Illustration showing how fast deoxygenation can happen

Left panel: Phytoplankton at the sea surface use nutrients from runoff and sunlight in photosynthesis to produce organic matter and oxygen.
Right panel: In the past, increased nutrients and warmer temperatures revved up this process, leading to large oxygen-depleted zones and mass extinctions of marine life. Unfortunately, todays ocean is exhibiting similar symptoms. (Illustration by Natalie Renier, © Woods Hole Oceanographic Institution)