On a warm summer day in august, you decide to go for a walk through a new housing development that was built near your home a few years ago. as you walk by the pond that's in the center of this development, you notice an unpleasant smell. then, you see that the pond is full of green algae, and there are dead fish floating on the surface. this is a pond that used to be clean and clear, and you used to play in it as a child. what could have most likely caused this change?