Abstract
Surveys of the California Current System in 1993 revealed high concentrations of photosynthetic pigment biomass at ∼200‐m depth, well below the euphotic zone. The deep fluorescence feature contained an estimated 2.2 × 10 4 metric tons of carbon and contained ∼2.5 times the amount of chlorophyll observed in surface waters directly above it. Deep phytoplankton assemblages may be a signature of water mass subduction, suggesting the possibility of using phytoplankton as water mass tracers. These field observations led to a laboratory study of the fluorescence characteristics of autotrophic cells as possible indices of acclimation to extended periods of darkness. In vivo multiexcitation Chl a fluorescence of the diatom Thalassiosira weissflogii was monitored for 2 months of total darkness. Numbers of living and dead cells were determined using the vital stain fluorescein diacetate (FDA). By the end of the dark incubation period, in vivo Chl a fluorescence and fluorescence per cell had leveled off to ∼45% and 65% of initial values, respectively. The contribution of accessory pigments to Chl a fluorescence, expressed as multiexcitation fluorescence ratios, was higher in the dark than prior to transfer to darkness but showed no significant changes during 2 months of darkness. The FDA assay indicated that ∼85% of the cells were alive for at least the first 3 weeks during the first dark experiment and for the entire 2 months of second dark incubation. Cell numbers decreased to 65% of initial values and then grew exponentially upon reexposure to a light: dark photoperiod. Our results for T. weissflogii suggest that extended light limitation of photosynthesis does not preclude the survival of subducted phytoplankton assemblages and the consequent accumulation of Chl a at depths below the euphotic zone. If these results extend to natural assemblages, it is not possible to estimate advective time scales based on a maximum persistence time of pigment fluorescence below the euphotic zone. Nevertheless, the deep phytoplankton assemblage we observed provides evidence for water mass subduction and suggests that large, intermittent pulses of phytoplankton carbon are a part of cross‐shelf exchange and vertical flux from surface waters to depth in this region.