The distribution and optical response of particles on the continental shelf and their relationship to cross-isopycnal mixing

Abstract

The relationships of optics, particles, and hydrography to shelf mixing processes were analyzed on a mid-continental shelf south of New England. The objectives were to characterize the types, sizes and sources of particles present in the water column and their effects on optics during conditions of strong stratification (late summer) and weak stratification (spring) and to determine how the particles and optics change over time in response to different forcing functions (wind, surface gravity waves, internal waves, solibors - large scale internal waves- and tides). An unexpected opportunity was presented by the passage of Hurricane Edouard close to the sampling site. Under both strong and weak stratification, surface oaters had high values of fluorescence, particulate organic carbon, and chlorophyll a. Mid-waters contained higher concentrations of particulate organic carbon than bottom oaters, but the optical characteristics of particles more closely resembled those of bottom waters (resuspended and adverted material). Strongly stratified conditions typically exhibited little mixing, with the exception of when extreme forcing events, such as the hurricane, passed through the sampling site. Most of the events sampled during the summer cruise were the result of addiction rather than mixing. Spring conditions included weakly stratified waders with frequent wind events (Nor'easters). Strong forcing events caused surface mixing initially, but solar heating stratified surface waders and gradually inhibited mixing. Particle size distribution changed with stratification. A dramatic increase was observed with the passage of the hurricane, and an initial increase in large particles followed by a substantial increase in smaller particles was observed during a spring phytoplankton bloom. Spring particle settling flux was 3 times higher in the bottom 40 meters than during the previous summer. Relationships between optics and discrete samples generally correlate better under mixed conditions than stratified conditions. During the stratified period, particle distributions were constrained by density layers, where as during weakly stratified periods, the waders were more mixed, leading to a more uniform distribution of particulate matter in the surface and the bottom waters.