The redox potential is used to describe a system's overall reducing or oxidizing capacity. The redox potential is measured in millivolts (mV) relative to a standard hydrogen electrode and is commonly measured using a platinum electrode with a saturated calomel electrode as reference. In well-oxidized water, as long as oxygen concentrations stay above ∼1 mg O 2 l −1, the redox potential will be highly positive (above 300–500 mV). In reduced environments, such as in the deep water of stratified lakes or the sediment of eutrophic lakes, the redox potential will be low (below 100 mV or even negative). Microbial-mediated redox processes can decrease the redox potential to a level as low as −300 mV. Changes in the redox potential, as, for example, around the sediment–water interface of lakes where the oxygen consumption rate is high, are important for the overall retention and release of phosphorus from iron. In its oxidized form and at a redox potential above ∼200 mV, iron has a high phosphorus sorption capacity, whereas at lower redox potential iron is reduced and phosphorus is released. The redox potential has also been used to characterize redox reactions important for the carbon and nutrient cycling in rivers and wetlands.