Dead Zone on the Baltic
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Ocean Dead Zones: The Water of Death

Algal Blooms and Hypoxia

We live on a blue planet dominated by oceans, and our biosphere is dependent on water. Without food, humans survive for weeks or months; without water, we can only live for about three days. But water doesn’t always bring life, and expansive dead zones appear more frequently in our oceans as global temperatures rise.

When you step outside on a warm summer morning and breath in the sweet air, you refresh your body’s blood supply with free oxygen. When you look over your property, you know the plants around you are photosynthesizing, taking in carbon dioxide, using the sun’s energy to produce food, and releasing oxygen back into the atmosphere. But plants are also constantly using oxygen to drive their cellular metabolism. Without free oxygen, most flora and fauna around you can’t survive.

The same oxygen requirements necessary for you and your garden also apply to the plants and animals below the surface of rivers, lakes, and oceans; oxygen means life to them also. These aquatic ecosystems depend on a constant supply of oxygen, and the waters where they exist need to continually absorb oxygen from the atmosphere to ensure a never-ending supply. Failure to maintain adequate oxygen levels makes the waters hypoxic, meaning that oxygen levels are too low to support life.

We refer to large areas of hypoxic water as ‘dead zones.’ These zones occur naturally, but their increasingly frequent presence in the world’s oceans and lakes is related to human activity exacerbating the underlying natural cycles.

Triggers for Dead Zones

Various sets of conditions trigger dead zones. Still, the heart of the problems is always the same — the oxygen demands of flora and fauna in the ecosystem outstrip the ability of the water to maintain sufficient levels of oxygen; life uses the available oxygen faster than it can be replaced.

The most common trigger for a dead zone is eutrophication, a condition where a body of water receives too many nutrients, usually phosphorous and nitrogen. These nutrients feed the growth of cyanobacteria (blue-green algae). Since these bacteria grow rapidly and reproduce, the excess nutrients let their population grow out of control. Thick zones of these algae block sunlight and decrease the exchange of oxygen with deeper zones.

As the algae die, they decompose, and the process requires a lot of oxygen. If enough dead matter accumulates, the decomposition process uses all available oxygen, thereby suffocating the rest of the flora and fauna in the ecosystem, leaving a dead zone.

The primary sources of excess nutrients are sewage, farm manure, and fertilizers used in agriculture. The collective runoff from thousands of farms accumulates in our rivers and pours into the ocean.

Dead Zones appear annually off the coast of Louisiana and Mississippi, fed by the agricultural runoff from the Mississippi River. In some years, the Gulf Coast dead zone encompasses as much as 8,500 square miles.

Cataloging Dead Zones for Research.

Other well-known reoccurring dead zones occur in the Baltic Sea and the Chesapeake Bay on the U.S. East Coast. The Chesapeake Bay problem is worsened by saltwater stratification as freshwater from inland rivers flows oceanward over denser, saline waters from the Atlantic Ocean. This stratification prohibits oxygen exchange between the atmosphere and the deeper saline water, so hypoxic conditions develop more quickly.

The Baltic Sea, home to seven of the world’s ten largest dead zones, suffers from eutrophication by sewage and agricultural runoff, but overfishing of cod has amplified the problem by altering the food chain and reducing the amount of zooplankton that keep the algae population in check.

Well over 400 dead zones exist around the globe, and marine scientists are calling for a global tracking system to aid in studying how these hypoxic zones develop. Limited experiences in places like the Black Sea, Boston Harbor, and the Mersey Estuary of the U.K. demonstrate how water quality improvements lead to the recovery of ecosystems damaged by ocean dead zones. Human activity is largely responsible for the current state of affairs, and human intervention is the only way to implement the changes needed to decrease the frequent occurrence of ocean dead zones and protect our marine habitats.

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Ocean scientists call for global tracking of oxygen loss that causes dead zones (Source: The Guardian)

Dead Zone (Source: National Geographic)

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.