What is it?
The concentration of dissolved oxygen [DO, units of milligram per liter (mg·L-1)] is perhaps the single most important feature of water quality. It is an important regulator of chemical processes and biological activity. Most forms of aquatic life require oxygen (DO). For example, certain combinations of low temperature and high DO concentrations are required for the maintenance of a cold water sport fishery (such as trout and salmon). Plant photosynthesis produces oxygen within the region below the water surface with adequate light (photic zone). Microbial (for example, bacteria) respiratory and organic decay processes consume oxygen. Near the reservoir surface, oxygen can move between the water and air. The rate and direction of this exchange is dependent on the wind speed and status of the surface waters with respect to the equilibrium or saturation concentration.
How is it measured?
Dissolved oxygen is measured using a DO probe. The DO probe consists of small silver anodes and a gold cathode. These electrodes are separated from the surrounding lake water by a Teflon membrane. Dissolved oxygen diffuses across the membrane and is reduced to OH- ions at the cathode and AgCl is formed at the anode. The current associated with this process is proportional to the DO in the surrounding water
Why is it important?
Saturation Conditions
Oxygen is moderately soluble in water. The solubility limit, or saturation concentration of DO is largely regulated by temperature. Concentrations that exceed the saturation value are described as supersaturated. Such conditions reflect high photosynthetic activity (i.e. during an algal bloom). Undersaturated conditions prevail when the DO concentration is less than the saturation value, indicating oxygen-demanding processes exceed the sources of DO.
Depletion in the Bottom Waters
 Upon the onset of thermal stratification the hypolimnion (see thermal stratification) becomes isolated from sources of oxygen. DO is widely observed to decrease progressively in the hypolimnion over the period of stratification, because the demand for oxygen associated with respiration and decay exceeds the sources. This is illustrated by the time plot of DO concentration in the hypolimnion
Oxygen demand in the hypolimnion is usually localized at the interface between the lake bottom (sediments) and the overlying water column. This bottom demand is described as sediment oxygen demand (SOD). DO  concentrations are widely observed to decrease progressively with depth in the hypolimnia of mesotrophic and eutrophic lakes as the sediments are approached, because of the localized demand at the bottom and the limited vertical mixing at these depths. DO concentrations in metalimnia and hypolimnia of oligatrophic (low productivity) lakes are often higher than in the overlying epilimnia. These differences are temperature based, as satuation DO values are higher than in the overlying epilimnia. These differences are temperatue based, as saturation DO values are higher at lower temperatures of the metalimnia and hypolimnia. DO minima and/or maxima may at times be observed in the metalimnia of certain lakes and reservoirs generally associated with high localized concentrations of plankton (see plot of metalimnetic DO minimum in profile to right)
Oxygen concentrations are replenished in the lower layers during fall turnover.  Reductions in oxygen concentrations may be observed in the upper waters of a lake during this interval if very low DO levels developed in the hypolimnion, reflecting the outcome of mixing of high (for example, saturated) and low (from the oxygen-depleted hypolimnion) DO concentrations. A particularly sever case of such dynamics for DO in an eplimnion is illustrated by the time plot on the right.
What to look for in our systems?
Progressive depletion of DO from the hypolimnion of mesotrophic Otisco Lake occure annually. Anoxia (zero DO) occurs first at the bottom and expands upward through late summer, until the entire layer is anoxic. Modest lake-wide DO depletion may be evident during fall turnover.
Higher DO concentrations occur in portions of the metlimnia and hyplimnia, compared to the epilimnia in summer intervals in Skaneateles and Owasco Lakes.
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