Almost all aquatic organisms require oxygen to live (some bacteria species do not), but many are unable to breathe atmospheric oxygen and must rely upon oxygen dissolved in the water. Although water molecules (H2O) contain one oxygen atom, this oxygen is bound and cannot be used by aquatic organisms. Dissolved oxygen is free oxygen (O2) present in water, but not bonded to other elements. This results from an exchange of the oxygen between the water and atmosphere, and the photosynthetic production of oxygen by plants and algae. Many springs have naturally low oxygen due to the length of time the water has been underground where no photosynthesis can occur. After leaving the spring vent, gas exchange with the atmosphere and photosynthesis from plants and algae increases the oxygen level as the water flows down the spring run.
Several factors, such as temperature, control how much oxygen can be dissolved in water. As water warms, the amount of oxygen that can be dissolved in water is reduced. Oxygen in water is measured as either mg/L of oxygen or percent saturation. Percent saturation corrects for temperature by taking into account the maximum amount of oxygen that could be dissolved in water at any particular temperature, and is the value currently reported by SRWMD. Due to naturally occurring seasonal changes in temperature and daily changes in photosynthetic production, dissolved oxygen typically shows both daily and seasonal fluctuations.
Florida has recently revised its surface water criteria for dissolved oxygen. The revised criteria are expressed as percent saturation instead of the older concentration-based criteria. These revised criteria are designed to recognize the inherent natural variability in dissolved oxygen. The revised criteria are for both fresh and marine waters. In portions of the Suwannee, Withlacoochee, Waccasassa, New, and Santa Fe Rivers inhabited by the endangered Gulf Sturgeon and Oval Pigtoe mussel, the proposed criteria were modified to assure that the sturgeon and mussel were fully protected. The new freshwater Class I and III criterion is: No more than ten percent of the daily average percent DO saturation values shall be below 67 percent in the Panhandle West bioregion, or 38 percent in the Peninsula and Everglades bioregions, or 34 percent in the Big Bend and Northeast bioregions.