Estimating relative flux of phytodetritus, fecal pellets, and bacterial biomass using compound-specific isotope analysis of amino acids at Station Papa (EXPORTS)

The vertical flux of organic matter to mesopelagic depths in the ocean contributes both to the sequestration of atmospheric carbon and to the diets of mesopelagic fauna. Heterotrophic microbes and metazoans both can act to reduce or enhance this vertical flux through chemical and physical transformations of particles, but the relative importance of these effects is difficult to constrain. A primary objective of the EXPORTS program is to determine the relative importance of various carbon flux pathways in ocean water columns, including those mediated by zooplankton and microbes. Recent work has demonstrated that the naturally-occurring nitrogen and carbon isotopic ratios of amino acids can differentiate patterns in particulate organic matter that are indicative of their net heterotrophic processing by microbes versus zooplankton. Here we apply this method to sediment trap samples and four size fractions of sinking and suspended particles spanning from the mixed layer to 500 m depth at Station Papa (subarctic Pacific EXPORTS cruise). Using a multivariate statistical framework, we estimate the proportions of phytodetritus, microbially hydrolyzed material, fecal pellets, and bacterial biomass within particles. Combined with carbon and nitrogen flux calculated from sediment traps and thorium disequilibria, we further estimate the quantity of vertical amino acid flux associated with these categories of biomass and detritus. We will discuss these results in the context of complementary particle and biological data from the EXPORTS program.