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Abstract Particulate organic matter settling out of the euphotic zone is a major sink for atmospheric carbon dioxide and serves as a primary food source to mesopelagic food webs. Degradation of this organic matter encompasses a suite of mechanisms that attenuate flux, including heterotrophic metabolic processes of microbes and metazoans. The relative contributions of microbial and metazoan heterotrophy to flux attenuation, however, have been difficult to determine. We present results of compound specific nitrogen isotope analysis of amino acids of sinking particles from sediment traps and size‐fractionated particles from in situ filtration between the surface and 500 m at Ocean Station Papa, collected during NASA EXPORTS (EXport Processes in the Ocean from RemoTe Sensing). With increasing depth, we observe: (1) that, based on theδ15N values of threonine, fecal pellets dominate the sinking particle flux and that attenuation of downward particle flux occurs largely via disaggregation in the upper mesopelagic; (2) an increasing trophic position of particles in the upper water column, reflecting increasing heterotrophic contributions to the nitrogen pool and the loss of particles via remineralization; and (3) increasingδ15N values of source amino acids in submicron and small (1–6μm) particles, reflecting microbial particle solubilization. We further employ a Bayesian mixing model to estimate the relative proportions of fecal pellets, phytodetritus, and microbially degraded material in particles and compare these results and our interpretations of flux attenuation mechanisms to other, independent methods used during EXPORTS.
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Microbial and Metazoan Activity in Monterey Bay, CA Recorded in Nitrogen Isotope Ratios of Sinking and Suspended Particles
Abstract Particulate organic matter supports pelagic food webs, and the activity of these food webs attenuates the flux of carbon into the ocean interior. Understanding the extent to which microbial and metazoan heterotrophs influence particle dynamics is essential to describing the biological carbon pump and nutrient delivery to deep ecosystems. We present results of bulk and compound‐specific nitrogen stable isotope analyses and a Bayesian mixing model of zooplankton fecal pellets (FP), phytoplankton, and microbial detritus end‐members on size‐fractionated particulate organic matter from 10 depths in the upper 500 m of Monterey Bay, CA. Our results suggest three distinct zones of plankton‐particle interactions in Monterey Bay: primary production and grazing from 0 to 60 m, intense microbial reworking from 60 to 200 m, and inclusion into metazoan food webs below 200 m. Zooplankton FP signatures were found in a <20 μm particle size fraction, both at the approximate depth to which zooplankton migrate at night (∼25–60 m) and in the mesopelagic at the approximate depth to which zooplankton migrate during the day (∼200 m). This finding indicates that fecal pellets were rapidly disaggregated at the depth at which they were produced, which has implications for estimates of zooplankton FP contribution to carbon export and modeling efforts. In some water columns, much of zooplankton FP production may be disaggregated and entrained in the epipelagic zone, above the export depth.
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- Award ID(s):
- 1830016
- PAR ID:
- 10638849
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 130
- Issue:
- 10
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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