Measuring dissolved oxygen in sediments

Posted on March 23, 2015

Although oxygen is one of the most important water quality variables, the direct measurement of oxygen concentration in some environments, such as the interstitial water of streambed sediments, has been problematic because of the technical drawbacks of conventional test equipment. As a result, most researchers have chosen either not to measure sediment oxygen at all, or to resort to proxy variables such as redox potential, which is allied to pH and gives a relative indication of sediment oxidation. The introduction of a new generation of handheld oxygen probes, in the form of optical fluorescence sensors, has overcome these difficulties. Optical flouresence probes contain an oxygen-sensitive dye trapped in a matrix with high gas permeability. They also house a light source, and the environmental oxygen concentration affects the fluorescence response of the sensor to light excitation. Unlike conventional oxygen probes, fluorescence sensors have a long life, are relatively immune to variations in environmental conditions, don’t consume oxygen, don’t require that the test water be flowing or stirred, and don’t need frequent calibration. In the first published study of in situ, directly measured dissolved oxygen in river substrates, a YSI ProODO meter with a stainless steel guard was used to measure sediment oxygen availability at 56 locations in 13 rivers in southern Ireland. For comparison, redox potential was also measured at the same time with a WTW modified pH instrument. The probes were inserted to depths of up to 10 cm. Similar measurements were also made in the lab, using tapwater and clean aquarium gravel. While the optical meter gave very consistent results both in the lab and the field, the readings from the redox meter were much more variable and drifted with time. Under lab conditions, for example, the average standard deviation was about 9.1% of the mean in the case of the redox datasets, but only 1.6% in the case of the optical data. Optical fluorescence technology should prove particularly useful in studies on the oxygen requirements of sediment-dwelling invertebrates and fish embryos, and on the ecological consequences of substrate clogging and oxygen limitation caused by high silt loads.

Reference: Neill, M. et al. 2014. Direct measurement of oxygen in river substrates. Water and Environment Journal 28, 566–571.