Besides heat and acidity, some of the kill-off owes to the seas losing oxygen. Most marine life needs oxygen to survive. Even slight decreases in oxygen content can have severe consequences on ocean ecosystems, especially when combined with other detrimental dynamics.
Earth’s oceans lost over 2% of their oxygen 1960–2015. Anoxia more than quadrupled over that period. Oceanic life is being starved of oxygen; a trend that is accelerating. Coral reefs in anoxic zones quickly die off.
Oxygen concentrations in both the open ocean and coastal waters have been declining since at least the middle of the 20th century. This oxygen loss is one the most important changes occurring in the ocean and may result in ecosystem collapses. ~ American marine biologist Denise Breitburg et al
The Arabian Sea is the largest and thickest dead zone in the world. The area of dead zone is vast and growing. The ocean is suffocating. ~ English marine biogeochemist Bastien Queste