A Large-Scale Comparison of Factors Influencing Phytoplankton Abundance in Rivers, Lakes, and Impoundments

We analyzed two large data sets (the National Stream Water Quality Accounting Network and the National Eutrophication Survey) to compare quantitatively the ecological structure and function of rivers, river impoundments, and natural lakes on a broad scale. Discriminant analysis with physical-chemical variables separated these three types of aquatic systems along a composite gradient that correlated with water residence time (r = 0.7), drainage area (r = 0.7), water depth (r = 0.6), stream flow (r = 0.6), and water clarity (r = 0.4). Natural lakes and rivers occupied opposite ends of this gradient, and reservoirs, on average, were intermediate, but natural lakes and impoundments overlapped extensively and did not form a useful dichotomy. Regression analysis showed algal abundance per unit phosphorus increasing in the sequence: rivers < impoundments < natural lakes, which paralleled intergroup differences in residence time. Algal abundance per unit phosphorus did not differ significantly among these three system types when systems with similar residence times were compared. Our results indicate that water residence time is a useful system-level index that has similar ecological implications for rivers, lakes, and reservoirs.