Oxygen in Black Sea has declined by more than a third since 1955

The Black Sea’s toxic underside is approaching the surface, new research finds.

Comparing measurements collected from 1955 through 2013, researchers discovered that the sea’s oxygen-rich top layer shrank by more than a third from 140 meters to 90 meters deep. That oxygenated layer supports a marine ecosystem and separates the atmosphere from the world’s largest reservoir of poisonous hydrogen sulfide gas.

The oxygen reduction probably resulted from rising regional temperatures and increased nutrient supplies, the researchers report October 2 in Biogeosciences Discussions. While a catastrophic release of hydrogen sulfide remains unlikely, “it’s not science fiction,”says study coauthor Arthur Capet, an oceanographer at the National Institute of Oceanography and Experimental Geophysics in Trieste, Italy.

A shallower top layer significantly reduces fish habitats, says marine ecologist Denise Breitburg of the Smithsonian Environmental Research Center in Edgewater, Md. If the oxygen decline continues, the region’s multibillion-dollar fishingindustry and food supply could be at risk, she says.

Two distinct layers make up the Black Sea: a dense, salty bottom layer and a less salty surface layer. Over time, microbes filled the bottom layer with hydrogen sulfide and pulled oxygen out, creating a dead zone where few life-forms survive.

Scientists worried when the sea’s upper layer waned in the 1970s and early 1980s, but a subsequent rebound between 1985 and 1995 suggested that the sea’s oxygen supply was recovering. Studies at the time, however, missed the recovery’s long-term context, Capet says.

Analyzing 4,467 ship-based measurements of the Black Sea’s oxygen, temperature and salinity, Capet and colleagues found that the sea’s bounce back was short-lived. The sea’s oxygen supply fell 36 percent within the studied interval. In addition, 2013 marked the record’s shallowest top layer.

The trend stems from warming temperatures, the researchers say. Cold water patches sink less frequently, so less oxygen is ferried to deep water. Nutrients from sources such as agricultural runoff can also cause microbial blooms that boost the amount of decaying matter pulling oxygen from the water.