APPLICATION OF WIGLEY-PLUMMER-PEARSON MODEL TO THE INTERPRETATION OF ISOTOPIC COMPOSITION OF DISSOLVED OXYGEN

Dissolved oxygen (DO) is a major indicator of the health of an aquatic ecosystem. The net concentration of DO depends on the photosynthesis-to-respiration ratio, P/R, in that system. Nutrient loading, light, temperature, organic matter, and gas exchange all play crucial roles in DO concentration.

The oxygen released from photosynthesis originates from the breakdown of water molecules, and the oxygen isotope composition of DO is slightly lower in O-18/O-16 ratio than that of the water. In contrast, the respiration process preferentially uses the DO with a lighter isotopic composition; as a result, the residual DO is enriched in heavier isotopes. For a system in which the DO production from photosynthesis equals the consumption from total respiration (community respiration), the net DO remains constant. However, the oxygen isotope composition of the final DO will be quite different from the original value.

The commonly used Rayleigh fractionation process cannot adequately model a system where oxygen is simultaneously produced from photosynthesis and consumed by respiration. Based on mass balance and simultaneous multiple Rayleigh processes, Wigley, Plummer, and Pearson (1978) formulated a general mathematical framework to model the evolution of carbon isotopes in a closed system (WPP model). This equation is also applicable to other elements with one dominant isotope, such as oxygen. The WPP equation is an overlooked but important model for the interpretation of isotope composition in closed system conditions. The purpose of this presentation is to demonstrate the applicability of WPP equation to the interpretation of the isotopic composition of DO in aqueous systems.