Compound-Specific Isotope Analysis of Zooplankton Fecal Pellets: Insights into Dietary and Trophic Processes and Characterization of Fecal Pellets as Organic Matter End-Member

Zooplankton fecal pellets are a major contributor to the vertical flux of particulate organic matter (POM) through the marine water column and into sediments. However, the contribution of zooplankton fecal pellets to POM is difficult to quantify. Current estimates and models often rely on visual identification of pellets to determine their relative input to POM pools, but these methods only capture fecal pellets that are intact or recognizable. Lipid biomarkers are also used to quantify fecal pellet contributions, but fecal pellets are not reliably biochemically distinct from zooplankton food sources. We used stable isotopic compositions to characterize zooplankton fecal pellets and detect their contribution to POM in the marine environment.

Zooplankton digestion alters the isotopic composition of its diet. Altabet and Small (1990) demonstrated, in the northeast Pacific and Sargasso Sea, that the bulk δ¹⁵N value of fecal pellets lies between that of zooplankton biomass and their food source. We suggest that compound-specific isotope analyses of amino acids (CSIA-AA) can determine whether bulk isotopic signatures are derived from increases in trophic position or enteric microbial activity. Here we present bulk and CSIA-AA data of carbon and nitrogen from zooplankton and fecal pellets from the northeast Pacific, Bermuda, and Antarctica. Our measurements distinguish among multiple size classes and taxonomic classes of zooplankton. These analyses provide an isotopic "fingerprint" of the fecal pellet end-member in POM derived from mixed sources. We present a preliminary quantitative model to estimate fecal pellet contributions based on these data. We also present corresponding concentration data of cholesterol, a commonly used zooplankton biomarker. We assess how variations in the relative concentration of cholesterol among zooplankton types and fecal pellets affect the use of this lipid as a zooplankton indicator, as compared to variation in our findings using CSIA-AA.

Altabet MA, LF Small (1990) Geochimica et Cosmochimica Acta 54, 155-163.