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Freshwater Diatoms: Indicators of Ecosystem Change


by
Donald Charles
Academy of Natural Sciences, Philadelphia
Pat Kociolek
California Academy of Sciences, San Francisco
Diatoms are photosynthetic unicellular organisms. They are found in almost all aquatic and semi-aquatic habitats and are of great ecological importance because they form an important part of the base of the food web. Although diatoms are widely distributed as a group, most species occur only in habitats with specific physical, chemical, and biological characteristics. Ecologists have long made practical use of this habitat specificity by collecting and analyzing individual species and community data to determine the quality or condition of aquatic habitats. Both long-term monitoring of specific lake and stream habitats and analysis of diatom remains (that become part of the sedimentary record of lakes) allow scientists to obtain a unique long-term historical perspective on these ecosystems. This perspective is especially valuable in assessing the long-term effects of human activities on aquatic and terrestrial ecosystems. Diatoms have been studied throughout the country, but no reasonably complete compilation or summary of these studies exists.
    Examples of diatoms (top to
bottom): Aulacoseira sp., Tabellaria sp., Gomphonema sp., and Stephananodiscus sp.
Photos courtesy J.P. Kociolek
Diatoms are divided into two groups based on overall symmetry of the cell walls; radially symmetrical forms are informally called "centric" diatoms while bilaterally symmetrical forms are referred to as "pennate" diatoms. One remarkable aspect of these organisms is that they have cell walls made of glass (silicon dioxide). The glass cell walls are perforated and ornamented with many holes, which are usually arranged in definite patterns. The nature of these perforations as well as their orientation and densities help in the identification of diatom species. Diatom cell walls come in two pieces that fit together the way a Petri dish or pill box does. When these organisms divide, each half reproduces a "daughter" half that, because of the rigidity of the glass walls, must be smaller than the original half.
   
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.
   
Fifteen centric and 63 pennate diatom genera have been reported from fresh water. No exact species counts have been made, but about 4,000 species have been described in the literature. 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, nearly 10% of those species are new to science. There is still a great need to document the variety 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 biological community and food web and are sensitive 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 number of local and regional-scale monitoring programs.

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 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
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
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
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
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.
For further information:
Donald Charles
Academy of Natural Sciences
1900 Benjamin Franklin Parkway
Philadelphia, PA 19103
References
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.


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