Optics, Acoustics and Stress in Situ (OASIS): Effects of Aggregation, Vertical Structure, and Relation to Physical Forcing

Nearbed optical and acoustical properties in coastal waters are determined by particle concentration, composition, and size distribution, all of which can change in response to erosion, deposition, advection, and particle aggregation and disaggregation. Aggregation and disaggregation rates depend on sediment concentration and turbulence in the water column, which in turn are functions of waveand current-generated shear stresses, distance from the bottom, and stratification. The overarching goal of this research is to use in situ observations to evaluate and improve models of the mechanistic links between physical forcing, particle concentration, particle size distribution, particle composition and optical and acoustical properties near the seabed. Over the past three years, we have gathered and analyzed a comprehensive time series of these varaibles at the Martha's Vineyard Coastal Observatory (MVCO). These data so far have revealed rapid changes in particle size distribution mediated by aggregation, disaggregation, and particle settling. The particle size distribution plays an important role in determining the wavelength dependence of optical beam attenuation as well as affecting the magnitude of error associated with the confusion of light scattered in the near forward direction with transmitted light. In this round of research we propose two field deployments at MVCO during which we will gather the same data as in the first round of research. In addition we propose to characterize particle sinking with in situ observations, to collect water samples to groundtruth optical and acoustical estimates of sediment concentration and to monitor compositional variability in suspension, and to gather vertical profiles of optical and particulate properties. The proposed research will be carried out collaboratively by Emmanuel Boss from the University of Maine, Paul Hill from Dalhousie University, Tim Milligan from the Bedford Institute of Oceanography, and John Trowbridge from the Woods Hole Oceanographic Institution. The four principal investigators are submitting companion proposals with nearly identical text but different budgets.