Ocean dead zones cause for concern

By Stuart Wakeham

Professor, Skidaway Institute of Oceanography

When you look at the activities in the waters of Coastal Georgia and you see an ocean seemingly teeming with life, it is difficult to comprehend that large sections of the world’s oceans are considered “dead zones.” These are areas of the oceans where low levels of dissolved oxygen have either killed or driven off most of the fish and shell fish sought by commercial and recreational fishermen.

Dr. Stuart Wakeham

Dr. Stuart Wakeham

Oxygen‑deprived areas of ocean around the world have spread exponentially since the 1960s. They now affect a total area of more than 95,000 square miles. Georgia’s waters may not be immune to the threat.

Dead zones are caused by both natural and man-made processes. It begins when excessive loads of nutrients are introduced to the water from sewage, storm runoff containing fertilizer and other sources. These nutrients promote the growth of microscopic marine algae called phytoplankton. When the phytoplankton die and decay, the process, if excessive, can consume much of the dissolved oxygen in the water. Oxygen deficit (hypoxia) occurs when dissolved oxygen in seawater falls below 0.2 mg of O2/liter.

Some dead zones occur naturally along the western boundaries of continents where nutrient-rich, cold water is occasionally upwelled from the deeper ocean. However, they also occur in areas where rivers deliver excess of nitrogen from farm fertilizers, sewage and emissions from vehicles and factories. The most infamous coastal dead zone is in the northern Gulf of Mexico, where the Mississippi River dumps fertilizer runoff from the Midwest.

Although not considered a dead zone, Georgia’s coastal waterways are showing discouraging signs. Skidaway Institute scientist Peter Verity has monitored local water conditions for more than 20 years and documented a steady decline in dissolved oxygen to the point where some areas approach hypoxic during the summer.

Dead zones strongly affect marine life and threaten fisheries. The expansion of dead zones may lead to diminished biodiversity and increase the distributions of organisms that have adapted to oxygen‑poor waters, such as jellyfish.  Motile animals such as fish might flee the suffocation of hypoxia, but slow moving or non-motile creatures that dwell on the bottom such as clams, lobsters and oysters, are less able to escape. The result is mass mortality due to low levels of dissolved oxygen.

Scientific studies suggest that in many areas of the oceans there is a small margin of safety against oxygen dropping to deadly levels. It is therefore of critical importance to develop a clear understanding of the functioning of oxygen‑deficient zones.

At Skidaway Institute, we have been studying how carbon cycles under conditions of low oxygen levels. This understanding is essential if we are to assess how coastal oxygen‑deficient zones occur and function as a result of man-made influences.

The need for a clear understanding of how dead zones develop and function is more pressing than ever as new research indicates global warming could aggravate the problem, leaving fish and other marine life in oxygen‑poor oceans for thousands of years to come if global warming continues unchecked.

Increases in the emissions of carbon dioxide could change rainfall patterns. In some areas, this could lead to increased levels of run‑off from rivers into the seas. While previous studies have established a link between climate change and dead zones, new computer simulations by Danish researchers, among others, suggest the dead zones could persist for millennia and lead to a considerable purge and restructuring of ocean life.

Dead zones currently make up less than two percent of the world’s ocean volume. The model predicts that global warming could cause dead zones to grow by a factor of ten or more by the year 2100.

In the worst‑case scenario, dead zones could encompass more than a fifth of the world’s oceans. The impact on global ecosystems will be substantial.

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