LIGHT-ABSORPTION BY PHYTOPLANKTON, PHOTOSYNTHETIC PIGMENTS AND DETRITUS IN THE CALIFORNIA CURRENT SYSTEM

Pigment-specific absorption by total particulates, detritus and phytop lankton was measured throughout the euphotic zone at > 275 stations on three cruises off California in late 1991 and early 1992. A new spect ral fluorescence method for assessing photosynthetically active absorp tion in natural samples was developed and applied. Spatial variability in specific absorption coefficients at the mesoscale was found to be as high as previously observed between mid- and high-latitudes, while differences between cruises were very low. In surface waters, the high est values of specific absorption were found in warm, low-pigment surf ace waters offshore and in the Southern California Eight. Vertical sec tions reveal that low values occur near the surface only where the pyc nocline and nitracline slope toward the sea surface. The highest value s of phytoplankton specific absorption occurred al shallow optical dep ths for stations with deep nitraclines, whereas the lowest values alwa ys occurred close to or below the depth of the nitracline. Specific ab sorption generally increased with increasing temperature, but there we re large differences in the relationships between cruises. In the cont ext of previous laboratory observations, these results imply that nutr ient availability plays a greater role than direct temperature effects in controlling natural variance in phytoplankton specific absorption. Specific absorption of photosynthetically active phytoplankton pigmen ts was found to be less variable than that of total phytoplankton and showed no systematic trends with temperature, optical depth, or distan ce from the nitracline. This result leads to a new version of a bio-op tical model for primary production which is based only on the photosyn thetically active component rather than total phytoplankton absorption .