Thousands of farms are located along the mississippi river and its tributaries through the central u. s. as water washes into the river after a heavy rain, it brings with it nutrients from fertilizers added to farmland. these materials flow downriver and eventually enter the gulf of mexico, where they create conditions for a dead zone — a region of the ecosystem that can’t support any living creatures. this is an example of a tragedy of the commons. why is the dead zone seasonal?