Final answer:
The dead zone in the Gulf of Mexico became smaller in 2012 most likely due to less fertilizer runoff into the water, which would have reduced algal bloom and subsequent oxygen depletion.
Step-by-step explanation:
The most likely reason the dead zone in the Gulf of Mexico became smaller in 2012 is because less fertilizer washed into the water. A drought likely resulted in reduced runoff from land, meaning that fewer nutrients from fertilizers were carried into the Gulf. These nutrients, primarily phosphorus and nitrogen, are needed for algae to grow. Without as much fertilizer runoff, there would have been less algal growth, leading to a smaller accumulation of dead algae on the bottom and therefore, a smaller dead zone. When algal blooms occur, algae consume a lot of oxygen as they grow, and when they die, their decomposition further depletes the water of oxygen, making it inhospitable for fish and other aquatic life—creating a dead zone. By reducing the nutrient load entering the Gulf, the size of the dead zone was able to decrease.
The most likely reason a dead zone in the Gulf of Mexico became smaller in 2012 is because less fertilizer washed into the water. Dead zones are created when large amounts of fertilizers from the land are washed into the gulf when it rains, causing excessive growth of algae. When the algae die, they sink to the bottom and use up the oxygen, creating a dead zone. So, if less fertilizer washed into the water, there would be less algae growth and a smaller dead zone.