Abstract
Abstract. Kaiser (2011) has introduced an improved method for calculating gross productivity from the triple isotopic composition of dissolved oxygen in aquatic systems. His equation avoids approximations of previous methodologies, and also accounts for additional physical processes such as kinetic fractionation during invasion and evasion at the air-sea interface. However, when comparing his new approach to previous methods, Kaiser inconsistently defines the biological end-member with the result of overestimating the degree to which the various approaches of previous studies diverge. In particular, for his base case, Kaiser assigns a 17O excess to the product of photosynthesis (17δP) that is too low, resulting in his result being ~30 % too high when compared to previous equations. When this is corrected, I find that Kaiser's equations are consistent with all previous study methodologies within about ±20 % for realistic conditions of metabolic balance (f) and gross productivity (g). A methodological bias of ±20 % is of similar magnitude to current uncertainty in the wind-speed dependence of the air-sea gas transfer velocity, k, which directly impacts calculated gross productivity rates as well. While previous results could and should be revisited and corrected using the proposed improved equations, the magnitude of such corrections may be much less than implied by Kaiser.
Highlights
IntroductionIn the manuscript “Consistent calculation of aquatic gross production from oxygen triple isotope measurements” Kaiser derives exact equations for calculating gross oxygen pro-
In the manuscript “Consistent calculation of aquatic gross production from oxygen triple isotope measurements” Kaiser derives exact equations for calculating gross oxygen pro-duction (GOP) from the triple oxygen isotopic composition of dissolved oxygen (17 )
The derived equations improve upon previous methods of calculating gross oxygen pro-duction (GOP) in that they avoid approximations and account for additional processes such as kinetic fractionation during air-sea evasion and invasion of oxygen
Summary
In the manuscript “Consistent calculation of aquatic gross production from oxygen triple isotope measurements” Kaiser derives exact equations for calculating gross oxygen pro-. The crux of the discrepancy is in the assumptions Kaiser uses to calculated the relation between 17δP and 18δP, where the subscript “P” refers to dissolved oxygen produced by photosynthesis. In his “base case” used for comparison of methods, Kaiser uses Eq (2) by assuming a 17 #P(λ = 0.518) = 249 ppm where 249 ppm is the biological endmember value reported by Luz and Barkan (2000). I will describe how 17δP and 18δP should have been defined using a slope of λBSS = 0.5154 instead of γ R With this correction, Kaiser’s “base case” 17 value would have been 58 ppm higher To communicate the difference between previous methodology and the proposed method, it is important to clarify the relative roles of (1) measured physical parameters used in calculations, such as 17δ and 18δ and fractionation factors 18εR and (2) the accuracy of various equations under varying conditions of metabolic balance (f ) and productivity (g) when the same physical parameters are used
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