Interpretation of particle spectra of electronic counters by microscopical methods

Abstract

1. Using monocultures or single species dominated natural phytoplankton, cell counts and volume estimations obtained by visual and electronic methods show reasonable agreement. Calibration seems possible (Figs. 1–2). 2. Further examples given (Fig. 3–4) show that microscopical identification of the volume peaks in electronic counter spectra of natural seston is not quite simple. Phytoplankton peaks may not be detected in the Coulter spectrum. Shifts of Coulter peaks to the left side of the visual spectrum may be found when cylindrical, elongated or needle-like phytoplankton dominate the sample. 3. Both visual and electronic methods include potentially large errors. Possibly particle volume is either overestimated by the microscope and/or underestimated by the Coulter counter (Figs. 1a, 2, 3b, 4b, c; Table 2). In grazing studies both methods should be employed. Mutual corrections may be possible, based on the type of the seston present (size and nature of phytoplankton cells and detritus particles). In each case both techniques can yield complementary information about the seston investigated. When performing multitube analysis, screening tests of the samples as described by VANDERPLOEG (1981) are recommended. 4. Detritus, especially the different types of aggregated particles, offers severe problems. In the analysis of detritus-rich samples both methods give unreliable results. 5. In most cases estimates of volume, obtained by microscopical and electronic methods, are used as auxiliary parameters. It is the relation between microscopical or Coulter volume and other parameters (e.g. Coulter volume versus POC and phytoplankton volume versus chlorophyll content) that can give useful ecological information.