygen constitutes 1/3 or more of the gas in these enclosed spaces. A network of air chambers inside the plant transports gases to distant tissues and provides buoyancy as well.

Aquatic plants can also contribute indirectly to fluctuations in oxygen levels. When plants that are not fully submerged block light from reaching the depths, they discourage algae growth and promote the growth of microbes that consume oxygen. Many floating plants block transfer of oxygen from atmosphere to water, reducing oxygen levels in the waters below. Floating plants also exchange gases directly with the atmosphere, so they don't contribute dissolved oxygen to the aquatic environment.