Latitudinal variations in the contribution by copepod fecal pellets to organic carbon and amino acid flux

摘要

Copepod fecal pellets can be an important vector for transporting organic carbon and amino acids from the surface waters to deep water and the sediments. Understanding the processes, e.g., fecal pellet production, organic carbon composition and concentration along with dissolved organic carbon release, that influence the contribution made by copepod fecal pellets to organic carbon flux is crucial for ocean carbon budgets. These processes were studied in different geographical locations, i.e., the Barents Sea, North Atlantic, Arabian Sea and Equatorial Pacific, to determine what environmental variables influenced fecal pellet carbon flux. Total organic carbon content of the fecal pellets was measured using two different methods. The first method involved collecting 100 pellets for C-H-N analysis. The second approach was done by injecting individual fecal pellets into a high temperature combustion system. Fecal pellet carbon was correlated with pellet volume. The slope of the carbon:volume ratio was not significantly different between study sites and for different food types. Food quality appeared to influence the concentration of total organic carbon and amino acid carbon within the fecal pellets such that increases in food quality resulted in a lower organic carbon and amino acid content in the pellets. Total amino acid carbon comprised 1-84% of the organic carbon while the dissolved amino acids made up 1-68% of the amino acid fraction.;Dissolved organic carbon (DO$sp{14}$C) release was traced using $sp{14}$C-labeled fecal pellets suspended in a simulated water column or suspended in rotating bottles. DOC and dissolved amino acids were released from copepod fecal pellets sinking through the water under both biotic and abiotic conditions. However, the molecular composition changed reflecting the effect of heterotrophic bacteria and flagellates on DOC release from fecal pellets. Under abiotic conditions the DOC was primarily of low molecular weight ($<$5000 daltons) and both dissolved free and dissolved combined amino acids accumulated in the water surrounding the pellets. In contrast, under biotic conditions the DOC was primarily $>$5000 daltons and only dissolved combined amino acids accumulated in the water. Up to 50% of the fecal pellet organic carbon could be lost to the DOC pool within the first 2 days.;Within these study sites, estimates based on measured fecal pellet carbon production (mgC m$sp{-2}$ day$sp{-1}$) within the euphotic zone indicated that copepod fecal pellets could contribute 1-100% of the sinking particulate organic carbon (POC) flux from the euphotic zone. The highest fecal pellet carbon production (mgC m$sp{-2}$ day$sp{-1}$) occurred when zooplankton abundance was $rm {>}10sp5 msp{-3}.$ Curiously, the area with the maximum, potential fecal pellet contribution to organic carbon flux occurred in the equatorial Pacific where the high water temperatures resulted in enhanced fecal pellet production rates per copepod. While estimates of copepod fecal pellet carbon contributions to sinking POC flux were highest in the equatorial Pacific, potential fecal pellet contribution to amino acid cycles and fluxes increased with latitude and fecal pellet production of amino acids was highest in the North Atlantic.