Use of fluorescence microscopy for quantifying phytoplankton, especially filamentous blue‐green algae 1

Lake water samples were stainedwith primuline and filtered on black membrane filters. The phytoplankton were visualized under wet mount at high magnification. For filamentous blue‐green algae of indefinite lengths, volumes were estimated from counts of the numbers of crosses that filaments made with a grid placed in the ocular. Identification of algae on the filters was relatively easy and the method was amenable to rapid counting with large volumes of water.