Autonomous, high-resolution observations of particle flux in the oligotrophic ocean

Abstract

Abstract. Observational gaps limit our understanding of particle flux attenuation through the upper mesopelagic because available measurements (sediment traps and radiochemical tracers) have limited temporal resolution, are labor-intensive, and require ship support. Here, we conceptually evaluate an autonomous, optical proxy-based method for high-resolution observations of particle flux. We present four continuous records of particle flux collected with autonomous profiling floats in the western Sargasso Sea and the subtropical North Pacific, as well as one shorter record of depth-resolved particle flux near the Bermuda Atlantic Time-series Study (BATS) and Oceanic Flux Program (OFP) sites. These observations illustrate strong variability in particle flux over very short (~1-day) timescales, but at longer timescales they reflect patterns of variability previously recorded during sediment trap time series. While particle flux attenuation at BATS/OFP agreed with the canonical power-law model when observations were averaged over a month, flux attenuation was highly variable on timescales of 1–3 days. Particle fluxes at different depths were decoupled from one another and from particle concentrations and chlorophyll fluorescence in the immediately overlying surface water, consistent with horizontal advection of settling particles. We finally present an approach for calibrating this optical proxy in units of carbon flux, discuss in detail the related, inherent physical and optical assumptions, and look forward toward the requirements for the quantitative application of this method in highly time-resolved studies of particle export and flux attenuation.

Carbon export by small particles in the Norwegian Sea

“…Data from Bio-Argo floats have already demonstrated that multiyear, high-resolution, vertically resolved observations can transform the way we understand ocean ecosystems and biogeochemistry [Bishop et al, 2004;Riser and Johnson, 2008;Boss and Behrenfeld, 2010;Johnson et al, 2010;Estapa et al, 2013]. The results presented here provide another example of how a global fleet of Bio-Argo floats could allow us a quantum leap in understanding of the ocean carbon cycle.…”

Changes in the Dust‐Influenced Biological Carbon Pump in the Canary Current System: Implications From a Coastal and an Offshore Sediment Trap Record Off Cape Blanc, Mauritania

“…Fluxes may change on short timescales, even within days (e.g., Estapa et al, ; Summerhayes et al, ) as seen in studies in the Sargasso Sea, the North Pacific and the Cape Blanc study area as well (Iversen et al, unpublished data). Off Cape Blanc, the event scale of upwelling favorable winds is a few days and the total upper water column may be influenced within one day only (Pradhan et al, ; Postel, ).…”

Direct Observations of Biological Carbon Export From Profiling Floats in the Subtropical North Atlantic

“…• Supporting Information S1 • Figure S1 Lagrangian float behavior (Bishop, 2009;Bishop et al, 2004Bishop et al, , 2016Bishop & Wood, 2009;Estapa et al, 2013Estapa et al, , 2017. All of these techniques require calibration against appropriate samples-either suspended particles in the case of the methods integrating particle stocks (e.g., Cetinić et al, 2012) or carbon flux samples from direct sediment traps (Estapa et al, 2017).…”

“…As with geochemical budget estimates, if one assumes that the floats behave in a Lagrangian manner, or that there is little horizontal variability, then the rates of change of these depth‐integrated stocks between consecutive profiles provide estimates of export fluxes (Briggs et al, ; Dall'Olmo & Mork, ) and sinking speeds (Jackson et al, ). Upward facing optical and imaging sensors or optical sediment traps (OSTs) are used to physically trap and detect sinking particles during single profiles, eliminating the need to assume Lagrangian float behavior (Bishop, ; Bishop et al, , ; Bishop & Wood, ; Estapa et al, , ). All of these techniques require calibration against appropriate samples—either suspended particles in the case of the methods integrating particle stocks (e.g., Cetinić et al, ) or carbon flux samples from direct sediment traps (Estapa et al, ).…”