Final answer:
A dangerous oceanic phenomenon known as dead zones are areas in the ocean where excessive growth of algae leads to low oxygen levels, killing aquatic organisms. These dead zones are accentuated in regions known as gyres that collect and concentrate floating materials, including plastic waste.
Step-by-step explanation:
A dangerous oceanic phenomenon is known as dead zone, which is an area in the ocean or other body of water where low oxygen levels following excessive growth of algae have killed virtually all aquatic organisms. These dead zones occur throughout the ocean, but are greatly accentuated in regions known as gyres.
These gyres are regions of the ocean in which a combination of ocean currents drives permanent vortices that tend to collect and concentrate floating materials, including plastic waste like in the Great Pacific Gyres.