Which environmental factor significantly influences the distribution of freshwater species?

Water quality parameters, such as pH, play a critical role in influencing the distribution and health of freshwater species. Different species have specific tolerances to various water quality factors, including pH, temperature, dissolved oxygen, and nutrient levels. For example, many aquatic organisms thrive within certain pH ranges; if the pH level falls outside of these ranges, it can lead to stress, reduced reproduction, or mortality.

Moreover, water quality directly impacts the availability of food sources and habitat conditions essential for species survival and growth. For instance, slight changes in pH can affect the solubility of nutrients and toxins, which in turn influences the types of algae and macrophytes that can grow, creating a cascading effect on the entire food web.

Physical barriers like dams can obstruct migration but do not inherently change the fundamental water quality characteristics. Similarly, while terrestrial species may indirectly influence freshwater environments, they do not directly affect the fundamental water quality parameters that govern aquatic life. Seasonal variations in precipitation can lead to changes in water levels and flow, but they do not alter the chemical composition of the water—something that directly affects species distribution like water quality does. Thus, water quality parameters are paramount in understanding freshwater species distribution.