Mapping phytoplankton in situ using a laser‐scattering sensor

A primary limitation of phytoplankton ecology research is the difficulty of describing patchiness and distributions of different phytoplankton groups. Chlorophyll fluorescence and optical backscatter are useful measurements that provide information about phytoplankton, but these measurements do not allow distinction of phytoplankton taxa. Traditional phytoplankton identification methods (such as microscopy, HPLC analysis, and flow cytometry) are labor intensive and therefore can provide only very limited coverage and resolution. Through lab experiments we show that the Laser In Situ Scattering and Transmissometer (LISST‐100) instrument can accurately quantify phytoplankton cell dimensions for some cell shapes. Pseudo‐spherical dinoflagellates are described with a single peak in the particle size distribution (PSD) at the cross‐sectional dimension of the cells. Pennate diatoms are described with peaks in the PSD at the major and minor axis dimensions of the cells. Diatom cells with minor axis dimensions that vary along the major axis are described with one peak across the range of minor axis dimensions and a second peak at the major axis dimension. Through field experiments, we show that mapping the PSD in situ at high resolution permits description of patchiness and evolution of phytoplankton populations. We present two examples: (1) growing dominance of Ceratium species during a red tide bloom, and (2) concentration of Pseudo‐nitzschia australis, a harmful algal bloom (HAB) species, at a water mass front. We conclude that synoptic mapping of the PSD can significantly advance phytoplankton ecology research in the coastal ocean.