Advances in underwater fluorometry: from bulk fluorescence to planar laser imaging

Abstract: Underwater measurements of photosynthetic pigment fluorescence have long been used as a proxy for phytoplankton biomass. While many useful results have been achieved, the usual measurements made on bulk samples restrict the inferences that can be drawn from the sample. Most importantly, there is no spatial resolution of the single measurement. As an alternative, we examine the utility of underwater imaging of phytoplankton via lab experiments and the analysis of field data. A planar laser imaging fluorometry (PLIF) system, assembled from readily available optical components, was used to image individual fluorescent particles in the lab and field. In addition, an underwater microscope was deployed in the field and showed a layer of large (250–300 μm), highly absorptive particles, inferred to be diatoms. These diatoms were also measured concurrently with a standard subsea fluorometer. The two profiles show that the intensity of fluorescence bears little relationship to the community structure of the phytoplankton. Only by the use of imaging can regions of high fluorescence intensity (high phytoplankton biomass) caused by many small phytoplankton, or a few large ones, be distinguished.