Modest net autotrophy in the oligotrophic ocean. Issue 4 (18th April 2017)

• Record Type: Journal Article
• Authors: Letscher, Robert T.; Moore, J. Keith
• Is Part Of: Global biogeochemical cycles. Volume 31:Issue 4(2017:Apr.)
• Page Start: 699
• Page End: 708
• Publication Date: 2017-04-18
• Subjects: net community production -- metabolic state -- oligotrophic gyres -- autotrophic -- heterotrophic; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1002/2016GB005503
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: The metabolic state of the oligotrophic subtropical ocean has long been debated. Net community production (NCP) represents the balance of autotrophic carbon fixation with heterotrophic respiration. Many in vitro NCP estimates based on oxygen incubation methods and the corresponding scaling relationships used to predict the ecosystem metabolic balance have suggested the ocean gyres to be net heterotrophic; however, all in situ NCP methods find net autotrophy. Reconciling net heterotrophy requires significant allochthonous inputs of organic carbon to the oligotrophic gyres to sustain a preponderance of respiration over in situ production. Here we use the first global ecosystem‐ocean circulation model that contains representation of the three allochthonous carbon sources to the open ocean, to show that the five oligotrophic gyres exhibit modest net autotrophy throughout the seasonal cycle. Annually integrated rates of NCP vary in the range ~1.5–2.2 mol O2 m −2 yr −1 across the five gyre systems; however, seasonal NCP rates are as low as ~1 ± 0.5 mmol O2 m −2 d −1 for the North Atlantic. Volumetric NCP rates are heterotrophic below the 10% light level; however, they become net autotrophic when integrated over the euphotic zone. Observational uncertainties when measuring these modest autotrophic NCP rates as well as the metabolic diversity encountered across space and time complicate the scaling up of in vitro measurements to the ecosystem scale and may partially explain the previous reports of net heterotrophy. The oligotrophic ocean is autotrophic at present; however, it could shift toward seasonal heterotrophy in the future as rising temperatures stimulate respiration. Key Points: Allochthonous organic carbon support for oligotrophic gyre net heterotrophic metabolism is tested in a 3‐D biogeochemical ocean model Annual euphotic zone integrated net community production is positive for all five subtropical gyres Scaling functions relating volumetric respiration to O2 production rates lead to spurious prediction of net heterotrophy at low productivity