Stable oxygen isotopic fractionation during photolytic O2 consumption in stream waters

Oxygen (O2) is required for life in higher organisms, however, processes such as respiration, the oxidation of reduced inorganic species, and the photolytic breakdown of dissolved organic matter (DOM) decrease the O2 concentrations in aquatic systems. Filtered, inoculated, and sterile samples of stream waters from Ontario, Canada, were incubated in natural sunlight to examine the effects of photolysis of DOM, respiration, and abiotic reactions on O2 consumption and δ18O of dissolved oxygen (δ18O–O2). Oxygen consumption rates in the light were up to an order of magnitude greater than in the dark, suggesting light-mediated processes controlled O2 consumption. Rates of O2 loss were the same for each treatment (i.e. filtered, inoculated, and sterile) indicating that photolysis was the dominant O2 consuming process over respiration in these incubations. O2 consumption rates were different between streams, even when normalized to the change in dissolved organic carbon (DOC), signifying that DOM photolability varied among streams. During DOM breakdown to CO2, the lighter 16O isotopomer was preferentially consumed. Fractionation factors observed for photolysis, respiration, and abiotic reactions ranged between 0.988 and 0.995, and were similar in both the light and in the dark incubations in all streams. These fractionation factors are not a function of O2 consumption rates, and are outside the range published for respiration (0.975–0.982). In current models of O2 and δ18O–O2, photolysis and respiration are not considered separately and the isotopic fractionation during respiration that is measured in the dark is used in the light. In these incubations, DOM degradation and abiotic reactions are important O2 consuming and δ18O–O2 fractionating processes. Current models of O2 and δ18O–O2 incorporate photolysis of DOM and other abiotic processes into the respiratory component of O2 consumption, thereby overestimating respiration and underestimating photosynthesis to respiration ratios. Consequently, photolysis and abiotic reactions should be considered separately, particularly in shallow aquatic systems with high DOC.