Relationships between optical and physical properties were examined based on intensive sampling at a site on the New England continental shelf during late summer 1996 and spring 1997. During both seasons, particles were found to be the primary source of temporal and vertical variability in optical properties since light absorption by dissolved material, though significant in magnitude, was relatively constant. Within the particle pool, changes in phytoplankton were responsible for much of the observed optical variability. Physical processes associated with characteristic seasonal patterns in stratification and mixing contributed to optical variability mostly through effects on phytoplankton. An obvious exception to this generalization occurred during summer as the passage of a hurricane led to a breakdown in stratification and substantial resuspension of non-phytoplankton particulate material. Prior to the storm, conditions of high stratification in summer were associated with mid-water column maxima in absorption, scattering and diffuse attenuation coefficients. These maxima coincided with high concentrations of phytoplankton pigments near the nutricline. In spring, stratification was much weaker, but increased over the 3-week sampling period and a modest phytoplankton bloom occurred. The bloom was associated with apparent nutrient removal from surface waters and with the appearance of surface layer maxima in absorption and scattering coefficients. These seasonal differences in the vertical distribution of phytoplankton and associated absorption and scattering were evident in surface reflectance spectra, which were elevated and shifted toward blue wavelengths in the summer. Some seasonal differences in optical properties, including reflectance spectra, suggest that a significant shift toward a smaller particle size distribution occurred in summer. Shorter time scale optical variability was consistent with a variety of influences including episodic events such as the hurricane, physical processes associated with shelfbreak frontal dynamics, biological processes such as phytoplankton growth, and horizontal patchiness combined with water mass advection.
Biology Department, MS 32
Woods Hole Oceanographic Institution
Woods Hole, MA 02543-1049