Answer:
Pollution of the oceans
Step-by-step explanation:
The oceans are the largest storage and producer of oxygen on the planet. They have by far the most producers, which use the carbon dioxide and release the oxygen as a waste product. That oxygen ends up into the ocean waters, and from the ocean waters into the atmosphere. The humans though are breaking up this natural cycle with the enormous pollution they cause in the oceans. Fossil fuels, plastics, electronics, very often end up in the oceans, harming the living organisms, including the producers, resulting in their decline, thus decline in the levels of oxygen produced and released into the atmosphere.