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.

This unique observation prompted a collaborative study explained in a new paper published today in Nature Communications analyzing what this foggy water layer is caused by, and the impact it has on life at the bottom of the seafloor.

«What we’re seeing are hypoxic ocean waters, meaning there is little to no oxygen in that area. All of the macro-organisms are trying to get away from this deoxygenated water, and those that cannot escape essentially suffocate. I have never seen anything like that on a coral reef,» said Johnson.

«There is a combination of stagnant water from low wind activity, warm water temperatures, and nutrient pollution from nearby plantations, which contributes to a stratification of the water column. From this, we see these hypoxic conditions form that start to expand and infringe on nearby shallow habitats,» explained Johnson.

Investigators suggest that loss of oxygen in the global ocean is accelerating due to climate change and excess nutrients, but how sudden deoxygenation events affect tropical marine ecosystems is poorly understood. Past research shows that rising temperatures can lead to physical alterations in coral, such as bleaching, which occurs when corals are stressed and expel algae that live within their tissues. If conditions don’t improve, the bleached corals then die. However, the real-time changes caused by decreasing oxygen levels in the tropics have seldom been observed.

At a local scale, hypoxic events may pose a more severe threat to coral reefs than the warming events that cause mass bleaching. These sudden events impact all oxygen-requiring marine life and can kill reef ecosystems quickly.

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Materials provided by Woods Hole Oceanographic Institution. Note: Content may be edited for style and length.