In its course from the source to the sea, the progressive eutrophication of a river water by drainage from cultivated and inhabited districts is an almost inevitable natural process. —Butcher 1947
Although current concerns about stream eutrophication mainly focus on nitrogen (N) and phosphorus (P) enrichment (e.g., Smith 2003), early water quality and nutrient enrichment studies in lotic systems focused on carbon (C) enrichment from untreated sewage. Excessive loading of biochemical oxygen demand (BOD) made rivers completely anoxic downstream of sewage treatment plants. Hynes (1960) considered the physical, biological, and chemical effects of sewage loading to create a general conceptual model on the basis of the research of Butcher (1946) and others. The conceptual model of Hynes in part considered the influence of increased organic C on dissolved oxygen (O2) and subsequently on hypoxia- and anoxia-sensitive animals. He noted that most animals immediately downstream from a sewage outfall disappear under anoxic conditions and that, as O2 enters the stream via aeration, high densities of pollution-tolerant fauna could be found. Eventually, as the influence of the sewage diminished downstream, Hynes predicted a return to the clean water animal communities found upstream of the sewage outfall.
Enrichment by N and P were also considered in the Hynes model. He noted a substantial increase in ammonium, phosphate, and nitrate immediately downstream from the sewage outfall that diminished farther downstream. In the anoxic zone, the prevalence of cyanobacteria (Oscillatoria and Phormidium) and Euglena, and further downstream substantial biomass of Cladophora, were predicted. A decade later, Hynes (1970) specifically noted that nutrient enrichment could occur in rivers and streams as a direct result of human alteration of land use (in addition to sewage input). He described the amounts of increase in fertilizer use and made preliminary calculations of how much fertilizer might reach rivers and streams. At that time, however, Hynes documented a paucity of studies on the effects of nutrient enrichment alone but predicted that planktonic algae in large rivers would be stimulated.
There has been conceptual progress related to defining trophic state and characterizing lotic eutrophication on several fronts in the last three decades. Omernik (1977) proposed that various regions are expected to have distinct baseline amounts of nutrients related to geology, topography, and land uses. He described areas of the United States that would be expected to have relatively greater concentrations of nutrients in streams, and he defined the concept of nutrient ecoregions. The idea that a reference baseline trophic level occurs naturally in a region forms the basis of many current efforts to regulate stream nutrients. Over the last three decades, numerous research programs were designed to link nutrient enrichment to increases in autotrophic biomass in rivers and streams by methods that included the ‘‘clay pot’’ nutrient–diffusing substrata experiments, experimental stream channel enrichment experiments, whole-stream enrichments, and a definition of nuisance amounts of algae (Welch et al. 1988).
At a more fundamental level, there have been few attempts to define the trophic state of lotic ecosystems and provide a comprehensive definition of eutrophication applicable to rivers and streams. Thus, I initially attempt to provide such a definition, and under this framework, I describe how prior research can be viewed given my definition.