FINAL REPORT - International Joint Commission

FINAL REPORT
Figure 39: Wetland plant community zones
In Lake Ontario, complete stabilization of levels would produce a sharp cutoff between the plants that grow
on land (upland vegetation last flooded > 30 years ago) and the plants that can grow under water
(submerged vegetation), with a very narrow band in the middle (meadow marsh and emergent vegetation).
When water levels rise, the woody plants near the water die back, as do the dominant emergent plants
(now submerged), and the amount of aquatic vegetation increases. Conversely, during extended dry
periods, water levels drop and new seeds take root in the newly uncovered bank, establishing meadow
marsh above and emergent marsh closer to the water, and the amount of aquatic vegetation decreases.
During these extended dry periods, cattails, which can dominate emergent marsh, are dewatered and die
off. Surveys conducted in this study found little or no cattails at elevations that had not been flooded for
five years or more. This cycle of high and low water levels creates more diverse wetland vegetation that is
more resilient to other stresses put on the system, and produces more diversified habitat for a broader
range of species (Wilcox, et al, 2005).
Plan 1958-DD has reduced the natural range and longer term cycles of Lake Ontario and this has negatively
impacted wetland habitat, as can be seen in the differences between Plan E and 1958-DD. Of the three
candidate plans, Plan B + is the clear winner for improving wetland habitat on Lake Ontario. Plan D +
provides some modest improvement, while Plan A + offers little to no improvement over 1958-DD.
While the results presented in the previous tables are based on the historical water supply sequence, the
Environmental Technical Work Group did run the wetland model portion of the Integrated Ecological
Response Model through the 50,000- year stochastic time series. The long-term wetland simulation
evaluated the annual elevation range and total wetland area associated with each of the four major Lake
Ontario plant communities (upland, meadow marsh, emergent marsh, and submerged vegetation) for each of
the candidate plans and 1958-DD, as well as Plan E for reference. The model results for the 50,000-year
simulation, as shown in figures 40 and 41, were found to be consistent with the results obtained for the
historical sequence. In some cases, the differences between the regulation plans appeared to be greater
70 Options for Managing Lake Ontario and St. Lawrence River Water Levels and Flows