In Situ Particle Measurements Deemphasize the Role of Size in Governing the Sinking Velocity of Marine Particles.

Sinking particles are important in delivering carbon to the deep ocean where it may be stored out of contact with the atmosphere. Whilst particle sinking velocities are known to be influenced by a multitude of factors, size‐based parameterizations remain common in biogeochemical models and in the methods used to determine particulate fluxes from autonomous platforms. Here we carried out an extensive literature review (62 data sets) into the size‐sinking velocity relationship, and find the relationship is much weaker for studies examining particles in situ (median R2 = 0.09) compared with ex situ studies (median R2 = 0.35). This discrepancy may be because particles examined in the laboratory have more uniform properties than those studied in situ. Our review highlights the shortcomings of using a simple relationship between size and sinking velocity to calculate sinking particulate fluxes in the ocean; considering additional particle characteristics will enable more accurate calculations of particulate fluxes. Plain Language Summary: Sinking particles are important in delivering carbon to the deep ocean where it may be stored out of contact with the atmosphere. Sinking particle speeds are an important control on sinking particle fluxes, and are known to be influenced by many factors. However, methods used to study particle fluxes from particle image data sets commonly place size as an important determinant of sinking speed. Here we carried out an extensive literature review into the size‐sinking speed relationship, and find the relationship is much weaker for studies measuring particle sinking speeds in the ocean (in situ studies) compared with studies measuring sinking speeds in the laboratory (ex situ studies). This may be because other particle characteristics that can also influence particle sinking speeds are more constant in ex situ studies, allowing size to exert a stronger control on particle sinking speed. These results have implications for the use of novel in‐situ imaging methods to calculate sinking particle fluxes, highlighting that it is important to incorporate information about particle characteristics as well as size when using these methods. Key Points: Ex situ studies typically find size to be a strong predictor of sinking velocity; strong correlations are rarely observed in situ howeverIncreased homogeneity of other particle characteristics ex situ (e.g., density, composition) are responsible for this discrepancyResults suggest importance of these other factors when predicting particle sinking velocities and fluxes using size‐scaling relationships [ABSTRACT FROM AUTHOR]