The emphasis of this chapter in general is oriented toward routine identification of genera and species by investigators primarily using light microscopy. The species dealt with are a selection of those which, according to the literature and our own experience, are regularly encountered in marine plankton.

Identification of diatoms usually must rely on the siliceous frustule. Shape, size, number, and arrangement of chloroplasts and the presence or absence of pyrenoids may, however, be used for identification on the generic and specific level. This is especially true in pennate diatoms (Cox, 1981) and for the centric genera Chaetoceros and Leptocylindrus. For example, many species of Chaetoc-eros and Rbizosolenia may be identified by their gross morphology as seen in water mounts or when embedded in a medium of higher refractive index than that of silica. The proper identification of other species, such as Thalassiosira, Coscinodiscus, and Pseudo-nitzschia, may require permanent mounts of cleaned single valves (see Methodology). Phase and/or differential interference contrast optics are especially helpful when working with weakly silicified diatoms.

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