The oceans generate most of the oxygen we breathe, provide food and medicine and help keep our climate stable, according to the day's organizers. They are also home to amazing animals and ecosystems, like whales and coral reefs, that make the earth a more wondrous place to live. But the world's marine environments face unprecedented threats. Here are five things to know about the state of our oceans in 2019.

1. Ocean Plastics Are on the Rise

It's well-known that eight million metric tons of plastics enter the world's oceans every year. But a study published in April gave new insight into how plastic pollution has proliferated in the past six decades. Researchers found that equipment used to collect plankton had increasingly been disrupted by plastic since it first got entangled with fishing gear in 1957.

"The message is that marine plastic has increased significantly and we are seeing it all over the world, even in places where you would not want to, like the Northwest Passage and other parts of the Arctic," Marine Biological Association in Plymouth, England researcher Clare Ostle told The Guardian.

2. Plastic Pollution Threatens Marine Oxygen Production

All that plastic floating in the ocean kills one million birds and more than 100,000 marine mammals every year, according to the UK government. But a study published in May found it could have a disturbing impact on some of the ocean's smallest life forms as well. Scientists exposed the ocean's most abundant photosynthetic bacteria to chemicals that leach from plastic bags. The chemicals made it harder for the bacteria to grow and produce oxygen. This is scary because these bacteria are responsible for 10 percent of the oxygen we breathe.

"This study revealed a new and unanticipated danger of plastic pollution," paper co-author and Macquarie University research fellow Lisa Moore told The Independent.

3. Global Warming Is Already Putting Fish in Hot Water

The oceans and the creatures in them are also threatened by climate change, and a groundbreaking study published in March found that rising ocean temperatures are already shrinking fish populations. A University of Rutgers-led team discovered that sustainable fish populations had declined by an average of 4.1 percent over 80 years. That might not sound like a lot, but it actually amounts to 1.4 million metric tons of fish lost between 1930 and 2010. And in some regions the decline was more extreme: sustainable fish populations fell by 34 percent in the northeast Atlantic and 35 percent in the Sea of Japan.

"We were stunned to find that fisheries around the world have already responded to ocean warming," study co-author and Rutgers' Department of Ecology, Evolution, and Natural Resources associate professor Malin Pinsky told Rutgers Today. "These aren't hypothetical changes sometime in the future."

4. Marine Heatwaves Act Like Underwater Wildfires

Ocean warming doesn't just damage individual species. It devastates entire ecosystems. A first-of-its-kind study published in March found that the number of ocean heat wave days per year is surging: The number has increased by more than 50 percent between two 29-year time chunks compared by the scientists. This has particularly harmed coral reefs in the Caribbean, Australian sea-grass beds and California's kelp forests.

"You have heatwave-induced wildfires that take out huge areas of forest, but this is happening underwater as well," lead author Dan Smale at the Marine Biological Association in Plymouth, UK told The Guardian. "You see the kelp and seagrasses dying in front of you. Within weeks or months they are just gone, along hundreds of kilometres of coastline."

5. Ocean Acidification Makes Life Even Harder for Coral Reefs

Marine heat waves threaten coral reefs by causing coral bleaching, in which corals expel the algae that give them color and nutrients. But the greenhouse gasses we are pumping into the atmosphere also endanger coral in another way. They cause ocean acidification, which is what happens when carbon dioxide dissolves in seawater and changes its chemical makeup. This reduces the amount of calcium carbonate that animals like corals use to repair themselves after stressful events like bleachings. In research published just last week, scientists found that some corals and algae they studied were not able to adapt to more acidic waters. This could alter the composition and function of reefs.

"We found that corals and coralline algae weren't able to acclimatize to ocean acidification," study author Malcolm McCulloch said.

Oceans Do Us a ‘Huge Service’ by Absorbing Nearly a Third of Global CO2 Emissions, but at What Cost? #OCEANS https://t.co/tsFCz6FGwP

— EcoWatch (@EcoWatch) March 18, 2019