Trophic Changes in Reservoirs; An Hypothesis Using Phosphorus Budget Models

Current empirical models used to predict phosphorus concentration from loading, retention and discharge data assume a phosphorus load which is constant in time. Such models are clearly inappropriate in relation to reservoirs. In a study of several lakes and a reservoir in western Labrador, the phosphorus budget model was found to be an excellent predictor of phosphorus concentration in the natural lakes, but seriously underestimated phosphorus concentration in the reservoir. Since leaching of nutrients from inundated soils and vegetation is generally considered to be the cause of trophic changes in reservoirs, a time dependent “leach loading” component was built into the steady state phosphorus budget model. This component not only allowed the budget to be balanced, but allowed the estimation of leaching coefficients describing the rate of phosphorus loss from submerged soils and vegetation. Using literature data from other reservoirs, the modified model predicted phosphorus dynamics consistent with other observed trophic status indicators.