Dangerous Ocean 'Dead Zones' May be Limited by Natural Cutoff Switch

Dead zones can impact ocean ecosystems in large ways, causing fish species to decline and populations of animals to move to different locations in order to survive. Now, scientists have found that there may be a natural limiting switch to keep these ocean systems from developing persistent dead zones--and it all has to do with iron.

Iron is a crucial catalyst for fueling biological productivity in the ocean. Without it, microscopic plants called phytoplankton cannot fully consume nitrates and phosphates. This, in turn, limits their growth. Iron can enter the ocean through river sediments, windblown dust and continental margin sediments. Yet this iron needs to be dissolved rather than locked up in sediments so that the phytoplankton can use it; it appears that oxygen, in this case, is the key.

In a high-oxygen environment, most of the iron that is dissolved in the water precipitates and turns into iron oxide coatings on particles, which sink to the seafloor; this makes it unavailable to phytoplankton. When a hypoxic environment occurs, though, the iron oxides dissolve and may diffuse back into the water column. This makes them available to fertilize plankton growth which can make a hypoxic state worse as the plankton decays and sinks to the seafloor.

"When this moderate hypoxic state occurs, the iron release fuels more biological productivity and the organic particles fall to the sea floor where they decay and consume more oxygen, making hypoxia worse," said Florian Scholz, one of the researchers, in a news release. "That leads to this feedback loop of more iron release triggering more productivity, triggering more iron release."

Yet there's apparently a limiting factor for hypoxia. The researchers examined concentrations of sediments dating back 140,000 years and made some interesting discoveries.

"But we found that when the oxygen approaches zero a new group of minerals, iron sulfides, are formed," said Scholz in a news release. "This is the key to the limit switch because when the iron gets locked up in sulfides, it is no longer dissolved and thus not available to the plankton. The runaway hypoxia stops and the hypoxic region is limited."

The findings reveal that there is a limit for hypoxic conditions. That said, dead zones still remain and issue, and we should continue to limit the amount of pollutants which enter our coastal waters.

The findings are published in the journal Nature Geoscience.