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

Sarah L C Giering,Nathan Briggs,Morten H Iversen,Brett Hosking

doi:10.3389/fmars.2020.00564

Abstract

In situ imaging of particles in the ocean are rapidly establishing themselves as powerful tools to investigate the ocean carbon cycle, including the role of sinking particles for carbon sequestration via the biological carbon pump. A big challenge when analysing particles in camera images is determining the size of the particle, which is required to calculate carbon content, sinking velocity and flux. A key image processing decision is the algorithm used to decide which part of the image forms the particle and which is the background. However, this critical analysis step is often unmentioned and its effect rarely explored. Here we show that final flux estimates can easily vary by an order of magnitude when selecting different algorithms for a single dataset. We applied a range of static threshold values and 11 different algorithms (7 threshold and 4 edge detection algorithms) to particle profiles collected by the LISST-Holo system in two contrasting environments. Our results demonstrate that the particle detection method does not only affect estimated particle size but also particle shape. Uncertainties are likely exacerbated when different particle detection methods are mixed, e.g., when datasets from different studies or devices are merged. We conclude that there is a clear need for more transparent method descriptions and justification for particle detection algorithms, as well as for a calibration standard that allows intercomparison between different devices.

Highlights

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)
We here show that the choice of threshold algorithm is critical in particle sizing and, particulate organic carbon (POC) concentration and flux estimates
(2) We used a conversion from equivalent spherical diameter (ESD) to POC contents that was based on a different camera system with an unknown threshold setting and calibration

Summary

Introduction

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 – 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 ocean carbon storage (see review by Giering et al, 2020). Particle size is a crucial parameter as it is used as input for various conversions, in particular, to estimate particulate organic carbon (POC) content and sinking velocities (e.g., Alldredge and Gotschalk, 1988; Alldredge, 1998; Iversen and Ploug, 2010; Laurenceau-Cornec et al, 2015). The shape of a particle (e.g., how round or solid a particle is) can inform about the particle’s density, drag and type (e.g., Laurenceau-Cornec et al, 2015)

Methods

Results

Conclusion