For most of the year, the Gulf of Mexico is teeming with marine life, from tiny crustaceans  to massive baleen whales. But every summer, disaster strikes. Around May,  animals begin to flee the area. And soon, creatures that can’t swim  or can’t swim fast enough begin to suffocate and die off in massive numbers. From late spring to early autumn, thousands of square kilometers  along the coast become a marine dead zone— unable to support  most forms of aquatic life. This strange annual curse isn’t unique; dead zones like this one  have formed all over the world. But to explore what’s creating  these lethal conditions, we first need to understand  how a healthy marine ecosystem functions. In any body of water that receives  sufficient sunlight, plant-like organisms such as algae  and cyanobacteria thrive. Clouds of algae streak the surface  of deep waters, and in shallower regions, large seaweeds  and seagrass cover the ground. Not only do these organisms form  the foundation of local food chains, their photosynthesis provides the oxygen necessary for aquatic animals to survive. Besides sunlight and C02, algae growth also depends on nutrients like phosphorus and nitrogen. While such resources  are typically in short supply, sometimes the surrounding watershed can  flood coastal waters with these nutrients. For example, a large rainstorm  might wash nutrient-rich sediment from a forest into a lake. These additional resources lead  to a massive increase in algae growth known as eutrophication. But rather than providing  more food and oxygen, this surge of growth  has deadly consequences. As more algae grows on the surface,  it blocks sunlight to the plants below. These light-deprived plants  die off and decompose in a process which uses up the water’s  already depleted oxygen supply. Over time, this can reduce  the oxygen content to less than 2 milligrams  of oxygen per liter, creating an uninhabitable dead zone. There are rare bodies of water  that rely on natural eutrophication. Regions like the Bay of Bengal  are full of bottom-dwelling marine life that has adapted  to low-oxygen conditions. But human activity has made eutrophication a regular and widespread occurrence. Nutrient-rich waste from our sewage  systems and industrial processes often end up in lakes, estuaries  and coastal waters. And the Gulf of Mexico is one  of the largest dumping zones on earth for one particular pollutant: fertilizer. American agriculture relies  heavily on nitrogen and phosphate-based fertilizers. 31 states, including America’s  top agricultural producers, are connected  to the Mississippi River Basin, and all of their runoff  drains into the Gulf of Mexico. Farmers apply most of this fertilizer  during the spring planting season, so the nutrient flood  occurs shortly after. In the Gulf, decomposing algae sinks into the band of cold saltwater near the seafloor. Since these dense lower waters don’t mix with the warmer freshwater above, it can take four months  for tropical storms to fully circulate oxygenated water back into the gulf. This dead zone currently costs  U.S. seafood and tourism industries as much as $82 million a year, and that cost will only increase  as the dead zone gets bigger. On average the gulf dead zone  is roughly 15,000 square kilometers, but in 2019 it grew  to over 22,000 square kilometers— approximately the size of New Jersey. Human activity is similarly responsible  for growing dead zones around the world. So what can be done? In the short term, countries can set  tighter regulations on industrial run-off, and ban the dumping of untreated  sewage into ocean waters. On farms, we can plant buffer zones composed of trees and shrubs  to absorb runoff. However, long term solutions will require radical changes to the way we grow food. Farmers are currently incentivized  to use techniques that reduce the health of the soil and rely heavily  on nitrogen-rich fertilizers. But there would be less need  for these chemicals if we restore the soil’s natural nutrients by planting diverse crops that manage soil erosion and fertility. Hopefully we can make  these fundamental changes soon. Because if we don’t, the future of our marine ecosystems  may be dead in the water.