The University of Maine Maine In-situ Sound & Color Lab
School of Marine Sciences
In Situ Suspended Particle Size Distributions and Their Influence on Water Column Optical Properties

Particle size distributions in the ocean are dynamic, and when they change, the optical properties of the water column are affected. Several physical processes influence particle size in the coastal ocean, including advection, size-dependent erosion and deposition, and aggregation and disaggregation of suspended sediments. The latter processes transfer particles, particularly small, optically active particles, into and out of large, fast-sinking agglomerates, typically referred to as "flocs". In the past, these large, relatively rare flocs have been assumed to play little or no role in defining the optical properties in the water. Recent in situ measurements of particle size extending from micrometers to centimeters, however, reveal that flocs can contribute significantly to the attenuation and backscattering of light. Flocs can be abundant enough to account for 25-50% of the particle projected area in suspension, with the percentage depending on floc internal structure, on overall sediment concentration and composition, and on water-column turbulence, which can aid both in the formation and destruction of flocs. Interestingly, the estimated contribution of flocs to attenuation in the water column depends on the geometry of the light sensor. Flocs, because they are large, scatter light in the near forward direction. Transmissometers with large acceptance angles characterize more of the light scattered from flocs as transmitted light, thus underestimating attenuation. Ongoing research is devoted to better characterization of floc internal structure and how it affects optical properties and elucidation of how particle size distributions respond to changes in seabed stress generated by waves and currents.

Hill, P.S., E. Boss, T. Milligan, K. Curran, W. Slade, and O. Mikkelsen, 2006. In Situ Suspended Particle Size Distributions and Their Influence on Water Column Optical Properties. Ocean Optics Conference 2006, Montreal, Quebec.

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