Abstract
AbstractNatural‐abundance stable isotope ratios provide a wealth of ecological information relating to food web structure, trophic level, and location. The correct interpretation of stable isotope data requires an understanding of spatial and temporal variation in the isotopic compositions at the base of the food web. In marine pelagic environments, accurate interpretation of stable isotope data is hampered by a lack of reliable, spatio‐temporally distributed measurements of baseline isotopic compositions. In this study, we present a relatively simple, process‐based carbon isotope model that predicts the spatio‐temporal distributions of the carbon isotope composition of phytoplankton (here expressed as δ13CPLK) across the global ocean at one degree and monthly resolution. The model is driven by output from a coupled physics‐biogeochemistry model, NEMO‐MEDUSA, and operates offline; it could also be coupled to alternative underlying ocean model systems. Model validation is challenged by the same lack of spatio‐temporally explicit data that motivates model development, but predictions from our model successfully reproduce major spatial patterns in carbon isotope values observed in zooplankton, and are consistent with simulations from alternative models. Model predictions represent an initial hypothesis of spatial and temporal variation in carbon isotopic baselines in ocean areas where a few data are currently available, and provide the best currently available tool to estimate spatial and temporal variation in baseline isotopic compositions at ocean basin to global scales.
Highlights
IntroductionNatural-abundance stable isotope analysis is a routine tool in ecology providing information on food web structuring, trophic interactions, and nutrient flux (e.g., Kelly 2000, Post 2002, Boecklen et al 2011, Layman et al 2012, Trueman et al 2014, Choy et al 2015), and offers a method for retrospective geolocation (e.g., Hobson 1999, Graham et al 2010, Hobson et al 2010, Wunder 2010, Trueman et al 2012, 2016, Vander Zanden et al 2015)
NEMO-MEDUSA We used the coupled physics–biogeochemistry model, NEMO-MEDUSA, to generate globalscale fields of relevant properties to estimate the carbon isotope fractionation occurring during photosynthesis
Within NEMO-MEDUSA, dissolved inorganic carbon (DIC) is always available in excess to phytoplankton and is, entirely assimilated via passive diffusion in the form of CO2(aq), in reality some phytoplankton groups may actively take up HCO3À and CO32À, when CO2(aq) is limiting
Summary
Natural-abundance stable isotope analysis is a routine tool in ecology providing information on food web structuring, trophic interactions, and nutrient flux (e.g., Kelly 2000, Post 2002, Boecklen et al 2011, Layman et al 2012, Trueman et al 2014, Choy et al 2015), and offers a method for retrospective geolocation (e.g., Hobson 1999, Graham et al 2010, Hobson et al 2010, Wunder 2010, Trueman et al 2012, 2016, Vander Zanden et al 2015). All ecological applications of stable isotope data require consideration of spatial and temporal variation in the isotopic compositions of nutrients at the base of the food web, or in local precipitation in the case of stable oxygen and hydrogen isotopes. Ocean basin-scale isoscapes have been constructed by spatial interpolation of published zooplankton d13C and d15N data (Graham et al 2010, McMahon et al 2013) Such opportunistic compilations of literature data have relatively few, unevenly distributed data points (i.e., approximately 550 data points for the Atlantic Ocean; see McMahon et al 2013), and are strongly influenced by single data points or specific cruises. Isotopic variation associated with season and year of sampling, taxa sampled, and processing methods cannot be controlled from opportunistic compilations of literature data
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