Optical backscatter of marine flocs

Marine flocs (defined here as aggregated particles >10 μm diameter) derived from phytoplankton culture (Phaeodactylum tricornutum; PT) and water-based drilling mud (WBDM) waste were generated in an enclosed upwelling system in order to examine how optical backscatter coefficients at six wavelengths changed as flocs matured. Equilibrium was reached after death of the phytoplankton cells and was identified as a consistent particle projected-area concentration of flocs (>10 μm diameter) despite compensating changes in the number and average size of flocs. As the suspension of flocs matured, the particle backscatter coefficients at six optical wavelengths increased as a function of increasing particle projected-area concentration, despite a drop in the mass of suspended particulate matter (SPM). In this flocculating suspension, the mass of suspended particles and the particle projected-area concentration were not co-variates. These results indicate that the strong response of optical backscatter (OBS) to particle size in many previous studies may simply be due to the inappropriate use of weight concentration as the independent variable, rather than particle projected-area concentration.