In situ High-resolution Optical Measurements of Taxon-specific Plankton Vertical Distributions: Evidence for Regulation by Water Density and Large Fluorescent Particles

Trophic interactions involving zooplankton and phytoplankton are difficult to assess in situ at spatial scales relevant to the individuals. This work presents high spatial resolution observations of the nighttime fine-scale (<1 m) vertical distributions of plankton and particles in summer stratified coastal waters of the Southern California Bight, obtained using a planar laser imaging fluorometer (PLIF) augmented with a shadowgraph zooplankton imaging system (O-Cam). Comparisons of the O-Cam with net samples suggest that this system generates reliable relative abundance estimates of several zooplankton groups. Relative zooplankton abundances obtained with the O-Cam indicated that cylcopoid copepods were located deeper in the water column than calanoid copepods, which were found in near-surface waters along with hydromedusae and appendicularians. Cryptic maxima in fluorescent particles were correlated with the relative distributions of zooplankton grazers and their predators. Using these optical sensors augmented with hydrographic profilers we found that phytoplankton and zooplankton distributions were more closely associated with water density than depth. That is, the apparent changes in vertical distributions in depth between consecutive profiles disappeared when the distributions were plotted versus density – a result that would not be apparent using more traditional sampling tools.