The phytoplankton of lakes, estuaries and oceans is composed of singlecelled algae. Cells of the plankton algae are largely microscopic in size ranging from a few microns to a few hundred microns in the longest dimension. Species may occur as single cells, or as colonies or filaments composed of many cells. The colonies/filaments of some species are often visible to the naked eye. For example, spherical colonies of the blue-green alga Gloeotrichia are 1 mm in diameter and clearly visible. The filamentous blue-green alga Aphanizomenon forms bundles of thichomes (filaments) that appear as grass clippings in the water. The phytoplankton are sometimes grouped by size into ultra-, nano-and net plankton (those caught with a net), with respective separations at about 10 and 50 µm (Wetzel, 1983). Phytoplankton are usually quantified by microscopic analysis of preserved water samples, rather than net samples, because of the significance of nanoplankton.