Temperature is more than just a number! Temperature is an important water quality parameter because it influences both biological and chemical characteristics of the water. Temperature impacts dissolved oxygen levels, rates at which algae and other plants photosynthesize, metabolic rates of organisms, and bacterial growth. Warmer temperatures tend to be associated with the most problems which is why summer often brings a decline in water quality.
Temperature directly affects dissolved oxygen levels. The relationship is inverse; as temperature goes up, dissolved oxygen goes down. Chances are you have heard the term "dead zone." Dead zones occur when the dissolved oxygen levels become too low to sustain life. These occur most frequently in the summer because warmer water is less capable of holding dissolved oxygen than cooler water.
The metabolic rate of aquatic organisms is also increased when temperature increased. As their metabolic rate increases, their bodies need more oxygen. At the same time, the increased temperatures are decreasing the water's capability to hold dissolved oxygen creating stress on the organisms. Additionally, temperatures that are too low or too high can make an organism more susceptible to pollutants and diseases caused by bacteria, parasites, and viruses.
In addition to the changing seasons, other factors can influence temperature. Turbidity increases the amount of heat that is absorbed from sunlight which is one of the reasons we care so much about water clarity. Thermal pollution also influences temperatures. In the South River watershed, most of the thermal pollution comes from stormwater runoff whereas other rivers may be impacted by thermal pollution from power plants. On hot days, stormwater heated up by impervious surfaces such as roads and roof tops before being carried directly into our waterways.