Despite the important roles diatoms play in aquatic ecosystems and their utility in evaluating and monitoring environmental change in these systems, intensive floristic or taxonomic studies on freshwater diatoms in North America have been limited. A two-volume work entitled The Diatoms of the United States (Patrick and Reimer 1966, 1975) considered a selected number 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 taxonomic treatments of diatoms in the United States. The focus has been on specific 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. |
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Lake Acidification |
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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 paleolimnological 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 µeq/L) have become more acidic since preindustrial times (Cumming et al. 1992). Lakes in other regions of the country have also acidified but not to the same extent (Charles et al. 1989). The amount of acidification inferred from diatoms is related to the level of atmospheric loading of strong acids and the ability of watersheds to neutralize those acids. Analysis of diatoms and sedimentary remains of other biological groups (e.g., chrysophytes, chironomids, Cladocera) reveals that acidic deposition has had significant effects on aquatic communities in many lakes. Numbers of taxa are reduced, but some acid-tolerant taxa have significantly increased in abundance. | ||
Lake Eutrophication |
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Population estimates of the numbers of lakes in New England and New York that are more eutrophic now than in presettlement times are being obtained from analyses of diatom assemblages from recent and preindustrial levels of sediment cores taken as part of the U.S. Environmental Protection Agency's Surface Water component of the Environmental Monitoring and Assessment Program (EMAP; Dixit and Smol 1994). The approach of examining lake eutrophication by using diatom assemblages has been widely applied in North America and throughout the world. | ||
Rivers and Streams |
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Many long-term diatom data sets exist that can inform us about trends in water quality. The monitoring program conducted by the Federal Water Pollution Control Agency in the 1960's tracked the status of major rivers throughout the country (Williams and Scott 1962). Monitoring of diatom assemblages in rivers and streams is just beginning as part of the U.S. Geological Survey's National Water Quality Assessment (NAWQA) and of the Environmental Monitoring and Assessment Program. The Academy of Natural Sciences of Philadelphia has long-term records for several rivers in the eastern United States. Many of these records show that the quality of water downstream from industrial effluent outfalls and sewage treatment plants has improved markedly, but others show worsening conditions, often due to the increased number of sources of stress along the river or in the watershed. Much more could be learned about trends by simply analyzing the immense data that already exist, especially by using new quantitative techniques developed in the past 5-10 years. | ||
Climate Change |
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Diatom assemblage composition is sensitive to changes in water level, salinity, ice cover, wind-mixing patterns, and other characteristics directly and indirectly affected by climate. Paleolimnological studies of sediment cores are providing valuable data on climate change over the past hundreds to thousands of years, which are essential for understanding the nature and magnitude of ecosystem change that can be expected in future years. | ||
Conclusions |
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The ability to infer ecosystem status and trends from diatoms is largely dependent on the availability of ecological data for the species occurring at study sites. The amount of such data is accumulating at an increasingly rapid rate, but it is in many separate data bases. These need to be coordinated so that users will have easier access to the data that already exist. | ||
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References | |
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Charles, D.F., R.W. Battarbee, I. Renberg, H. Van Dam, and J.P. Smol. 1989. Paleoecological analysis of lake acidification trends in North America and Europe using diatoms and chrysophytes. Pages 207-276 in S.A. Norton, S.E. Lindberg, and A.L. Page, eds. Acidic precipitation. Soils, aquatic processes, and lake acidification. Vol. 4. Springer-Verlag, New York. Cumming, B.F., J.P. Smol, J.C. Kingston, D.F. Charles, H.J.B. Birks, K.E. Camburn, S.S. Dixit, A.J. Uutala, and A.R. Selle. 1992. How much acidification has occurred in Adirondack region lakes (New York, USA) since preindustrial times? Canadian Journal of Fisheries and Aquatic Sciences 49:128-141. |
Dixit, S.S., and J.P. Smol. 1994. Diatoms and indicators in the Environmental Monitoring and Assessment Program --Surface Waters (EMAP--SW). Environmental Monitoring and Assessment 31:275-306. Patrick, R., and C.W. Reimer. 1966, 1975. The diatoms of the United States. Vols. 1 and 2. The Academy of Natural Sciences of Philadelphia. Vol. 1:688 pp. Vol. 2:213 pp. Williams, L.G., and C. Scott. 1962. Principal diatoms of major waterways of the United States. Ecology 7:365-379. |