Oxygen and Triple Oxygen Isotope Measurements Provide Different Insights into Gross Oxygen Production in a Shallow Salt
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Oxygen and Triple Oxygen Isotope Measurements Provide Different Insights into Gross Oxygen Production in a Shallow Salt Marsh Pond Evan M. Howard 1 Cara C. Manning 6
Amanda C. Spivak 2 & Jennifer S. Karolewski 3 & Kelsey M. Gosselin 4 7 & Rachel H. R. Stanley
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Zoe O. Sandwith 5
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Received: 10 January 2020 / Revised: 5 May 2020 / Accepted: 8 May 2020 # Coastal and Estuarine Research Federation 2020
Abstract The metabolism of estuarine environments is often estimated by measuring changes in dissolved oxygen concentrations. A central assumption of common oxygen-based approaches is that oxygen consumption rates (primarily respiration) are similar under light and dark conditions. Evaluating this assumption is critical, especially in benthic-dominated systems, because differences between daytime and nighttime respiration could result in underestimation or overestimation of ecosystem productivity. We evaluated rates of gross oxygen production over hourly to seasonal time scales in a shallow, temperate salt marsh pond. To assess whether a dissolved oxygen diel mass balance underestimated gross oxygen productivity, we compared rates using this traditional approach and using the triple oxygen isotope tracer of photosynthesis. This is a powerful combination because the triple oxygen isotope approach is theoretically insensitive to respiration. The methods agreed well over daily to seasonal time scales. However, during midday periods of peak light and productivity, the triple oxygen isotope approach resulted in higher hourly scale gross oxygen production rates. The timing and magnitude of this short-term difference is consistent with lightdependent oxygen uptake fluxes including photoreduction and/or light-stimulated community respiration. Finally, aquatic vegetation was associated with variability in productivity across the pond. Such small-scale environmental heterogeneity is evidence that this shallow pond was not laterally well mixed, and likely contributes to the dynamism of these common estuarine environments. Keywords Metabolism . Light-dependent respiration . Oxygen . Triple oxygen isotopes . Salt marsh . Pond
Introduction Dissolved oxygen (O2) fluxes are widely used to estimate aquatic ecosystem metabolism rates, including in lakes (Staehr et al. 2010), streams (Demars et al. 2015), estuaries
(Caffrey 2004), and the ocean (Yang et al. 2017). However, attribution of these fluxes to photosynthesis and respiration is complicated because the physical and biological processes that affect net oxygen fluxes vary over time and space and may be difficult to disentangle without additional tracers. In
Communicated by Isaac Santos Electronic supplementary material The online version of this article (https://doi.org/10.1007/s12237-020-00757-6) contains supplementary material, which is available to authorized users. * Evan M. Howard [email protected] 1
School of Oceanography, University of Washington, Seattle, WA, USA
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Department of Marine Sciences, University of Georgia, Athens, GA, USA
3
MIT-WHOI Joint Program in
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