Light absorption by phytoplankton, photosynthetic pigments and detritus in the California Current System

Pigment-specific absorption bq total particulatea. detritus and phytoplankton was measured throughout the euphotic zone at > 275 stations on three cruises off California in late IYY 1 and early 1992. A new spectral Ruoresccncc method for assessing photosynthetically active absorption in natural samples was developed and applied. Spatial variability in specific absorption coefficients at the mesoscalc was found to be as high as previously observed between midand highlatitudes, while differences between cruises wcrc very low. In surface waters, the highest values of specific absorption were found in warm. low-pigment surface waters offshore and in the Southern California Bight. Vertical sections reveal that low \alucs occur near the surface only where the pycnocline and nitracline slope toward the sea surface. The highest values of phytoplankton specific absorption occurred at shallow optical depths for stations with deep nitraclincs, whereah the lowest values always occurred close to or below the depth of the nitracline. Specific absorption generally increased with increasing temperature. but thcrc WC’IK large differences in the relationships between cruises. In the context of previous laboratory observations. these results imply that nutrient availability plays a greater role than ducct tcmpcraturc cffccts in controlling natural variance in phytoplankton specific absorption. Specific absorption of photosynthetically active phytoplankton pigments was found to lx ICES variable than that of total phytoplankton and showed no systematic trends with tcmpcrature. optical depth, or distance from the nitraclinc. This result leads to ;I new version of a hio-optical model for primary production which is based only on the photosynthetically active component rather than total phytoplankton absorption.