Abstract
AbstractDiel variations in oxygen concentration have been extensively used to estimate rates of photosynthesis and respiration in productive freshwater and marine ecosystems. Recent improvements in optical oxygen sensors now enable us to use the same approach to estimate metabolic rates in the oligotrophic waters that cover most of the global ocean and for measurements collected by autonomous underwater vehicles. By building on previous methods, we propose a procedure to estimate photosynthesis and respiration from vertically resolved diel measurements of oxygen concentration. This procedure involves isolating the oxygen variation due to biological processes from the variation due to physical processes, and calculating metabolic rates from biogenic oxygen changes using linear least squares analysis. We tested our method on underwater glider observations from the surface layer of the North Pacific Subtropical Gyre where we estimated rates of gross oxygen production and community respiration both averaging 1.0 mmol O2 m−3 d−1, consistent with previous estimates from the same environment. Method uncertainty was computed as the standard deviation of the fitted parameters and averaged 0.6 and 0.5 mmol O2 m−3 d−1 for oxygen production and respiration, respectively. The variability of metabolic rates was larger than this uncertainty and we were able to discern covariation in the biological production and consumption of oxygen. The proposed method resolved variability on time scales of approximately 1 week. This resolution can be improved in several ways including by measuring turbulent mixing, increasing the number of measurements in the surface ocean, and adopting a Lagrangian approach during data collection.
Highlights
Already in 1930, Butcher et al reported that the photosynthetic production of O2 during daytime and its continuous consumption through respiration resulted in diel oscillations in O2 concentration in three British rivers (Butcher et al 1930)
A typical rate of gross O2 production (GOP) in the North Pacific Subtropical Gyre (NPSG) on the order of 1 mmol O2 m−3 d−1 is associated with a diel peak amplitude in O2 concentration of only 0.5 mmol O2 m−3 which is two orders of magnitude lower than the baseline O2 concentration of ~200 mmol m−3 (Williams et al 1983)
The average diapycnal O2 flux through the base of the surface layer accounted for a volumetric concentration change in the 0.02–0.19 mmol m−3 d−1 range, for diapycnal diffusivities from 10−5 to 10−4 m2 s−1. This change was smaller than the rates of GOP and community respiration (CR), but similar to the net O2 production reported for the mixed layer from the O2/Ar saturation ratio not corrected for diapycnal fluxes (Quay et al 2010; Ferrón et al 2015)
Summary
Considering the specific interest in the surface layer, we only used Winkler measurements from the upper 30 m of the water column (two thirds of the samples were collected at ~5 m, one third of the samples at ~25 m), and only for shipboard and glider measurements within a distance of 10 km from each other, and a time difference of less than 0.2 d. This procedure used four and eight Winkler measurements during the two summer missions (A and B, respectively), but only one Winkler measurement during each of the shorter spring missions (C and D). As described in a previous section, we started from vertically resolved O2 measurements (Fig. 3A) and ended with a corrected surface layer O2 time-series and the curve that approximates it (Fig. 3B, Supporting Information Table S1)
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