What happens to marine life when oxygen is scarce?

In September of 2017, marine biologists were conducting an experiment in Bocas del Toro, off the Caribbean coast of Panama. After sitting on a quiet, warm open ocean, they snorkeled down to find a peculiar layer of murky, foul-smelling water about 10 feet below the surface, with brittle stars and sea urchins, which are usually in hiding, perching on the tops of coral. This observation prompted a collaborative study analyzing what this foggy water layer is caused by, and the impact it has on life at the bottom of the seafloor.

source https://www.sciencedaily.com/releases/2021/07/210726165845.htm