Abstract
In situ laser diffractometry (LD) is increasingly used in oceanographic studies to estimate sediment transport, particle fluxes and to assess the concentration of marine phytoplankton. It enables an accurate characterization of the size distribution of suspended particles from the scattering signal produced by their interaction with a collimated laser beam. LD reliably reflects the sizes of suspensions dominated by nearly spherical particles; however, when complex particle morphologies dominate the suspension (e.g. phytoplankton) the resulting particle size distribution (PSD) may present significant variations attributed to different factors. In particular, the orientation of non-spherical particles - which abound in the sea - modifies LD measurements of PSDs. While this may be interpreted as a drawback for some studies (i.e. when precise measurement of the volume concentration is required), we propose that detailed analysis of this signal provides information on particle orientation. We use PDMS micropillars with prescribed elliptical cross-sections to experimentally determine the dependence between the spatial orientation of elongated particles and changes in the PSD measured with a LISST laser diffractometer. We show that LD can be used to adequately characterize the different dimensions of the non-spherical particles at specific orientations. Using this property, we describe and validate a method to infer the preferential orientation of particles in the sea. Our study opens new perspectives in the use of in-situ LD in ocean research.
Highlights
The characterization of biotic and abiotic particle size distributions (PSD) in natural waters is essential for a wide variety of scientific disciplines, ranging from ocean optics [1] and biological oceanography [2,3] to sediment dynamics and marine geochemistry [4,5]
In situ laser diffractometry is a method with high applicability to the study of suspended particles in the marine environment
We expand the sampling capabilities of laser diffractometry (LD) to the estimation of preferential particle orientation in the sea that can contribute to the understanding of key processes in the fields of marine optics and plankton ecology
Summary
The characterization of biotic and abiotic particle size distributions (PSD) in natural waters is essential for a wide variety of scientific disciplines, ranging from ocean optics [1] and biological oceanography [2,3] to sediment dynamics and marine geochemistry [4,5]. Suspended particles influence trophic interactions within the planktonic community, provide a habitat for microbial communities [8,9] and modulate vertical fluxes of major biogeochemical components, pollutants, and contaminants [10,11,12]. These effects have enormous interest for the understanding, prediction and management of the marine environment. Several commercial in-situ operating instruments such as the Multi-Angle Scattering Optical Tool (MASCOT) or the Laser In-Situ Scattering and Transmissiometry (LISST) are based on this principle [19,20] These instruments are routinely used for sediment and plankton studies [21,22,23]
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