Comment on: "Technical note: Consistent calculation of aquatic gross production from oxygen triple isotope measurements" by Kaiser (2011)

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 (¹⁷&delta;_P) 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 (<i>f</i>) and gross productivity (<i>g</i>). A methodological bias of ±20 % is of similar magnitude to current uncertainty in the wind-speed dependence of the air-sea gas transfer velocity, <i>k</i>, 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.