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A model of the phosphorus cycle and phytoplankton growth in Skaha Lake, British Columbia Fleming, William M.


Phosphorus is recognized as a key nutrient in the cultural eutro-phication of lakes. A simulation model of the phosphorus cycle in eutro-phic Skaha Lake shows total phosphorus to be a useful indicator for the prediction of trophic states. Difference equations and a daily time scale are used in a mass balance model which accounts for the dynamic stratification regime of the lake. Total phosphorus movement between epilimnion, hypolimnion, and sediments is detailed in a series of submodels. An eddy diffusion submodel predicts loading from the hypolimnion to the epilimnion which can equal external loading for short periods of the summer. A phosphorus sedimentation submodel predicts organic sedimentation on the basis of primary production and inorganic sedimentation from adsorption considerations. A regeneration submodel considers the temperature-dependent decomposition rates of sedimented phosphorus. A primary production submodel accounts for temperature, light and phosphorus dependency, as well as respiration, grazing, sinking and advection losses. Based on known phosphorus loading and three years of limnological data, reasonable agreement was found between real and simulated total phosphorus concentration, phytoplankton biomass, and hypolimnetic dissolved oxygen. Results show that three to four times more phosphorus apparently returns to the lake from deep-water sediments than possible by bacterial decomposition alone. Improved simulation of phytoplankton production could probably be achieved with the inclusion of a zooplankton submodel and exten sion to include the specific growth dynamics of more than one algal group. The Michaelis-Menton half-saturation constant appears to be the most sensitive coefficient in the primary production submodel. The probable effects of four phosphorus management policies are assessed using 20 years of hydrologic data (1949-69) and the eutrophic conditions of 1970 as a starting point. While no attempt is made to predict the trophic status of the lake for the next 20 years, definite trends are apparent. With no phosphorus removal and no increase in loading over the hypothetical 20-year period, phytoplankton blooms increase in intensity and hypolimnetic dissolved oxygen approaches zero. With 60 per cent removal of municipal phosphorus and conditions of either low or high economic growth in the Penticton region, the eutrophic conditions of 1970 are reached within 12 to 14 years. Algal blooms and hypolimnetic dissolved oxygen deficits are particularly serious during dry years. With 100 per cent municipal phosphorus removal, trophic conditions appear to improve significantly, with the possibility of minor algal blooms during only dry years. These results indicate that complete removal of the phosphorus from municipal sources appears to be the most rational long-range management policy. These conclusions demonstrate that a theoretical model to predict trophic indicators in a lake can be useful as both a research tool and a practical planning aid for decision-making.

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