The Role of pH in a Freshwater Aquatic Ecosystem

Freshwater ecosystems are in large part regulated by pH levels that exist naturally in the range of 6 to 8. The pH of freshwater ecosystems can fluctuate daily and seasonally, and most freshwater life has evolved to tolerate these fluctuations. However, freshwater life can falter and fail when exposed to extremes in pH, as well as rapid changes in pH, even if the change occurs within a range that is normally tolerated by the ecosystem.

High pH

• When plants or algae grow rapidly in a freshwater environment more carbon dioxide is added each day by photosynthesis than is removed each night by respiration. As a result, pH levels may rise to levels that cannot be tolerated by the marine life. Juvenile fish and shallow water crustaceans are particularly susceptible to damage from high pH levels. Mature fish are better able to regulate high pH levels, typically by moving to deeper waters where the pH is lower.

Low pH

• Scientists most often attribute low pH levels to acid rain. A freshwater ecosystem reaches a low pH when pH levels approach 5 and below. As the pH begins to fall basic forms of food die. For example, aquatic bugs that serve as an important food source for fish will fail to reproduce and will cease to survive. As pH continues to fall, fish cannot reproduce, juvenile fish begin to die and mature fish die of suffocation. The ecosystem continues to change as new species of plants and algae invade and the system is entirely void of its original plant, fish and crustacean life.

Indirect Effects of Low pH

• As pH levels drop, acidity increases. As a result of the increased acidity, heavy metals present in the soils, such as aluminum ions, are dissociated and released into the water. Heavy metal ions burn the gills of fish, accumulate in their organs and eventually lead to death.

Sudden Changes in pH

• Sudden changes in pH can stress or kill aquatic life, even when the changes occur within a pH range they typically tolerate. Aquatic life is unable to adjust to the sudden change and is unable to maintain osmoregulation. As a result, aquatic life loses the delicate balance of salt in its tissues that is required to sustain life. Certain species, such as catfish, are more susceptible to this phenomenon, and all juvenile fish are particularly vulnerable.