New research has found that in the past 50 years, the amount of ocean areas with zero oxygen has gone up more than fourfold. Low-oxygen sites, as they are called, are regions where water has lost its usual oxygen levels. As warming trends continue, oxygen levels are likely to drop further.

Climate change and nutrient pollution are seen as the major causes of this drop in oxygen levels. Nutrient pollution happens when the water is too rich in organic fertilizers like nitrogen and phosphorus, which aid the excessive growth of algae. Nutrients can run off of land in urban areas where lawn fertilizers are used. Algae growth blocks light that is needed for underwater plants, such as seagrasses, to grow. When the algae and seagrass die, they decay. In the process of decay, the oxygen in the water is used up and this leads to low levels of dissolved oxygen in the water. This affects the population of fish, crabs, oysters and other marine animals.

"Oxygen is fundamental to life in the oceans," said Denise Breitburg, lead author of the research and marine ecologist with the Smithsonian Environmental Research Center in a release Thursday. "The decline in ocean oxygen ranks among the most serious effects of human activities on the Earth's environment."

"It's a tremendous loss to all the support services that rely on recreation and tourism, hotels and restaurants and taxi drivers and everything else," Lisa Levin from Scripps Institute of Oceanography, said in the release. "The reverberations of unhealthy ecosystems in the ocean can be extensive."

"Approximately half of the oxygen on Earth comes from the ocean," said Vladimir Ryabinin, executive secretary of the International Oceanographic Commission that formed the Global Ocean Oxygen Network. "However, combined effects of nutrient loading and climate change are greatly increasing the number and size of 'dead zones' in the open ocean and coastal waters, where oxygen is too low to support most marine life."

Areas in Chesapeake Bay and the Gulf of Mexico have shown a large increase in low oxygen zones. Fish naturally tend to avoid these zones causing a shrinkage in habitat. But, this is not the only problem of low oxygen. Even in areas where there is a slight decline in oxygen levels, it causes a stunt in animal growth, reproduction and also causes several diseases. It also can trigger the release of dangerous chemicals such as nitrous oxide, a greenhouse gas up to 300 times more powerful than carbon dioxide, and toxic hydrogen sulfide. While some animals can thrive in dead zones, overall biodiversity falls.

Warm water makes it harder for oxygen to reach the ocean interior. Furthermore, as the ocean as a whole gets warmer, it holds lesser oxygen the team found. According to the team, the only way out of this conundrum is to address the causes:  nutrient pollution and climate change. While neither issue has a simple, overnight cure, even small steps make a difference, they say.

Cutting down pollution and surface run-off is seen as the first step. The oceans have become one giant, watery dump-yard for our waste, be it organic or inorganic. Tannery and other industry run-off only make things worse and have to be curtailed.

"This is a problem we can solve," Breitburg said. "Halting climate change requires a global effort, but even local actions can help with nutrient-driven oxygen decline."

As proof, Breitburg points to the ongoing recovery of Chesapeake Bay, where nitrogen pollution has dropped 24 percent since its peak thanks to better sewage treatment, better farming practices and successful laws like the Clean Air Act. While some low-oxygen zones persist, the area of Chesapeake Bay with zero oxygen has almost disappeared.

"Tackling climate change may seem more daunting," she added, "but doing it is critical for stemming the decline of oxygen in our oceans, and for nearly every aspect of life on our planet."