Limitations of laser diffraction for measuring fine particles in oligotrophic systems: Pitfalls and potential solutions

摘要

Sampling of suspended participates in oligotrophic environments is inherently difficult because of low ambient concentrations. The performance of a Laser In Situ Scattering and Transmissometry (LISST) series particle size analyzer is evaluated for use in such low-concentration environments. Five issues are studied for their effect on recorded particle size distributions: the instrument limits of detection, the kernel matrix used in the inversion of the scattered light distribution, the effect of particles sized outside the instrument inversion limits (1.25 μm-250 μm), the effect of ambient light conditions, and the effect of sampling in density stratified conditions. We evaluate a LISST-100X type B instrument using a series of small-scale lab experiments and concentrate mainly on fine particles (<20 μm). Kernel matrix processing effects are shown to have large impacts on fine particle measurements, but accurate results can be obtained with the correct matrix choice. Concentration detection limits may be sufficient for the clearest fresh water environments but may be too high for accurate sampling in the oligotrophic open ocean. Large out of range particles (>250 μm) show little effects on the size distribution, but small particles (<1.25 μm) have large impacts. High ambient light conditions, typical in clear oligotrophic systems, create erroneously high concentrations of fine particles, and profiling in density stratified conditions is found to create incorrectly low concentrations. We present recommendations for the next generation of in situ laser diffractometers and advise consideration of the issues covered here before using current instruments in oligotrophic environments.