Variable carbon isotope fractionation of photosynthetic communities over depth in an open-ocean euphotic zone

Author:

Henderson Lillian C.1ORCID,Wittmers Fabian23ORCID,Carlson Craig A.4ORCID,Worden Alexandra Z.2ORCID,Close Hilary G.1ORCID

Affiliation:

1. Department of Ocean Sciences, Rosenstiel School of Marine, Atmospheric, and Earth Science, University of Miami, Miami, FL 33149

2. Josephine Bay Paul Center for Comparative Molecular Biology and Evolution, Marine Biological Laboratory, Woods Hole, MA 02543

3. Faculty of Mathematics and Natural Sciences, Christian-Albrecht University of Kiel, Kiel SH 24118, Germany

4. Department of Ecology, Evolution, and Marine Biology/Marine Science Institute, University of California, Santa Barbara, CA 93106

Abstract

Marine particulate organic carbon (POC) contributes to carbon export, food webs, and sediments, but uncertainties remain in its origins. Globally, variations in stable carbon isotope ratios (δ 13 C values) of POC between the upper and lower euphotic zones (LEZ) indicate either varying aspects of photosynthetic communities or degradative alteration of POC. During summertime in the subtropical north Atlantic Ocean, we find that δ 13 C values of the photosynthetic product phytol decreased by 6.3‰ and photosynthetic carbon isotope fractionation (ε p ) increased by 5.6‰ between the surface and the LEZ—variation as large as that found in the geologic record during major carbon cycle perturbations, but here reflecting vertical variation in δ 13 C values of photosynthetic communities. We find that simultaneous variations in light intensity and phytoplankton community composition over depth may be important factors not fully accounted for in common models of photosynthetic carbon isotope fractionation. Using additional isotopic and cell count data, we estimate that photosynthetic and non-photosynthetic material (heterotrophs or detritus) contribute relatively constant proportions of POC throughout the euphotic zone but are isotopically more distinct in the LEZ. As a result, the large vertical differences in ε p result in significant, but smaller, differences in the δ 13 C values of total POC across the same depths (2.7‰). Vertical structuring of photosynthetic communities and export potential from the LEZ may vary across current and past ocean ecosystems; thus, LEZ photosynthesis may influence the exported and/or sedimentary δ 13 C values of both phytol and total organic carbon and affect interpretations of ε p over geologic time.

Funder

Simons Foundation

Publisher

Proceedings of the National Academy of Sciences

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