Definitely Tundra Swans, can pretty much ID these birds without looking at them, by numbers (Trumpeter Swans don't gather in these large flocks, at least not in Ontario), location (Long Point area is a well known staging area for Tundra Swans in March, while Trumpeter Swans are uncommon there) and habitat (Tundra Swans regularly feed in fields and I haven't seen Trumpeter Swans away from water, although they do feed in fields on the West Coast).
And as pointed out above, when examining the picture you can also ID them as Tundra Swans.
/Thomas