Dynamics of dissolved oxygen isotopic ratios: a transient model to quantify primary production, community respiration, and air–water exchange in aquatic ecosystems

Abstract

Dissolved O(2) is an important aquatic ecosystem health indicator. Metabolic and gas exchange (G) rates, which control O(2) concentration, are affected by nutrient loading and other environmental factors. Traditionally, aquatic metabolism has been reported as primary production:community respiration (P:R) ratios using diel measurements and interpretations of dissolved O(2) and/or CO(2) concentrations, and recently using stable isotopes (delta(18)O, Delta(17)O) and steady state assumptions. Aquatic ecosystems, such as rivers and ponds, are not at steady state and exhibit diel changes, so steady state approaches are often inappropriate. A dynamic O(2) stable isotope model (photosynthesis-respiration-gas exchange; PoRGy) is presented here, requiring a minimum of parameters to quantify daily averaged P, R, and G rates under transient field conditions. Unlike steady state approaches, PoRGy can address scenarios with 100% O(2) saturation but with delta(18)O-O(2) values that are not at air equilibrium. PoRGy successfully accounts for isotopic G when applied to an oxygen isotope equilibration laboratory experiment. PoRGy model results closely matched the diel O(2) and delta(18)O-O(2) data from three field sites with different P:R:G ratios and various P, R and G rates. PoRGy provides a new research tool to assess ecosystem health and to pose environmental impact-driven questions. Using daily averaged rates was successful and thus they can be used to compare ecosystems across seasons and landscapes.

Intra- and inter-annual variability in metabolism in an oligotrophic lake

“…3 in Venkiteswaran et al (2007). For each day, NEP was calculated as the difference between temperature-corrected GPP and R. The autocorrelation (ACF) and partial autocorrelation (PACF) functions for all three metabolic rates for each year indicated significant autocorrelation, especially at the 1-3 day lag for all models.…”

Impact of tributary DOM and nutrient inputs on the nearshore ecology of a large, oligotrophic lake (Georgian Bay, Lake Huron, Canada)

“…The P/R ratios should be viewed with caution, as physical processes and diel variations can make the steady state model unreliable (Venkitsewaran et al 2007;Wang et al 2008), but comparisons with the results of oxygen light and dark bottle experiments in this study area in 2004 (S. Bocaniov unpublished data) and in Hamilton Harbour (western Lake Ontario) in 2003 and 2004 (Bocaniov et al 2012) indicated a substantial agreement between methods. Rather than an overriding influence of allochthonous DOM, the P/R ratios indicated a dual influence of allochthonous DOM and inorganic nutrients.…”

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

“…Unfortunately, both methods relied on timeconsuming incubations that would prevent large-scale studies such as those conducted across the Louisiana continental shelf. Furthermore, in some instances, the study by Venkiteswaran et al (2007) had to use extremely low ɛ values (−2.0‰) to produce a good agreement between observed and modeled data.…”

“…Recently, two alternative models have been developed that use oxygen concentrations and stable isotopes values to characterize productivity in aquatic systems (Tobias et al 2007;Venkiteswaran et al 2007). These models attempt to fit model parameters to measured diurnal oxygen patterns.…”