Carbon and Calcium Carbonate Export Driven by Appendicularian Faecal Pellets in the Humboldt Current System off Chile

A Eduardo Menschel,Humberto E González

doi:10.1038/s41598-019-52469-y

Abstract

The role of appendicularian faecal pellet (FPa) size fractions on coccolithophore-derived particulate organic carbon (POC) and calcium carbonate (CaCO3) export to the deep sea was assessed from sediment traps within a period of ten years (1995–2004) off Coquimbo (CQ, 30°S) and five years (2005–2009) off Concepción (CC, 36°S) in the Humboldt Current System (HCS) off Chile. The composition and size distribution of 1,135 FPa samples from sediment traps deployed at 2,300 and 1,000 m depths showed non-linear, inverse relationships between the FPa size-fractions and their volume-specific POC and CaCO3 contents, which were up to ten times higher for small (<100 µm in diameter) than large (>100 µm) FPa. On average, 13 and 2% of the total POC and CaCO3 fluxes, respectively, were contributed mainly by small FPa (90%), with maxima during the autumn and summer. Thus, a non-linear, exponential model of volume-specific POC and CaCO3 contents of FPa substantially improved vertical flux rate estimates. In the HCS, annual carbon flux based on a non-linear FPa carbon load was double the estimate assuming a linear-volume to carbon load for FPa (345 and 172 kton C y−1). We recommend a widespread consideration of this non-linear model in global carbon estimates.

Highlights

The wind stress that favours upwelling along the coast off Chile predominates during spring-summer and contributes to fertilization that promotes high primary production (>20 g C m−2 d−1)[1] and phytoplankton biomass (>5 mg m−3) in the surface waters of the HCS6
Efficiency of the total particulate organic carbon (POC) flux temporally and spatially depend on changes in zooplankton and phytoplankton community compositions, as well as on changes in the biological processes that affect the dynamics of particle flux from the euphotic zone in oceanic provinces[16] and upwelling areas off central Chile[12]
We found that 89% of the FPa were between 10 and 100 μm in diameter, highlighting the pivotal role of small FPa in fluxes of carbon and calcium carbonate in the oceanic region of the Humboldt Current System (HCS) off central Chile (Fig. 2)

Summary

Introduction

The wind stress that favours upwelling along the coast off Chile predominates during spring-summer and contributes to fertilization (e.g. nitrate, phosphate) that promotes high primary production (>20 g C m−2 d−1)[1] and phytoplankton biomass (>5 mg m−3) in the surface waters of the HCS6. Efficiency of the total POC flux temporally and spatially depend on changes in zooplankton and phytoplankton community compositions, as well as on changes in the biological processes (i.e. zooplankton grazing rate, faecal pellet production rate, phytoplankton aggregation, microbial degradation) that affect the dynamics of particle flux from the euphotic zone in oceanic provinces[16] and upwelling areas off central Chile[12]. Several hundred sediment trap studies since the 1970s have reported the pivotal role of zooplankton faecal pellets (FP) in the carbon biogeochemical cycle[14,21,22,23] This plethora of information demonstrates the importance of zooplankton FP in the export of particulate organic carbon (POC) from the photic zone to the deep sea[24,25]. Appendicularians usually represent a minor fraction of total mesozooplankton biomass; they can process large amounts of food in a short time and produce numerous FP that contribute to export production[27,28], playing significant roles in the export of the POC14 and calcium carbonate (CaCO3) to the deep sea[6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29] in many disparate areas of the world’s oceans

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Conclusion