Evaluation of community respiratory mechanisms with oxygen isotopes: A case study in Lake Kinneret

Abstract

Gross and net O2 production between May 1996 and February 1999 was determined in bottle incubation experiments with H218O spike and from the change in O2 concentration. Carbon fixation rates were obtained from 14C incubations. In general, production rates determined using the H218O‐spike were about twice the primary production determined by the 14C method, where the latter was close to net oxygen evolution. These relationships are similar to results for the open ocean. During the spring bloom, when the dinoflagellate Peridinium was abundant, the ratio of gross O2 production to carbon fixation was about 7.5, and net O2 production was greater than carbon fixation. The difference between O2 gross production and carbon fixation results, at least in part, from uptake by Mehler reaction and from recycling of the 14C tracer by dark respiration and the alternative oxidase (AOX). We used the difference in isotopic discrimination against 18O, occurring during O2 consumption by various biological pathways, to place constraints on the relative engagement of these pathways. We estimated the overall discrimination against 18O in the lake from O2 isotopic mass balance as 20.5—29‰. The only mechanism that can explain the strong overall fractionation in the lake is AOX, which strongly discriminates against 18O (~31‰). Our results show, for the first time, that uptake by AOX is widespread and quantitatively important to oxygen consumption in aquatic systems.