Potential Use of Rare Diatoms as Environmental Indicators in U.S.A. Rivers

Marina G. Potapova and Donald F. Charles

Patrick Center for Environmental Research, The Academy of Natural Sciences
1900 Benjamin Franklin Parkway, Philadelphia, Pennsylvania 19103-1195, U.S.A.

ABSTRACT.— Common diatoms are usually considered superior to rare or narrowly distributed species as indicators of environmental conditions because their ecology is better known or is easier to quantify. Rare diatoms, on the other hand, are often confined to oligotrophic and less polluted waters, and are possibly valuable bioindicators because of their sensitivity to pollution. The proportion of native species is another possible measure of ecosystem degradation, because competitive exclusion by introduced organisms might result from the human impact.

We sought answers to the following questions: are rare or native diatoms more likely to indicate natural or undisturbed conditions in rivers? Could their presence or relative abundance be considered as a good indicator of river health? We defined as rare those diatoms that had low frequency of occurrence in rivers at the continental scale, but that reached relatively high numbers in the benthic assemblages of at least some individual sites. The latter criterion was used to help distinguish true inhabitants of rivers from those washed in from other habitats. The second category used in our analyses was ‘native’ diatoms. For the purposes of this study we categorized as ‘native’ diatoms generally confined to the Americas.

We analyzed the U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) Program dataset, which consists of 3,200 quantitative algal samples collected from 1100 sites on U.S. rivers. We showed a significant trend of decreasing number and relative abundance of rare and ‘native’ benthic diatom species along gradients of human impact, such as concentration of nutrients and percent of watershed area in agricultural and urban land-use. The number of rare taxa also tended to drop at higher elevations. The correlations were very low, but significant because of the large number of samples. Average relative abundance of rare taxa was highest in the Southeast and some western mountainous areas. The number of ‘native’ species was highest in the Southeast and California, and lowest in the northern and agriculturally intensive areas, such as the upper Midwest. A large proportion of the rare and ‘native’ taxa in the NAWQA dataset have not yet been described in literature. Four species described here provide examples of rare and possibly endemic diatoms either confined to waters of good quality (Sellaphora californica sp. nov.) or able to tolerate moderate concentrations of nutrients (Navicula geronimensis sp. nov., Nitzschia kimberliensis sp. nov., N. rhombiformis sp. nov.). We concluded that the number and abundance of rare and native species, like other measures of diatom community diversity, cannot be used as sole predictors of water quality. Human disturbance is only one of the many factors affecting distribution of rare and native diatoms. Nevertheless, when coupled with autecological information, presence of these diatoms can serve as additional evidence of ecosystem health or degradation.