Phosphorus Transport in Lake Erie
The role of phosphorus as a causal factor in the eutrophication of Lake Erie is reviewed and the phosphorus budget analyzed. Recent studies by others provide quantitative estimates of the phosphorus loading to Lake Erie. In 1975 the total phosphorus loading to the lake was estimated to be 35,307 metric tons—3 percent from the atmosphere, 43 percent from shoreline erosion, 7 percent from the Lake Huron outflow, 24 percent from point sources within the Lake Erie drainage basin, and 25 percent from diffuse sources in the Lake Erie drainage basin. The above estimates are assumed to include phosphorus in all of its forms, i.e. total phosphorus. The disposition of total phosphorus in Lake Erie includes a mean value of 879 mg/kg in the sediments (based on dry sediment weight) and a mean value of 0.026 g/m3 in the water column (although there were significant variations with position in the lake).
Mathematical representations for phosphorus budgets are reviewed and an adaptation of an earlier model by Lorenzen et al. (1976) is described in detail. The model incorporates sedimentation of particulate phosphorus to the sediments, release of phosphorus from the sediments, and retention of phosphorus in the sediments. Phosphorus uptake by phytoplankton is not specifically included. Specific rate constants are estimated for a three-basin version of Lake Erie based on available field data and assuming equilibrium conditions existed in 1975.
The concept of phosphorus retention in lakes is discussed and various analytical expressions for computing phosphorus retention are reviewed. It is shown that significant errors may result in computing phosphorus retention if equilibrium is assumed when not applicable. Phosphorus loading limits are discussed in relation to achieving a de sired trophic status in Lake Erie. The findings strongly indicate that diffuse source loading of total phosphorus will need to be reduced if the desired improvements in the trophic status of Lake Erie are to be achieved. Alternative phosphorus loading reduction plans are presented to illustrate the application of the trophic status approach towards rehabilitating Lake Erie. Much research and data collection will be needed to more accurately quantify the phosphorus budget of Lake Erie and predict the response of the Lake Erie ecosystem to changes in phosphorus loading.
One important avenue of research is the coupling of the time-dependent hydrodynamic transport equations with the time-dependent material balance equations including the biochemical transformations resulting from phytoplankton growth. Present limitations to the development of such models include inadequacy of field data for model calibration purposes, limitations in computational capability, and insufficient knowledge regarding the analytical representation of the biochemical transformation and growth kinetics for phytoplankton.
KeywordsTotal Phosphorus Total Phosphorus Concentration Army Corps Central Basin Phosphorus Loading
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