Summary Analysis Of The North American (US Portion) OCED Eutrophication Project: Nutrient Loading - Lake Response Relationships And Trophic State Indices

Rast, Walter
Lee, G. Fred
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The US participation in the OECD Eutrophication Program of having 20 investigators prepare reports on the nutrient load-lake and impoundment response relationships for their respective water bodies. This report presents a critical review of these overall relationships with particular emphasis given to evaluation of the Vollenweider nutrient load-trophic state formulations. This review includes consideration of the nutrient load response relationships for 38 water bodies, or parts of water bodies, located throughout the US, with the preponderance located in the northern half of the US. It has been found that the Vollenweider nutrient load relationship involving water body mean depth, hydraulic residence time and phosphorus load correlates well with the trophic states assigned by the US OECD eutrophication study investigators. A good correlation has also been found between phosphorus loading, normalized as to hydraulic residence time and mean depth, and the average chlorophyll and water clarity (as measured by Secchi depth) for the US OECD water bodies. In general, phosphorus and nitrogen loads to US OECD water bodies were within a factor of + two of the loads predicted on the basis of average nutrient concentrations within the water bodies and on the land use patterns within the water body watersheds. Generalized nutrient export coefficients have been developed in this study, enabling estimates of nutrient loads to be made on the basis of land use patterns within the watershed. The relationships developed in this study can be used to predict the improvement in water quality that will result from a change in the phosphorus load to a water body for which phosphorus is the key chemical element limiting planktonic algal growth. The US OECD water bodies all show approximately the same trophic status when evaluated by several recently-proposed trophic state index systems. A new trophic state index system has been developed in this study which is based on the relationship between the actual phosphorus loading and permissible phosphorus loading as defined in the Vollenweider phosphorus loading and mean depth/hydraulic residence time relationship. This relationship has been modified to enable water quality managers to determine the appropriate phosphorus load for a particular water body in order to yield a certain chlorophyll content from planktonic algae and its corresponding water clarity. It is recommended that these relationships be used as a basis for establishing critical phosphorus loads to lakes and impoundments. This report was submitted in fulfillment of Contract No. R-803356-01-0 and Contract No. R-803356-01-3 under the sponsorship of the U.S. Environmental Protection Agency. Work was completed as of August, 1977.
OCED Eutrophication Project , Nutrient Loading , Lake Response