Phytoplankton appearance in particle size spectra - deriving conversion functions between microscopic and particle counter measurements.
Analysis of aquatic field samples by particle counters are a widespread method but the representation of phytoplankton abundance and of size classes in which phytoplankton appears in the resulting size spectra is not well studied. To address this gap, two freshwater phytoplankton species were analysed in a particle counter and using a microscope: the colony forming Asterionella formosa (Bacillariophyceae) and the single-celled Microcystis aeruginosa (Cyanobacteria). Field samples, growth experiments and model approaches were used to study the image of phytoplankton derived by two different commonly used and standardized counting methods. In our results, the colony forming A. formosa had to be considered in units of colonies because the counting device enumerated only 23% of the single cells but 85% of the colonies that were determined under the microscope. Furthermore, the size class representation in the particle counter of both taxa appeared in much smaller ranges than expected from microscopic size measurements. Model simulations of movements and rotations of phytoplankton in the measuring device can explain half of the size shift. We deduce that about 86% of the cell areas of both studied species are transparent from two approaches. First, areas derived from simulations of rotated phytoplankton colonies equal the measured particle spectra of the laboratory cultures when the shadow areas are reduced to 14%. Secondly, field counts of A. formosa can be integrated into particle size spectra of the total particulate material when the same reduction factor is applied. For the considered optical counting device, field samples of A. formosa can be detected in particle size spectra when colony sizes as well as transparency of the cells and reduction of cell sizes by rotations are taken into account.