Development of Productivity Models for the Northern Gulf of Mexico Based on Oxygen Concentrations and Stable Isotopes

Oxygen concentrations have been used for decades to estimate primary production (P) and respiration (R) in aquatic ecosystems. Yet, this approach cannot separate the effects of biological and physical processes affecting oxygen dynamics; therefore, it is now often complemented with the analysis of stable oxygen isotopes. Existing algorithms for calculating primary production and P/R have been developed for closed systems and steady-state open systems. None of these formulations are applicable to productive aquatic ecosystems where diurnal changes in oxygen concentrations and isotope values are usually large. Here, we describe a novel P/R model that includes algorithms for air-sea gas exchange and is not constrained by steady-state conditions. Our objective was to test model sensitivity to variations in input parameters for values commonly observed in coastal waters of the northern Gulf of Mexico. The model was highly sensitive to variations in fractionation factor for respiration (epsilon) but less sensitive to variations in wind speed, oxygen isotope values of source-water, or oxygen flux through the picnocline. This model is easily transferable to other coastal ecosystems, with a caveat that system-specific values for epsilon are needed to obtain realistic estimates of P/R.