which eventually adds to the available nutrient supply. Thus, it may well be 

 that the zooplankton is principally responsible for high recycling rates esti- 

 mated by Stadelmann and Fraser in Lake Ontario. 



Control of spatial gradien ts - While the above analyses provide information 

 regarding the major controls of biological production in Lake Ontario, they were 

 based on intentionally crude physical segmentation. The two-layer representation 

 reduced the complexity of physical interactions and thus allowed focus on bio- 

 logical and chemical interactions. However, there can often be vast differences 

 between lakewide averaged conditions and nearshore conditions, especially during 

 the spring-summer transition period. Because it is nearshore conditions that 

 most often affect people, one must be concerned with spatial variations. 



The spatial distribution of biological and chemical properties in the Great 

 Lakes is determined by variations of water depth, sunlight, and temperature, 

 by the loctions of rivers, and by currents. Separating these physical factors 

 from in situ transformation is a difficult problem, a problem which numerical 

 simulation can help solve. 



During the transition period between spring and summer both vertical mixing 

 and large-scale circulation are important. Also, the temperature and current 

 patterns of Lake Ontario are relatively two-dimensional (i.e., small longshore 

 gradients); therefore, variations along the long axis of the lake are negligible. 

 We (Scavia and Bennett 1980) simulated flow, temperature, and biological and 

 chemical processes for a north-south transect of the lake. Our approach was as 

 fo Hows . 



Temperature calculations of a hydrodynamic model (Bennett 1971, 1974) were 

 compared to observations, and the model was adjusted until computed and observed 

 temperature contours were in general agreement. We then repeated the calcula- 

 tions, this time including the chemical and biological processes from the 

 ecological model decribed above (Scavia et al . 1976, Scavia 1980a), and compared 

 them to observations. 



During the spring transition period a combination of strong heating and 

 low wind speeds causes the thermocline to form. Because the lake begins the 

 spring colder than 4°C (the temperature of maximum water density), this process 

 starts in shallow water. Thus the lake is divided into two hydrodynamic regions 

 — a deep region where the water is less than 4°C and where surface heating 

 causes vertical mixing, and a shallow region near the coast with temperature 

 greater than 4°C, which may stratify. 



In figure 9, simulated temperature is compared with observations. The 

 model correctly simulates this general spring temperature pattern and the depth 

 of the thermocline. In addition, thermocline extension further off the north 

 shore than the south is also reproduced. 



Wind and buoyancy combine to cause simple but interesting flow patterns. 

 The wind tends to drive a one-cell pattern, with upwelling at shore to the left 

 of the wind and downwelling near the opposite shore. Heating drives a two-cell 

 pattern, with warm water rising near both shores and colder water sinking in 

 the deep region. 



69 



