Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i |
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Analytical MethodForward light scattering can be used for rapid determination of in situ particle size distribution and particle concentration based on an inversion of the volume scattering function at small forward angles. One advantage of this technique is that it can capture continuous (1-Hz) in situ data. The LISST-100X measures the near-forward angular scattering distribution between 0.0017 to 0.34 radians (0.097-19.48^{o}; Type-B), at 670 nm, which is a region where scattering is strongly influenced by particle size. Using Mie scattering theory, the LISST estimates a volumetric particle size distribution (V(D_{i}) in units of mL L^{-1}) for 32 logarithmically spaced size classes with geometric mean diameters (D_{i}) ranging from 1.36 - 230.14 mm (for spherical particles). The shape of the PSD is based on an inversion of the angular pattern of forward scattering, and the concentration of particles is derived by the magnitude of scattering that reaches the detector. Before the measured light scattering distribution is inverted to obtain the particulate volume distribution, the signal must be corrected for background scattering due to pure water. After the inversion the data are corrected for the difference in laser power between the factory calibration and the in situ data, and an instrument-specific correction factor is applied to obtain the calibrated particle volume concentration, in volume particles per volume of water. The areal size distribution (A(D_{i})) is then calculated from the volume size distribution (µL L^{-1}) by assuming spherical geometry: A(D_{i}) = ^{3}/_{2} V(D_{i})D_{i}^{-1}. The mean particle size (D_{AVG}), the slope of the particle size distribution, and the total particle number (S 1.36mm - 230.14mm size classes) can then be calculated. ResultsTo investigate variability of particle size at Station ALOHA we have examined changes in the volume concentration of particles (in µL/L) over time via laser diffraction. By this method, a laser beam illuminates an in situ sample volume containing particles and the particle volume distribution between 1.25-250 µm is determined by inversion of the volume scattering function (VSF) at small forward angles. Particle volume estimated via the LISST was grouped into size bins of roughly 1.25-2 µm, 2-20 µm, and 20-100 µm. Particles in the 1.25-2 µm size range (Figure 72, top panel) generally exhibit maxima at depths of 100-140m at or near the depth of the deep chlorophyll maxima. Conversely, 2-20 µm particles are maximal in the upper water column, typically within the mixed layer (middle panel). Notably, within this size class particles with an equivalent spherical diameter of ~5 µm are the most significant contributors to particle volume. No persistent depth profile was apparent for particles in the 20-100 µm size class (bottom panel). These data are being used to map shifts in particle size over multiple temporal and spatial scales and relate findings to phytoplankton standing stocks and primary productivity. This study is a collaborative effort between the College of Oceanic and Atmospheric Sciences of Oregon State University and the Laboratory for Microbial Oceanography of the University of Hawai'i at Manoa. |