In September of 2017, Woods Hole Oceanographic Institution postdoctoral scholar Maggie Johnson was conducting an experiment with a colleague 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 unique observation prompted a collaborative study explained in a new paper published on July 26, 2021, 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.
The study looks closely at the changes occurring in both coral reef and microbial communities near Bocas del Toro during sudden hypoxic events. When water drops below 2.8mg of oxygen per liter, it becomes hypoxic. More than 10% of coral reefs around the world are at high risk for hypoxia (Altieri et al. 2017- tropical dead zones and mass mortalities on coral reefs).
A glimpse at the bottom of the seafloor in shallow waters off the coast in Bocos del Toro after a hypoxic event occurs. Hypoxic ocean waters are those with little to no oxygen, which research suggests is caused by stagnant, warm waters and nutrient pollution. Brittle sea stars, which usually are in hiding, perch on top of coral to attempt to escape from the deoxygenated environment. Credit: Maggie Johnson © Woods Hole Oceanographic Institution
“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.
A sea sponge after a hypoxic event occurs. Hypoxic waters are those with little to no oxygen. It has lasting impacts on marine life at the seafloor of shallow, tropical waters, like this (species name), as well as coral and macro-organisms like urchins. Credit: Maggie Johnson © Woods Hole Oceanographic Institution
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.
Investigators reported coral bleaching and mass mortality due to this occurrence, causing a 50% loss of live coral, which did not show signs of recovery until a year after the event, and a drastic shift in the seafloor community. The shallowest measurement with hypoxic waters was about 9 feet deep and about 30 feet from the Bocas del Toro shore.