During photosynthesis, terrestrial plants extract carbon dioxide (CO2) from the air, combine it with water, and use the sun to power a chemical reaction that allows them to feed themselves. But when plants are growing in water, the amount of CO2 needed to perform photosynthesis increases 10-20-fold. In lakes especially, the presence of microscopic algae can reduce the amount of CO2 dramatically compared to that of air.
Often, the best way to overcome this lack of CO2 is to use bicarbonate. According to Professor Ole Pedersen, “Use of bicarbonate is energy demanding and much less efficient than CO2 use, when concentrations are the same. However, in bicarbonate-rich lakes, the photosynthesis yield is much higher when water plants can use bicarbonate. Globally, this can account for the increasing abundance of bicarbonate users relative to non-users in lakes located in calcareous catchments."
Freshwater plants that receive sunlight release gaseous oxygen into the water, while simultaneously producing bicarbonate (HCO3) ions. HCO3 cause the pH to increase, sometimes to toxic levels. When there is no sunlight, there is no photosynthesis, the plants don’t produce new bicarbonate ions, resulting in a decrease in pH levels.
Professor Kaj Sand-Jensen says, "In order to establish the ability, or lack of ability to use bicarbonate of many species, we first had to diagnose several tropical species and then establish their abundance in relation to water chemistry.” Sand-Jensen’s research suggests that both the richness and composition of water plants will change as bicarbonate concentrations in lakes change as a result of acidification, use of nitrogen fertilizers, and other factors. When the balance between two plant types shifts, it changes the underwater meadows, which then jeopardizes the safety of small animals and fish. These smaller species and juveniles use those meadows as cover from predators.