Sinking Organic Particles in the Ocean—Flux Estimates From in situ Optical Devices

Abstract

Optical particle measurements are emerging as an important technique for understanding the ocean carbon cycle, including contributions to estimates of their downward flux, which sequesters carbon dioxide (CO 2) in the deep sea. Optical instruments can be used from ships or installed on autonomous platforms, delivering much greater spatial and temporal coverage of particles in the mesopelagic zone of the ocean than traditional techniques, such as sediment traps. Technologies to image particles have advanced greatly over the last two decades, but the quantitative translation of these immense datasets into biogeochemical properties remains a challenge. In particular, advances are needed to enable the optimal translation of imaged objects into carbon content and sinking velocities. In addition, different devices often measure different optical properties, leading to difficulties in comparing results. Here we provide a practical overview of the challenges and potential of using these instruments, as a step toward improvement and expansion of their applications. Keywords: sinking particle fluxes, sinking velocities, carbon content, size, image processing, automated classification, in situ optical particle measurements, biological carbon pump * Different magnifications available. Quoted details are for the magnification that is most suitable for marine snow.

The Interpretation of Particle Size, Shape, and Carbon Flux of Marine Particle Images Is Strongly Affected by the Choice of Particle Detection Algorithm

“…We therefore urge a collective effort to develop calibration routines and specifications to accurately describe particle size. Optical particle measurements are rapidly emerging as a major tool for understanding biogeochemical cycles and food webs in the ocean (Lombard et al, 2019;Giering et al, 2020). To fully leverage these exciting technological advances and the insights they provide, the research community needs a framework that encourages increased transparency in image analysis routines (including threshold choices) and allows merging data from the plethora of optical devices that are now used to explore the ocean.…”

“…Optical measurements of particles in the ocean are rapidly establishing themselves as powerful tools to investigate ocean biogeochemical cycles and food webs (Lombard et al, 2019;Giering et al, 2020). One research area that has greatly benefited from the use of underwater camera and optical sensors is the ocean carbon cycle -specifically the biological carbon pump.…”

“…Vertical profiles of particle images can elucidate the processes that determine particle size, type and distribution. Combined with information on carbon content and sinking velocity, particle profiles can provide high-resolution information on carbon fluxes and hence ocean carbon storage (see review by Giering et al, 2020).…”

Grand Challenges in Microbe-Driven Marine Carbon Cycling Research

“…Although the ocean's total primary production is very high (up to 50 Gt C/year), only a small fraction (<10%) is transported to the deep ocean via the BCP and even a smaller fraction (<1%) is sequestered for millennia (Henson et al, 2011;Bach et al, 2019;Fender et al, 2019;Giering et al, 2020). The majority of the marine primary production is converted back to CO 2 in the ocean's twilight zone via community respiration, to which the microbes usually contribute the most (∼50% to >90%) (Rivkin and Legendre, 2001;Sanders et al, 2016).…”

“…The BCP (∼0.2-0.5 Gt C/year) and MCP (∼0.2 Gt C/year) may make similar contributions to long-term organic carbon sequestration (Guidi et al, 2015;Legendre et al, 2015;Giering et al, 2020), and both show climate geoengineering potentials (Le Moigne, 2019; Richardson, 2019). However, the BCP export efficiency has reduced ∼1.5% over the past 33 years of climate warming (Cael et al, 2017), and warmer conditions will induce larger reductions (Boyd, 2015;Barange et al, 2017).…”