| The distribution and transport of suspended particulate matter (SPM) in Elliott Bay, an embayment of Puget Sound, Washington, was compared for dry (August) and wet (February) seasons of 1979–1980. During both survey times, the SPM distribution throughout the bay consisted of 1) a thin <5 m) surface layer of variable SPM concentration dominated by phytoplankton growth in summer and Duwamish River runoff in winter, 2) a uniform mid-depth minimum-SPM zone, and 3) a bottom nepheloid layer of concentrations and thickness highly variable in space and time. The total mass of SPM in Elliott Bay was about 20% higher in February (15.7 x 108 g) than in August (13.0 x 108 g). Scatter plots of salinity vs. SPM for both seasons indicate a strong negative correlation (r = –.95) in the surface water and a weaker positive correlation (r = .52) in the bottom waters. Vertical and horizontal transport of SPM was measured with sediment traps and current meter/transmissometers deployed at two stations. Accumulation of settled SPM 5 m above the bottom was 16%–30% higher in summer (~34.5 g/m2/day) than in winter (26.8 g/m2/day). Organic matter made up 6.9%–12.3% of the trapped sediment. Cross-spectral analysis between near-bottom velocity and SPM concentration showed significant coherency at tidal frequencies. Transport of the SPM was dominated by the mean flow; diffusion components had little influence. The high positive correlations between SPM and salinity concentrations and low negative correlations between SPM concentrations and current speed imply that advection plays a larger role than resuspension in maintaining the bottom nepheloid layer in Elliott Bay. |
