Concentrations, ratios, and sinking fluxes of major bioelements at Ocean Station Papa

Montserrat Roca-Martí,Margaret L Estapa,Samantha J Clevenger,Claudia R Benitez-Nelson,Abigale M Wyatt,Blaire P Umhau,Laure Resplandy,Ken O Buesseler,Tristan J Horner,Steven Pike

doi:10.1525/elementa.2020.00166

Abstract

Fluxes of major bioelements associated with sinking particles were quantified in late summer 2018 as part of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign near Ocean Station Papa in the subarctic northeast Pacific. The thorium-234 method was used in conjunction with size-fractionated (1–5, 5–51, and &gt;51 μm) concentrations of particulate nitrogen (PN), total particulate phosphorus (TPP), biogenic silica (bSi), and particulate inorganic carbon (PIC) collected using large volume filtration via in situ pumps. We build upon recent work quantifying POC fluxes during EXPORTS. Similar remineralization length scales were observed for both POC and PN across all particle size classes from depths of 50–500 m. Unlike bSi and PIC, the soft tissue–associated POC, PN, and TPP fluxes strongly attenuated from 50 m to the base of the euphotic zone (approximately 120 m). Cruise-average thorium-234-derived fluxes (mmol m–2 d–1) at 120 m were 1.7 ± 0.6 for POC, 0.22 ± 0.07 for PN, 0.019 ± 0.007 for TPP, 0.69 ± 0.26 for bSi, and 0.055 ± 0.022 for PIC. These bioelement fluxes were similar to previous observations at this site, with the exception of PIC, which was 1 to 2 orders of magnitude lower. Transfer efficiencies within the upper twilight zone (flux 220 m/flux 120 m) were highest for PIC (84%) and bSi (79%), followed by POC (61%), PN (58%), and TPP (49%). These differences indicate preferential remineralization of TPP relative to POC or PN and larger losses of soft tissue relative to biominerals in sinking particles below the euphotic zone. Comprehensive characterization of the particulate bioelement fluxes obtained here will support future efforts linking phytoplankton community composition and food-web dynamics to the composition, magnitude, and attenuation of material that sinks to deeper waters.

Highlights

The biological carbon pump redistributes atmospheric carbon dioxide into the ocean interior principally through the formation and export of particulate organic carbon (POC) via a combination of physical and biological processes (Boyd et al, 2019)
Profiles of major bioelements and 234Th in size-fractionated particles POC and particulate nitrogen (PN) concentrations in small particles were an order of magnitude higher than those measured in midsized and large particles (Figures 2 and 3)
We have presented the distribution of size-fractionated POC, PN, total particulate phosphorus (TPP), biogenic silica (bSi), particulate inorganic carbon (PIC), and 234Th concentrations in the upper 500 m of the water column during the EXPORTS field campaign in the subarctic NE Pacific

Summary

Introduction

The biological carbon pump redistributes atmospheric carbon dioxide into the ocean interior principally through the formation and export of particulate organic carbon (POC) via a combination of physical and biological processes (Boyd et al, 2019). The biological carbon pump redistributes many other bioelements, such as particulate nitrogen (PN), total particulate phosphorus (TPP), biogenic silica (bSi), and particulate inorganic carbon (PIC) that play critical roles in a variety of ecological and chemical processes, such as biological production, scavenging of particle-reactive trace elements, and particle export. As these bioelements are cycled through mesopelagic food webs, they may be transformed back into dissolved phases (i.e., remineralized), though not necessarily at the same rate (Arıstegui et al, 2009; Twining et al, 2014; Steinberg and Landry, 2017). Accurately quantifying particle flux and attenuation is crucial in order to better understand the factors that control the transfer of material to depth in marine systems.

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Conclusion