Optics, particles, stratification, and storms on the New England continental shelf

In situ beam attenuation and chlorophyll fluorescence were correlated with concentration and bulk composition of particles in shelf waters during summer and spring under different physical forcing conditions to determine if optical parameters could be used as an additional tracer in examining the process of mixing in shelf waters. Time series measurements were made for two 18 day periods during high stratification (late summer 1996, Delta sigma (i) = similar to3.0 kg m(-3) surface to bottom) and low but rapidly developing stratification (spring 1997, Delta sigma (i) = 0.05 to 0.5 kg m(-3)) in 70 m of water in a midshelf environment south of Martha's Vineyard, Massachusetts. When defined by hydrography and optical profiles, four layers were identified during the summer: the surface mixed layer, the particle/chlorophyll maximum, the midwater particle minimum, and the bottom nepheloid layer. Fast moving solitons perturbed the water column briefly, but no storms perturbed the system until large surface swells From Hurricane Edouard intensified and thickened the nepheloid layer. Bulk composition and optics of particles in and above the nepheloid layer were distinctly different after the passage of Hurricane Edouard. The hurricane passage demonstrated that intense atmospheric forcing greatly influences both hydrographic and optical properties in the entire water column, even when highly stratified (Delta sigma (i) = similar to3.0 kg m(-3), decreasing to 0.8 kg m(-3) post hurricane), and causes massive resuspension, due initially to wave shear stress that was later dominated by current shear. Restratification progressed rapidly after the hurricane passed. During spring the water column started as a weakly stratified two-layer system hydrographically and optically but evolved into three layers as stratification developed. Strong spring storms affected both surface and bottom layers but with decreasing impact as the water column stratified.