Verification Analysis Of Lake Ontario And Rochester Embayment Three Dimensional Euthophication Models

Thomann, Robert V.
Winfield, Richard P.
Segna, John J.
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A three dimensional time variable model of the phytoplankton and nutrients of Lake Ontario and the Rochester Embayment is examined in detail. The data from the International Field Year on the Great Lakes (IFYGL) are used as the primary data base. The data are summarized and statistically analyzed on a three dimensional grid and segment averages using a 67 segment representation of the lake and a 72 segment representation of Rochester Embayment, are calculated. In addition, averages for eight regions of the lake and lake wide averages for two depth layers are computed. Average phytoplankton levels during the period May, 1972 and June, 1973 in the near shore region are approximately 3 ug/l higher than open lake values. Similarly, near shore open lake total phosphorus gradients of about 5 ug P/L appear to persist for a substantial part of the year. The data base collected during IFYGL exhibited significant spatial and temporal variations at scales of 10 x 40 km. The two data bases available, Canadian Centre for Inland Waters (CCIW) and Environmental Protection Agency (EPA), only agree within certain limits. The verification analysis of the models indicates that the median relative error for the results of calculated versus observed chlorophyll on the segment to segment level is about 30%. The inclusion of diatoms and nondiatoms and silica limitation in the kinetic structure, only marginally improved the three dimensional credibility of the model. The Rochester Embayment model indicated that about 90% of the total phosphorus input to the embayment is transport of nutrients from the west of the embayment and about 10% is from direct input from the Genesee River and municipal input from the City of Rochester. The question of model credibility is examined in detail and it is concluded that as one progresses to smaller spatial scales, especially to the scale of the Rochester Embayment, hydrodynamic transport and local dispersion become increasingly significant. On the larger spatial scales, system kinetics dominate and the importance of the hydrodynamic structure is decreased. Chlorophyll verification status, of the model ranges from an average of 10% relative error on the whole lake scale to 50% error at the local embayment scale. In general, the results indicated that the ability of complex three dimensional models to capture the temporal and spatial variability of phytoplankton dynamics is relatively marginal given the existing data base and present kinetic structures. Only as the spatial scale of the problem is increased, do the models appear to accurately reflect the observed variations.
Lake Ontario , Rochester , Embayment , Eutrophication Models , Verification Analysis