Biome-specific scaling of ocean productivity, temperature, and carbon export efficiency

PUBLICATIONS Geophysical Research Letters RESEARCH LETTER 10.1002/2016GL068778 Key Points: • Optimized models predict carbon export efﬁciency from net primary production and sea surface temperature • Biome-speciﬁc relationships impact global export inferred from satellite-derived variables • Individual biomes respond differently to simulated net primary production and sea surface temperature changes Supporting Information: • Supporting Information S1 • Data Set S1 • Data Set S2 Correspondence to: G. L. Britten, gbritten@uci.edu Biome-speciﬁc scaling of ocean productivity, temperature, and carbon export efﬁciency Gregory L. Britten 1 and Francois W. Primeau 1 Department of Earth System Science, University of California, Irvine, USA Abstract Mass conservation and metabolic theory place constraints on how marine export production (EP) scales with net primary productivity (NPP) and sea surface temperature (SST); however, little is empirically known about how these relationships vary across ecologically distinct ocean biomes. Here we compiled in situ observations of EP, NPP, and SST and used statistical model selection theory to demonstrate signiﬁcant biome-speciﬁc scaling relationships among these variables. Multiple statistically similar models yield a threefold variation in the globally integrated carbon ﬂux (~4–12 Pg C yr 1 ) when applied to climatological satellite-derived NPP and SST. Simulated NPP and SST input variables from a 4×CO 2 climate model experiment further show that biome-speciﬁc scaling alters the predicted response of EP to simulated increases of atmospheric CO 2 . These results highlight the need to better understand distinct pathways of carbon export across unique ecological biomes and may help guide proposed efforts for in situ observations of the ocean carbon cycle. 1. Introduction Citation: Britten, G. L., and F. W. Primeau (2016), Biome-speciﬁc scaling of ocean pro- ductivity, temperature, and carbon export efﬁciency, Geophys. Res. Lett., 43, 5210–5216, doi:10.1002/2016GL068778. Received 21 MAR 2016 Accepted 29 APR 2016 Accepted article online 2 MAY 2016 Published online 24 MAY 2016 ©2016. American Geophysical Union. All Rights Reserved. BRITTEN AND PRIMEAU Ocean biology plays a signiﬁcant role in the global carbon cycle by exporting organic carbon to deep waters where it is remineralized back to CO 2 and may remain sequestered for timescales of tens to thousands of years [Kwon et al., 2009; DeVries et al., 2012]. However, ocean regions are characterized by unique ecological biomes with variable ecosystem structure that exhibit distinct pathways of carbon uptake, trophic transfer, and remineralization [DeVries and Deutsch, 2014; Teng et al., 2014; Henson et al., 2015] and likely respond differently to climate-driven changes in temperature and primary productivity. Satellite-derived oceanic net primary productivity (NPP) and sea surface temperature (SST, here used as a proxy for euphotic zone temperature) are thought to provide a basis to infer oceanic carbon export at global scales [Dunne et al., 2005; Henson et al., 2011; Laws et al., 2011; Siegel et al., 2014], yet the relationships used for global extrapolation remain uncertain, and very little is known about how the functional form and/or rate parameters vary across unique biomes and ecosystem types [Henson et al., 2015]. Cost and logistical difﬁculty has limited the observational basis from which to study and extrapolate biome-speciﬁc variability to the global scale. An upcoming 5 year NASA study [Siegel et al., 2015] aims to address this uncertainty with increased ﬁeld-based carbon export sampling. Here we adopt a statistical approach to evaluate current evidence for biome-speciﬁc drivers of export by synthesizing available in situ observations of NPP, SST, and EP and using model selection theory to deter- mine whether these relationships depend on ecosystem type. We then investigate the spatial pattern of these relationships and evaluate the consequences for global extrapolation of export ﬂux based on NPP and SST. The most common empirical models of carbon export parameterize the fraction of NPP exported (ef) as an increasing function of NPP and a decreasing function of SST [Laws et al., 2000, 2011; Dunne et al., 2005; Henson et al., 2011]. Ecological theory states that heterotrophic respiration temporally lags autotrophic production such that an increasing fraction of NPP can be exported at high productivity by way of sinking par- ticles [Sarmiento and Gruber, 2013]. The SST effect, however, negatively scales EP by increasing the rate of respiratory biochemical reactions more quickly than autotrophic reactions such that the ratio of autotrophic production to heterotrophic metabolism declines as temperature increases [Lopez-Urrutia et al., 2006], leading to lower EP at high SST. A variety of scaling models that predict ef as a function of NPP and SST, denoted here ef(NPP, SST), have been developed to extrapolate global EP from satellite-derived NPP and SST [Laws et al., 2000, 2011; Dunne et al., 2005; Henson et al., 2011] and thus provide a relatively simple means to predict and monitor key ocean carbon cycle processes from space. These models have also been used as simpliﬁed “subeco- system scale parameterizations” [Galbraith et al., 2015] of carbon cycling processes within larger biogeochemical and climate models [Bopp et al., 2001; Deutsch et al., 2007] as well as in food web models for ﬁsheries [Friedland et al., 2012], demonstrating the wide utility of ef(NPP, SST) relationships. CARBON EXPORT ACROSS OCEAN BIOMES