Gardner et al.: Stratification, storms, particles, and optics during the late summer and spring on the New England continental shelf

Abstract

The effects of mixing on the distribution of particles and the optical reseponse was investigated by making time-series measurements for 18-days at a fixed location on the continental shelf during the highly stratified late summer, 1996 and the weakly stratified spring, 1997. Hydrographic, optical, and particulate measurements and samples were collected up to 12 times during a day at the central Coastal Mixing and Optics site south of Cape Cod, Massachusetts at 70^o30' west and 40^o30' north in 70 meters of water. Cross-shelf transects were made to survey the larger scale conditions. during the summer cruise the entire water column was highly stratified (delta density = ~3.0 kg m^-3), with a subsurface chlorophyll maximum and a strong, but very thin bottom nepheloid layer. Fast-moving solitons perturbed the water column briefly, but no storms perturbed the system until the latter part of the cruise when large surface swells from Hurricane Edouard intensified and thickened the nepheloid layer. The onset of swells was coincident with the dissipation of the subsurface cholorphyll maximum, but may have resulted from advection rather than physiological disturbances. The hurricane created a two-layer system (delta density = 0.8 kg m^-3), which started restratifying quickly after Edouard's passage. Sediments resuspended to within 20 m of the surface quickly settled out or, more likely, were replaced by clearer water, and subsurface chlorophyll increased slightly. In the early spring, the water was probably mixed completely due to winter storms, but a weak two-layer system existed when we arrived (delta density = 0.05 after our first storm). Despite strong storms of decreasing intensity every few days, the water column developed into a three-layer system with increasing stratification in surface waters (delta density = 0.5 kg m^-3 at cruise end). Optical measurements can quickly quantify the distribution of biological and resuspended particles, but differentiating between the effects of mixing and advection is more challenging.

Wilf Gardner
Department of Oceanography
Texas A & M University
College Station, TX 77843-3146
Phone: 979-845-7211
Fax: 979-845-6331
e-mail: wgardner@ocean.tamu.edu

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