Microscale variability in the distributions of large fluorescent particles observed in situ with a planar laser imaging fluorometer

It has been known for decades that particle-size and biomass spectra show regular patterns in the ocean, and that these patterns often show systematic variations with other properties such as total biomass, nutrient concentration, season, and distance (both vertical and horizontal). The recent finding of the ubiquitous nature of layers of phytoplankton < 1 m thick prompted us to explore the fine- and microscale vertical variations of size- and fluorescence-abundance spectra in the ocean. Using a two-dimensional planar laser imaging system mounted on a free-falling platform, we quantified the properties of large fluorescent particles (∼ 20 μm–2 cm) through the water column, obtaining images every 10–30 cm. These images showed systematic relationships of the spectral properties to total chlorophyll: increased proportions of the smallest particles at high chlorophyll concentrations, and a lengthening of the spectral size range at high total chlorophyll concentrations (more large particles at high chlorophyll concentrations). Further, we observed significant variations of the spectral properties over scales of 1 m and less, and recorded the frequent occurrence of unusual layers of large particles. Our new instrument, which is sensitive to thin layers of enhanced phytoplankton biomass, shows the planktonic community to be highly structured vertically on scales of 1–2 m, particularly within the DCM.