Microscale distributions of phytoplankton: initial results from a two-dimensional imaging fluorometer, OSST

A digital camera system was designed to photograph laser-stimulated fluorescence of phytoplankton in situ at 0.67 x 0.67 cm resolution over an area of 70 x 70 cm. In a series of vertical profiles, 1200 images were collected, Criteria were developed to reject images smeared by camera motions (due to ship heave), and images taken when vertical velocities were > 0.2 m s(-1), leaving 240 good images. Horizontal velocities calculated using ancillary data from the FishTV imaging sonar showed the water to be directed toward the imaging plane during the deployment, supporting the hypothesis that images were uncontaminated by mixing from the instrument package. Images passing all the criteria show strong isotropic spatial variability. The averaged spatial spectrum was flat, suggesting an underlying random distribution of brightly fluorescent particles-probably phytoplankton aggregates. Such observations have important implications for our understanding of the food environment of herbivorous zooplankton, and the mechanisms creating and maintaining patchiness in the ocean.