USGS NAWQA: The Ecological National Synthesis (ENS) Project

Four Primary Objectives of the ENS 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.

Distribution of Benthic Diatoms in U.S. Rivers in Relation to Conductivity and Ionic Composition

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

SUMMARY

We quantified relations between diatom relative abundance and water conductivity and ionic composition, using a dataset of 3239 benthic diatom samples collected from 1109 river sites throughout the U.S. (U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program dataset). This dataset provided a unique opportunity to explore the autecology of freshwater diatoms over a broad range of environmental conditions.
Conductivity ranged from 10 to 14,500 mS cm^-1, but most of the rivers had moderate conductivity (interquartile range 180 - 618 mS cm^-1). Calcium and bicarbonate were the dominant ions. Ionic composition, however, varied greatly due to the influence of natural and anthropogenic factors.
Canonical Correspondence Analysis (CCA) and Monte Carlo permutation tests showed that conductivity, and abundances of major ions (HCO₃^-+CO₃²-, Cl-, SO₄^2-, Ca²⁺, Mg^2+-, Na⁺, K⁺) all explained a statistically significant amount of the variation in assemblage composition of benthic diatoms. Concentrations of HCO₃^-+CO₃^2- and Ca²⁺ were most significant sources of environmental variance.
CCA showed that the gradient of ionic composition explaining most variation in diatom assemblage structure ranged from waters dominated by Ca²⁺ and HCO₃^-+CO₃^2- to waters with higher proportions of Na⁺, K⁺, and Cl^-. CCA also revealed that the distributions of some diatoms correlated strongly with proportions of individual cations and anions, and with the ratio of monovalent to divalent cations.
We present species indicator values (optima) for conductivity, major ions, and proportions of those ions. We also identify diatom taxa characteristic of specific major-ion chemistries. These species optima may be useful in future interpretations of diatom ecology and as indicator values in water-quality assessment.