Abstract
Abstract. It has been proposed that the disagreement traditionally reported between in vitro incubation and in situ estimates of oxygen net community production (NCP) could be explained, at least partially, by undersampling episodic pulses of net autotrophy associated with mesoscale dynamics. In this study we compare in vitro incubation estimates of net community production with in situ estimates, derived from oxygen profiles and a 1-D model, within a cyclonic eddy investigated in the Sargasso Sea in summer 2004. The in vitro NCP rates measured at the center of the eddy showed a shift from net autotrophy (7±3 mmol O2 m−2 d−1) to net heterotrophy (−25±5 mmol O2 m−2 d−1) from late June to early August. The model-derived NCP rates also showed a temporal decline (19±6 to −3±7 and 11±8 mmol O2 m−2 d−1), but they were systematically higher than the in vitro estimates and reported net autotrophy or balance for the sampling period. In this comparison episodic pulses in photosynthesis or respiration driven by mesoscale eddies can not explain the discrepancy between the in vitro and in situ estimates of NCP. This points to methodological artefacts or temporal or submesoscale variability as the mechanisms responsible for the disagreement between the techniques, at least in this dataset.
Highlights
Net community production (NCP), the difference between gross primary production (GPP) and total respiration (R), constitutes a crucial term for comprehending the role of the marine biota as source or sink for CO2
An excess of photosynthesis over respiration (NCP>0), called net autotrophy, implies a net synthesis of organic carbon that can be exported to the deep ocean or to higher trophic levels
Measurements of instantaneous oxygen production and respiration rates by in vitro bottle incubations indicate net oxygen consumption, or heterotrophy, in the sunlit zone prevailing throughout large areas (Duarte et al, 2001; Robinson et al, 2002; Williams et al, 2004), whereas in situ geochemical budgets estimate that the upper layer of the ocean is a net source of oxygen (Emerson et al, 2008; Najjar and Keeling, 2000; Riser and Johnson, 2008)
Summary
In recent years numerous studies have been conducted to determine the metabolic state of subtropical regions. Determining net community production by measuring oxygen changes in situ relies on the ability to sample consistently within the same water mass and to account for non-biological processes that can alter oxygen concentration. Measurements of net community production (NCP) and respiration (R) based on in vitro changes in oxygen evolution were conducted during two of the four cruises (Mourino-Carballido and McGillicuddy, 2006). In order to investigate if undersampling of enhanced NCP within mesoscale eddies could explain at least part of the disagreement previously reported between in vitro and in situ techniques, we used a 1-D model to: 1) compare in vitro and in situ estimates of oxygen NCP in the cyclonic eddy C1, and 2) estimate the relative contribution of biological and physical processes to the changes in oxygen concentration observed at the eddy center through the sampling period
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