Our Livin^i Rcstnirct's — Aqiianc Ecosystems 



257 



The Diatoms of the United States (Patrick and 

 Reimer 1966. 1975) considered a selected num- 

 ber of genera, and in those genera treated only 

 those species reported from the United States up 

 to 1960. There are only a few regional or 

 statewide taxononiic treatments of diatoms in 

 the United States. The focus has been on specif- 

 ic habitats; areas receiving the most attention 

 have been the Northeast, upper Midwest, the 

 Great Lakes, and isolated areas in the West. 

 Only a few checklists of diatom taxa exist. 



Fifteen centric and 63 pennate diatom gen- 

 era have been repotted from fresh water. No 

 exact species counts have been made, but about 

 4.()()() species have been described in the litera- 

 ture. This number is undoubtedly a conservative 

 estimate because in two areas where intensive 

 research has been conducted, in Dickinson 

 County. Iowa (around the Iowa Lakeside 

 Laboratory), and the Laurentian Great Lakes, 

 about 1.200 and 2.000 species, respectively, 

 have been recognized. In the Great Lakes, near- 

 ly 10% of those species are new to science. 

 There is still a great need to document the vari- 

 ety and distribution of freshwater diatoms in the 

 United States. 



Diatom assemblages provide the basis for 

 many important assessments of trends in the 

 status of freshwater ecosystems. These versatile 

 indicators tell us about the acidification {see 

 glossary) of lakes caused by acidic deposition, 

 the eutrophication {see glossary) of lakes 

 caused by human impacts and changing land 

 use. improvements and declines in the quality of 

 our rivers and streams, and changes in climate 

 over the past thousands of years. Because 

 diatoms are important components of the bio- 

 logical community and food web and are sensi- 

 tive to changes in water quality, they provide 

 information on both the biological integrity of 

 the ecosystem and those factors likely to be 

 causing any observed changes. Researchers are 

 rapidly developing new techniques for using 

 diatoms to provide even more quantitative and 

 accurate inferences of ecosystem condition, and 

 diatoms are being included in a growing num- 

 ber of local and regional-scale monitoring pro- 

 grams. 



Lake Acidification 



The extent, magnitude, timing, and causes of 

 lake acidification in acid-sensitive regions of 

 the country have been inferred from analysis of 

 diatom assemblages in the stratigraphic record 

 of dated lake sediment cores. These paleolim- 

 nological studies show, for example, that about 

 25%-35% of the lakes in the Adirondack 

 Mountains with the lowest ability to neutralize 

 acids (acid neutralizing capacity < 400 |.teq/L) 

 have become more acidic since preindustrial 



Examples of diatoms (top to 

 bottom): 



Aulacoseira sp.. 

 Tcibellaria sp.. 

 Gomplionema sp.. and 

 Sleplumanodisciis sp. 



