Planar Laser Imaging Fluorometry In Situ: Microscale Distributions of Phytoplankton in Relation to Their Physical Environment

Using a Planar Laser Imaging Fluorometry (PLIF) system mounted on a free-falling platform, we have observed the spatial distribution patterns and size-frequency spectra of large fluorescent particles in the upper ocean. In drops of the system off San Diego, CA, we found that the size-frequency spectrum was strongly related to the total phytoplankton biomass (measured as chlorophyll a). Differences of the observed spectra from those predicted based on biomass showed regions suggesting aggregation and sinking of particles, systematic and sudden changes in spectra with depth, and associations of anomalously large particles with hydrographic features. The relative spatial distributions of particles in the images showed the particles to be non-randomly distributed on scales of a few cm, except in regions of high concentration (the subsurface chlorophyll maximum). Some images in every profile were found to be streaky, indicating that the water moved while the camera shutter was open. A careful analysis of the source of the streaking showed that the length and angle of the streaks, along with the heading and pressure records of the platform, give information on the vertical shear of the water column. Regions of high vertical shear were mostly confined to pycnoclines and micro-pycnoclines. Sudden changes in the biological microstructure were more often associated with density gradients than regions of high vertical shear.