Figure 10 shows mean circulation for three 24-day periods and the mean for 

 the entire 72-day simulation. During the first two 24-day periods the wind 

 came predominantly from the east; because the Coriolis force deflects the water 

 to the right, water near the surface flows north and deep water flows south in 

 compensation. Upwelling near the south shore and downwelling near the north 

 shore closes this circulation. During the last 24-day period the wind and 

 circulation reverse. For the whole 72-day period the wind's effects tend to 

 cancel and circulation looks like the thermally driven pattern. 



The simple picture of a combination of wind and buoyancy effects should be 

 considered an average circulation pattern. At any given time flow is dominated 

 by wind, and it is only because the thermally driven flow is more persistent 

 that it is as important as the wind-driven flow. 



In general, nutrient concentrations had slight offshore gradients and were 

 homogeneous vertically in early April. These data were used to initiate the 

 simulation. By the end of May, distinct offshore gradients had developed; 

 nutrients, especially phosphorus and silica, were severely depleted in regions 

 within 10 km of both shores (fig. 11). Also, at this time, no strong vertical 

 gradients were obvious in either the model output or the data. At the end of 

 the period of simulation (corresponding to the June 20-22 cruise) the symmetry 

 of the north and south shore contours was lost. The region of nutrient depletion 

 along the north increased to greater than 25 km and vertical stratification was 

 not as strong there. The spatial and temporal progression of the region of 

 nutrient depletion is demonstrated for phosphorus in figure 11; inorganic 

 nitrogen and soluble reactive silica have similar patterns. The comparison 

 between observed total dissolved phosphorus (TDP) and modeled available phosphorus 

 (AP) is made because previous simulations of phosphorus cycling in Lake Ontario 

 (Scavia 1979) indicate that in spring AP is approximated best by TDP, due, 

 presumably, to production of easily hydro lyzed phosphorus compounds during the 

 previous winter. 



Biomass parameters (chlorophyll a and particulate organic carbon) in April 

 were relatively homogeneous vertically, with higher values nearshore. Patterns 

 in May generally showed offshore gradients and little vertical structure (fig. 

 12), except for evidence of nearshore subsurface chlorophyll peaks, which the 

 model did not reproduce. The simulations indicated that between May and June 

 cruises, upwelling moved the higher nearshore concentrations offshore, creating 

 a lens of high biomass about 15 km from the north shore. (See figs. 9 and 10.) 

 By the June cruise, increased nearshore production apparently created higher 

 biomass again close to shore. A similar structure was produced along the south 

 shore with the exception that, like the nutrient contours, the biomass contours 

 were constrained closer to shore by the wind-driven flow. 



Given this model as an adequate representation of major dynamics in Lake 

 Ontario, we performed numerical experiments to find out which of the physical 

 mechanisms was most important. In these experiments we ran two simulations and 

 compared the results to the original calculations discussed above (henceforth, 

 the normal case). In the first simulation, we eliminated mass transport by 

 advection and diffusion. In the second simulation, only vertical diffusion was 

 included. In all cases in situ biological and chemical processes and sinking 

 were included and temperature distributions were kept the same as in the normal 

 case. The results of these experiments are summarized in figure 13 for available 

 phosphorus and particulate organic carbon. 



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