Annual Net Community Production of Particulate and Dissolved Organic Carbon From a Decade of Biogeochemical Profiling Float Observations in the Northeast Pacific

Abstract

Carbon export out of the surface ocean via the biological pump is a critical sink for atmospheric carbon dioxide. This process transports organic carbon to the deep ocean through sinking particulate organic carbon (POC) and the downward transport of suspended POC and dissolved organic carbon (DOC). Changes in the relative contribution of each pathway can significantly affect the magnitude and efficiency of carbon export to depth. Net community production (NCP), an analog of carbon export under steady state assumptions, is typically estimated using budgets of biologically important chemical tracers in the upper ocean constrained by ship-board or autonomous platform observations. In this study, we use measurements from biogeochemical profiling floats, the Ocean Station Papa mooring, and recently developed algorithms for carbonate system parameters to constrain budgets for three tracers (nitrate, dissolved inorganic carbon, and total alkalinity) and estimate NCP in the Northeast Pacific from 2009 to 2017. Using our multiple-tracer approach, and constraining end-member nutrient ratios of the POC and DOC produced, we not only calculate regional NCP throughout the annual cycle and across multiple depth horizons, but also partition this quantity into particulate and dissolved portions. We also use a particle backscatter-based approach to estimate POC attenuation with depth and present a new method to constrain particle export across deeper horizons and estimate in situ export efficiency. Our results agree well with previously published estimates of regional carbon export annually and suggest that the approaches presented here could be used to assess the magnitude and efficiency of carbon export in other regions of the world's oceans. Plain Language Summary "Carbon export" refers to the amount of carbon dioxide that is removed from the atmosphere by organisms in the surface ocean and subsequently transported into the deep sea, either through sinking particles (more efficient process) or downward mixing (less efficient process), making the ocean a natural sink for atmospheric carbon dioxide and significantly influencing ocean chemistry. The relative proportion of export through each pathway significantly affects the overall efficiency of this process and has implications for the pattern of carbon export globally. Measuring carbon export throughout the year traditionally requires persistent ship-based observations, which can be costly and perilous for researchers. Instead, carbon export is often estimated by budgeting nutrient distributions and changes through time, as they are also controlled by the same processes. These measurements can now be made remotely using autonomous biogeochemical profiling floats. Here, we present a new approach utilizing multiple chemical budgets to estimate carbon export over a decade in the Northeast Pacific, which can be combined to partition export occurring through sinking particles and downward mixing. Our results are supported by previously published estimates of carbon export and sugges...

An operational overview of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) Northeast Pacific field deployment

“…These deeper fractional isolume depths reflect the lower Chl a concentrations observed during EXPORTS compared with typical Line P observations for the period from 2007 to 2017. Since 2008, a series of BGC profiling floats have collected data on the physical and biological properties of the water column in the vicinity of Station P (Figure 7; Bushinsky and Emerson, 2015;Plant et al, 2016;Yang et al, 2018;Bif et al, 2019;Haskell et al, 2020). Mixed layer temperatures change on average by 6 C to 9 C from winter to summer, with the EXPORTS cruise happening during the warmest time of the year.…”

Geophysical and biogeochemical observations using BGC Argo floats in the western North Pacific during late winter and early spring, Part 2: Biological processes during restratification periods in the euphotic and twilight layers

“…To observe this in detail, it will be necessary to investigate the behavior of DOC and dissolved inorganic carbon, which were not quantified in this study. Fortunately, BGC Argo floats can be equipped with additional sensors, such as pH and fDOM, that may allow for the quantitative evaluation of various biogenic carbon pools (e.g., Haskell et al, 2020). To observe biogeochemical processes in the ocean and clarify the cycling of materials, it is essential to observe and analyze combined physical, chemical, and biological datasets.…”

Impact of the Elemental Composition of Exported Organic Matter on the Observed Dissolved Nutrient and Trace Element Distributions in the Upper Layer of the Ocean

“…In the subpolar North Pacific at OSP Haskell et al. (2020) use multiyear (2009–2017) profiling float and mooring based NO 3 , pCO 2 , and O 2 measurements to conclude that turbulent diffusion rates at the base of the photic layer estimated from heat budgets at OSP (Cronin et al., 2015) supplied sufficient NO 3 and DIC to support annual OM export. The increasing availability of continuous T, S, NO 3 , O 2 , and pCO 2 measurements provided by floats, moorings and seagliders will significantly improve estimates of turbulent mixing rates at the base of the photic layer (e.g., Cronin et al., 2015; Pelland et al., 2018), frequency of isopycnal displacement events (e.g., Johnson et al., 2010; Nicholson et al., 2008) and yield highly resolved vertical gradients of NO 3 , DIC, and O 2 in the upper ocean (e.g., Haskell et al., 2020) which together will help answer the question about importance of vertical versus horizontal nutrient supply in the surface ocean.…”

“…ΔDIC/ΔNO 3 /ΔPO 4 toward 199/20/1 Haskell et al (2020). came to a similar conclusion at OSP, based on multiyear(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017) float and mooring based photic layer NO 3 and DIC budgets, that an observed decrease in the C/N of exported OM with increasing depth was likely the result of a shift from DOM to particle degradation.…”