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

Pigment-specific absorption by total particulates, detritus and phytoplankton was measured throughout the euphotic zone at > 275 stations on three cruises off California in late 1991 and early 1992. A new spectral fluorescence method for assessing photosynthetically active absorption 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 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 values 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 nitraclines, whereas the lowest values always occurred close to or below the depth of the nitracline. Specific absorption generally increased with increasing temperature, but there were 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 direct temperature effects in controlling natural variance in phytoplankton specific absorption. Specific absorption of photosynthetically active phytoplankton pigments was found to be less variable than that of total phytoplankton and showed no systematic trends with temperature, optical depth, or distance from the nitracline. This result leads to a new version of a bio-optical model for primary production which is based only on the photosynthetically active component rather than total phytoplankton absorption.