Hydroelectric projects have the potential to alter the flow of nutrients (e.g., phosphorous in sediments, algal bound nitrogen) and energy flow in river systems. Because of the long hydraulic residence times and high particle trapping efficiency of reservoirs, they often provide ideal conditions necessary for algal blooms (i.e., excess nutrients and sunlight, no flushing of algae growing in suspension) with subsequent reservoir eutrophication (water pollution caused by these excess plant nutrients) becoming a common problem associated with ongoing hydroelectric operations.
The trophic state of a lake or reservoir is based on overall system productivity and is a function of physical features (e.g., latitude and elevation as they affect sunlight and air temperatures; ratio of watershed to waterbody areas; reservoir depth; or hydraulic residence time), chemical features (e.g., nutrients, oxygen), and biological responses (e.g., primary productivity, zooplankton and fish assemblage food webs and biomass). In low-oxygen conditions of many reservoir sediments, the adsorption bond between phosphorus and sediment particles becomes unstable and often results in the transformation and release of the adsorbed phosphorus into orthophosphate (i.e. previously precipitated phosphorous becomes dissolved into the water column again). Phosphorous is often a limiting nutrient and when orthophosphate it allows an increase it algal productivity. The decomposition following algal blooms absorb oxygen out of the water, leading to an oxygen deficit. Seasonal changes in reservoir stratification are often accompanied by periods with higher and lower amounts of oxygen, leading to periods of adsorption and desorption of phosphorus, or "internal cycling," which can lead to large algal blooms during spring and fall when large exchanges of surface and bottom waters occur.
Although the most common nutrient limiting the primary production in freshwater is dissolved phosphates, nitrogen compounds can limit production in alpine and arid climates where many hydroelectric developments are situated and where watershed productivity and soil nitrogen retention is low. This has implications for both the influence of air deposition of nitrogen compounds from atmospheric sources, and also of fish passage on nitrogen arriving in river systems with migratory salmonid populations.
The following references are recommended for additional information on eutrophication:
Horne, A., and C. Goldman. 1994. Limnology. McGraw-Hill, New York.
Triska, F. J., J. R. Sedell, and S. V. Gregory. 1982. Coniferous forest streams. Pages 292-332 in R. L. Edmonds, editor. Analysis of coniferous forest ecosystems in the western United States. Hutchinson Ross, Stroudsburg, Pennsylvania.
USACE (U.S. Army Corps of Engineers). 1997. Hydrologic engineering requirements for reservoirs. Washington, D. C. EM 1110-2-1420, CECW-EH-Y.
Vollenweider, R. A., and J. Kerekes. 1982. Eutrophication of waters: Monitoring, assessment and control. OECD Cooperative programme on monitoring of inland waters (Eutrophication control),Environment Directorate, OECD, Paris.
Wetzel, R. G. 2001. Limnology: lake and river ecosystems. Third edition. Academic Press, San Diego, California.
Various approaches for evaluating nutrient exchange and reservoir eutrophication are used by researchers, a few of which are discussed below.
