Final answer:
The use of nitrogen-rich fertilizers is a human activity that most directly contributes to the decrease in the amount of oxygen in ocean water by causing eutrophication. Reducing fertilizer runoff and carbon emissions is essential to protect ocean oxygen levels.
Step-by-step explanation:
One human activity that most directly contributes to the decrease in the amount of oxygen present in ocean water is the use of nitrogen-rich fertilizers which, after running off into bodies of water, lead to a process known as eutrophication. This process causes the growth of microorganisms and algae blooms that consume the dissolved oxygen in the water, leading to the formation of hypoxic or dead zones where aquatic life cannot be sustained.
Ocean acidification, another consequence of human activity, occurs when excess carbon dioxide from the burning of fossil fuels is absorbed by the oceans, lowering the pH and harming marine life. These activities together decrease the carrying capacity of various ecosystems, including marine environments which are crucial for supporting human welfare through oxygen production by organisms such as phytoplankton.