In a small bowl, combine lemon zest, lemon juice, and mayonnaise. Spread lemon mayonnaise over one side of each of 6 bread slices. Break bacon in half and place 4 to 6 pieces on each of those slices (depending on size of the bread). Layer 2 ounces salmon on top of bacon, add about 1/4 cup arugula, and top with about 2 tomato slices. Spread mayonnaise on each of the other 6 bread slices and place atop tomatoes, mayonnaise side down.