Scanning Electron Micrographs of Modern and Late Holocene Chrysomonad Cysts From Harden Lake Meadow, Yosemite National Park, California

Adam, David; USGS

This report is one of a series illustrating siliceous cysts from various localities. Although these cysts have been known for many years* they are so small that they cannot be observed in detail with optical microscopes. The recent development of the scanning electron microscope (SEM) has made possible much more detailed observations of the external features of these cysts* but as yet relatively few forms have been recorded in this way. A major difficulty is taxonomic confusion. Many and perhaps all of these cysts are the resting stages of various algae of the phylum ChrysophytaJ they will be referred to in the rest of this report as chrysomonad cysts, although other groups in addition to the Chrysomonadinae may be represented. Modern forms are found primarily in fresh water, and numerous authors have reported chrysomonad cysts from Holocene sediments (for example, see Nygaard* 1956). Older fossils have been recovered mostly from marine deposits, and are known as archaeomonads/ whether the two groups are as distinct as this terminology suggests is not clear. For an introduction to the literature on chrysomonad cysts and siliceous algal scales* the reader is referred to Adam and Mahood (1979), a preliminary annotated bibliography on the subject. The fossil archaeomonads have been described and named entirely on the basis of their cysts. This is not advisable with modern forms/ because the cysts are the remains of one staqe of the life cycle of atqae that presumably already have legitimate taxonomic names. Proper cyst nomenclature therefore depends on establishing which cysts are produced by which algae. At the moment/ we have only a very limited knowledge of the forms that exist/ and almost no knowledge of the phycological pedigrees of the various forms. The present work is directed towards expanding our knowledge of the various cyst forms and their geographic and environmental distributions. Taxonomic problems are ignored/ and the various cyst forms are simply given numbers/ which have been assigned arbitrarily. These numbers are consistent throughout all reports in this series/ and are being used to tabulate where the various forms occur. (A list of the previous reoorts in this series is given in Appendix A). The approach used has been that of "so splitting"/ as opposed to "lumping"; it may well be desirable to lump together many of the forms described here when more is known about them. The SEN photographs are the most important part of this paper/ and no attempt has been made to reduce them to words. Supporting data have been placed in the captions. Sample preparation techniques are generally the same as those used for preparing diatom samples/ details may be found in Mahood and Adam (1979). The purpose of these initial reports is to provide primary documentation of the occurrence of particular cyst forms at particular localities, and to provide a means by which the SEN photographs of the cysts may be placed in a permanent depository. Counts of the relative abundance of the various forms and interpretations of their significance have not yet been attempted, but must await a more complete understanding of the range of cyst morphologies.

Scanning Electron Micrographs of Modern and Late Holocene Chrysomonad Cysts From Harden Lake Meadow, Yosemite National Park, California

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