Quantitative Estimates of Filamentous Algae

One measure of the filamentous algae present in a microscope field is the total length of all the algal filaments. This is usually estimated by superimposing a measuring grid upon the field either by a micrometer eyepiece or a ruled counting cell. The inherent difficulty in such a method is in estimating lengths of segments making an acute angle with the lines of the grid. Just how this should be done is seldom stated in the directions for making counts. For instance, in the Areal Standard-unit Method, Whipple (1927) merely states: Filamentous forms of constant width can be measured in lengths and then reduced to units by multiplying by the average diameter. The same idea is expressed by Welch (1948): Some filamentous plankters have very constant diameters, hence counting is facilitated by determining the length in linear units of the individual filaments and then multiplying by the number representing the common diameter. Chandler (1940) likewise leaves the method of determining total length entirely up to the individual making the measurements. He employs for his phytoplankton analysis a unit system where, for example, one cell of Xavicula, four cells of Scenedesmus, 300 micra of Melosira, or 100 micra of Fragilaria each count as one unit.