National Water-Quality Assessment (NAWQA) Project
Assess the status and trends of aquatic ecological conditions (invertebrates, fish, algae and habitat) in rivers and wadeable streams.
Relate ecological conditions to chemical stressors (such as nutrients and pesticides), physical disturbances (such as habitat and hydrologic alterations) in the context of different environmental settings and land uses.
Enhance understanding of factors that influence the biological integrity of streams and how these stream ecosystems may respond to diverse natural and human factors.
Develop key ecological indicators of aquatic health.
Marina G. Potapova and Donald F. Charles
Patrick Center for Environmental Research, The Academy
of Natural Sciences of Philadelphia,
1900 Benjamin Franklin Parkway, Philadelphia, Pennsylvania 19103-1195, USA
Aim: To study large-scale patterns of benthic diatom assemblages in rivers, to assess the relative importance of environmental and geographical factors affecting their composition, and to evaluate the implications of these patterns for the use of diatoms as indicators of water quality, particularly nutrient enrichment.
Location: The United States Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Programme data set covers the conterminous United States.
Methods: We employed gradient analysis to understand the floristic structure of the data set and to discover major ecological gradients underlying variation in species composition at different spatial scales (entire US, Omernik Level1 and Level 2 ecoregions). We used variance partitioning to separate the effect of environmental and spatial characteristics.
Results: At the national scale, three major ecological gradients were evident. The first was a complex downstream gradient from fast-flowing, mostly oligotrophic, highland rivers to predominantly eutrophic rivers of high- and low-elevation plains. The second was a gradient integrating water mineral content and pH, which separated the soft and often more acidic waters of the humid eastern part of US from the alkaline waters of arid western regions. The third obvious gradient was related to latitudinal and altitudinal variation of temperature. Up to one-third of the total explainable variation in species data was attributed solely to geographic factors not correlated with measured environmental characteristics. We present several examples of species with complex patterns of spatial distribution.
Main conclusions: Although environment consistently plays
the most important role in structuring diatom assemblages in rivers,
spatial factors also explain some variation in diatom distribution,
especially at the continental scale. Most of the species that are
confined to limited geographical areas are not yet described and
await future taxonomic work. We show that selection of species which
could serve as indicators of nutrient enrichment in US rivers is
not a straightforward procedure. The existence of complex environmental
gradients, and still poorly understood spatial patterns of species
distributions, precludes attempts to develop uniform diatom-based
metrics that would be applicable everywhere in the US. We advocate
the development and calibration of metrics based on data sets collected
from more limited geographical area, and that include sites having
relatively narrow ranges of environmental characteristics other
than that which the metrics are designed to indicate.