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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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Distinguishing zooplankton fecal pellets as a component of the biological pump using compound‐specific isotope analysis of amino acids
Abstract Zooplankton contribute a major component of the vertical flux of particulate organic matter to the ocean interior by packaging consumed food and waste into large, dense fecal pellets that sink quickly. Existing methods for quantifying the contribution of fecal pellets to particulate organic matter use either visual identification or lipid biomarkers, but these methods may exclude fecal material that is not morphologically distinct, or may include zooplankton carcasses in addition to fecal pellets. Based on results from seven pairs of wild‐caught zooplankton and their fecal pellets, we assess the ability of compound‐specific isotope analysis of amino acids (CSIA‐AA) to chemically distinguish fecal pellets as an end‐member material within particulate organic matter. Nitrogen CSIA‐AA is an improvement on previous uses of bulk stable isotope ratios, which cannot distinguish between differences in baseline isotope ratios and fractionation due to metabolic processing. We suggest that the relative trophic position of zooplankton and their fecal pellets, as calculated using CSIA‐AA, can provide a metric for estimating the dietary absorption efficiency of zooplankton. Using this metric, the zooplankton examined here had widely ranging dietary absorption efficiencies, where lower dietary absorption may equate to higher proportions of fecal packaging of undigested material. The nitrogen isotope ratios of threonine and alanine statistically distinguished the zooplankton fecal pellets from literature‐derived examples of phytoplankton, zooplankton biomass, and microbially degraded organic matter. We suggest that δ15N values of threonine and alanine could be used in mixing models to quantify the contribution of fecal pellets to particulate organic matter.
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- PAR ID:
- 10375047
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 66
- Issue:
- 7
- ISSN:
- 0024-3590
- Page Range / eLocation ID:
- p. 2827-2841
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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