Interpreting LISST-100X measurements of particles with complex shape using digital in-line holography

Measurements of suspended sediment volume concentrations, particle size and number density are routinely collected in marine and fresh-water environments with LISST-100X instruments to understand sediment transport, biological processes and fundamental opto-acoustic problems. A LISST-100X was simultaneously deployed with a novel holographic camera (holocam) in UK coastal waters to assess the performance of the laser diffraction technique when measuring natural suspensions. Volume distributions from the LISST-100X, truncated to exclude non-overlapping size bins with the holocam, exhibit an increase in small particles and median particle size is elevated in comparison to the holocam by 20-40%. We observe positive offsets between LISST-100X and holocam number distributions of up to 2 orders of magnitude for particle sizes between 58-218 mu m, with discrepancies rising to 4 orders of magnitude for finer and coarser sizes. To explain these differences, a novel multiscale representation of particle size is used. The method quantifies individual dimensions that make up any two-dimensional geometrical structure, it can be used as a metric for particle complexity, and offers a plausible explanation for an apparent increase in small particles (<58 mu m) reported by the LISST-100X. The results suggest that for non-spherical natural suspensions the LISST-100X may be sensitive to optical scattering from sub-scales within larger particles, reporting them as individual particles regardless of the way in which they may be packaged into particles of larger overall size. We urge caution in over interpretation of LISST size distributions obtained in natural suspensions without verification with independent particle imaging.