The Effects of CO2 on the pH in Lakes

Carbonic Acid

• When carbon dioxide dissolves in a lake, much of it reacts with water molecules to form carbonic acid. Carbonic acid reduces lake water pH. If no other factors were involved, the CO2 in the atmosphere and the dissolved CO2 levels in the lake would eventually reach an equilibrium, where CO2 was dissolving in the lake water just as fast as dissolved CO2 was escaping. At equilibrium levels, the CO2 concentration would cease to change. In reality, however, other factors are involved, so dissolved CO2 levels are not typically at equilibrium.

Photosynthesis and Respiration

• When organisms like fish or decomposers release carbon dioxide into the lake as an end product of cellular respiration, they increase the CO2 concentration and cause pH to decrease. Photosynthetic organisms like algae, however, take up carbon dioxide, decreasing lake CO2 concentrations and causing pH to rise. The pH of lake water tends to be higher in the summer, since more algae and aquatic plants are present at this time of year.

Thermal Stratification

• Lake water pH can also vary with depth. Warmer water is less dense than cold water, so during the summer a layer of warm water can form at the surface of the lake; this layer does not mix well with the colder waters beneath. Organic matter like dead algae sinks to the floor of the lake, where bacteria that decompose it release CO2, increasing CO2 concentration and thus decreasing pH in the lower layer of water. In the upper layer of water, by contrast, photosynthetic activity is high, CO2 levels are lower and pH is consequently higher. During autumn and spring, by contrast, the lake water will be more fully mixed, and pH will vary less with depth.

Buffers

• As long as the lake water pH remains relatively close to 7, the dissolved carbon dioxide in lake water serves as a buffer that helps to moderate changes in the pH of the system. There are other buffer compounds in the water as well that help to soak up excess acidity or alkalinity. The buffer capacity of the lake water is a measure of how well it resists change in pH (changes caused, for example, by acid rain introducing acidic compounds into the lake water).