Berry Lemonade

Prepare sugar syrup: In 4-quart saucepan, heat 4 cups water with sugar and lemon peel to boiling over high heat, stirring occasionally. Cover saucepan and boil 3 minutes. Remove saucepan from heat.
Meanwhile, in food processor, pulse berries until pureed. Pour into medium-mesh sieve set over large bowl and press berry mixture with back of spoon to remove seeds; discard seeds.
Remove peel from syrup; stir syrup into berry puree with lemon juice and remaining 4 cups water. Makes 13 cups. Pour into large pitcher with tight-fitting lid. Cover and refrigerate until cold, at least 3 hours or up to 2 days. Serve over ice. Add seltzer to taste; garnish with berries and lemon slices.