Eutrophication and trophic state in rivers and streams

Abstract

Many natural streams are net heterotrophic, so I propose that trophic state be divided into autotrophic and heterotrophic state. This division allows consideration of the influence of external carbon sources as well as nutrients such as nitrogen and phosphorus. Empirical results suggest that phosphorus and nitrogen are the most important nutrients regulating autotrophic state in flowing waters and that benthic algal biomass is positively correlated to gross primary production in streams. Reference (minimally influenced by human activities) nutrient concentrations and correlations of nutrients with algal biomass are used to characterize reference distributions of stream autotrophic state. Only when reference nutrient concentrations are in the upper one third of those expected in the United States, is maximum benthic chlorophyll projected to exceed 100 mg m Ϫ2 (a concentration commonly used to indicate nuisance levels) Ͼ30% of the time. Average reference nutrient concentrations lead to sestonic chlorophyll concentrations above those considered typical of eutrophic lakes (Ͼ8 mg m Ϫ3 ) less than half the time. Preliminary analysis suggests that autotrophic state is variable in small pristine streams because it is influenced by canopy cover (light), but heterotrophic state is less variable because it can be based on allochthonous or autochthonous production. Nitrogen and phosphorus enrichment can influence both heterotrophic and autotrophic state, and these effects could cascade to animal communities. Stoichiometry should be considered because carbon, nitrogen, and phosphorus are all involved in trophic state. The proposed definition of trophic state offers a starting conceptual framework for such considerations. The evolution of concepts regarding enrichment in streamsIn 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 1947Although 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 (O 2 ) 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 O 2 enters the stream via aeration, high densities of pollution-tolerant fauna 1 Corresponding author (wkdodds@ksu.edu).

Eutrophication of freshwater and marine ecosystems

“…Building on the trophic state classification systems for lakes (Carlson 1977;OECD 1982), Dodds (2006) developed a new trophic state classification system for rivers and streams. Dodds' study revisits some of the ideas discussed 30 yr ago by Gene Likens, who presented a conceptual diagram of relationships between allochthonous inputs, autochthonous inputs, and the trophic state of freshwater lakes (see fig.…”

Modifications of ecological trophic structures on chemical gradients in lotic ecosystems and their relations to stream ecosystem health

“…N:P ratios are widely used as a key indicator for diagnosis of trophic state and limiting nutrients in the ecosystems (Shannon & Brezonik 1972). The high N:P mass ratios of 17 indicate phosphorus limitation in the ecosystem and low N:P ratios of B4 indicate nitrogen limitation (Forsberg & Ryding 1980;Dodds 2006). This reference indicates that our streams largely have severe phosphorus limitation (Elwood et al 1981), based on the N:P ratio calculations on the trophic components.…”

Trends in chlorophyll-a concentration in urban water bodies within different man-used basins

“…Since the initial review of Vollenweider (1968) relating nutrient supply to lake trophic state, subsequent explorations have been done and several eutrophication indicators have been employed. Chlorophyll-a, as a proxy of phytoplankton biomass, merits its use owing to its importance in trophic relationships and ease of analysis (OECD 1982, Dodds 2006. To constitute a good trophic level indicator, a comprehensive knowledge of aquatic system dynamics and functioning is needed, given that it is not only influenced by nutrient supply but also by other environmental factors.…”