There's a reason parks post signs asking visitors not to feed the wild waterfowl: It can be harmful to them. White bread — as well as chips, popcorn and doughnuts — do not provide any benefits to ducks.
According to a Sugarland, Texas, government website, giving ducks and other wild birds people foods is the equivalent of "feeding your child nothing but candy." If they do not eat a balanced diet, they "will quickly succumb to disease and death."
And even if they do not eat the bread, it will still affect them and other animals. You are polluting their home, as well as the homes of other animals, like small fish in the water. It can also be harmful to humans. If left to rot, leftover food will bring more insects and rats.