On August 3-5, 1998, a variety of optical data were collected aboard the R/V Barnes in three areas of the San Juan Islands, WA: West Sound (Orcas Island), East Sound (Orcas Island), and the Strait of Juan de Fuca. Among these data were vertical profiles of absorption, total attenuation, upwelling radiance, and downwelling irradiance.
In order to isolate the fluorescent contribution to the underwater light field, comparisons were made between measurements of radiance reflectance (including the effects of fluorescence) and model estimates of radiance reflectance (excluding the effects of fluorescence). The difference between these two was used to derive estimates of fluorescent irradiance as a function of depth.
In each of the three sampling areas, a characteristic profile of fluorescence-associated irradiance was observed, showing surface inhibition, a subsurface maximum, and then a marked decline with depth. Profiles of absorption-specific fluorescent irradiance revealed much more dramatic differences among the three sampling areas, reflective of differences in the phytoplankton physiology and biomass (i.e., water clarity). These observed differences were consistent with the physical mixing structure of the Sounds and Strait, as indicated in profiles of temperature and salinity.