What exactly are ocean dead zones?

Ocean dead zones are just what they sound like, areas of the ocean which are ‘dead’, large areas of the ocean where animal populations are greatly reduced due to a lack of oxygen in the water. They typically occur in the bottom 1.5 metres along the seafloor where there are countless small creatures living on or in the seabed, sediment dwelling animals like worms, bivalves (such as clams and mussels), sponges, crabs, urchins and starfish all live on the seafloor. These smaller creature which feed on tiny plankton and small particles are the beginning of the food chain in the ocean. The bottom dwelling creatures are eaten by slightly bigger animals and so on until you reach the top predators like whales, dolphins, sharks and seals. So as you can tell, the creatures living on the seafloor play an important role in the overall health and survival of the oceans.

How are ocean dead zones formed?

It all starts with human activity on the land. In nature, landscapes are filled with a wide range of plants and animals, all working in harmony together. But over the past 100 years, through intense industrialized agricultural practices like mono-cropping and clearing land for animal production, the land has been stripped of its natural barriers. What once was forest or prairies is now field after field of crops or cattle fields. And to keep continuous production high throughout the year, many farmers are using synthetic chemicals on the degraded soils. Made mainly of phospherous and nitrogen, these excessive fertilizers find their way into the water system in the form of runoff. These build up and become increasingly concentrated closer to the mouth of the river where it is met by the ocean.

Once these excess nutrients flow out into the ocean, combined with the warming spring temperatures, it leads to excess growth of phytoplankton, known as an algal bloom. This is where a much larger amount of phytoplankton feeds off the nutrients and forms a concentrated mass of algae in the water. In the summer months, the weather is calmer and ocean winds have dropped, meaning there is not much mixing of the ocean layers from the surface to the bottom. Once the phytoplankton dies, it sinks to the seafloor where microbes and bacteria breaks it down. This process, which uses up oxygen, forms an hypoxic layer about 1.5 metres high, where there is a significant depletion of oxygen and no life can live.

These areas can stay in this hypoxic state for weeks and even months, causing long term effects on the ocean. When autumn begins and wind pick up again, the ocean layers are mixed and the dead zone dissipates.

Where are ocean dead zones found?

Found all over the world, dead zones are most prominent where large rivers flow into the oceans. Chesapeake Bay was one of the first areas indentified as a dead zone in the 1970s. Urbanization and agriculture, particularly poultry farming, are two factors which contribute to the high dissolved nitrogen levels in that area. Dead zones in the Baltic Sea have grown to over 60,000 square kilometers in recent years due to wastewater treatment plants, sewage, large animal farms and overuse of fertilizers.

One of the most studied dead zones on the planet is found in the Gulf Of Mexico, where the Mississippi River flows into the ocean. The Mississippi River Basin covers a 41% of the continental US, stretching from New York in the East, to Minnesota in the North, to Montana in the West. Think of all the cities and farms within the Mississippi Basin. The majority of farm land within the Basin is used for three main products, corn-soybean rotation which is used for animal feed, animal farming of poultry, pigs and cattle, and ethanol for fuel. Since the 1950s, studies have seen nitrogen levels increase three fold.

The US Geological Survey reported 1.15 million tons of nitrogen pollution, enters the Gulf of Mexico, to put that in perspective, that’s almost double amount of oil that entered the ocean in BP oil spill.

Other increases in pollutants include unchecked and excessive usage of fertilizers, sewage, plastic pollution and animal manure pollution. All of these chemicals and pollution flows out to the Louisiana coast and forms the dead zone we find today. It covers over 8,000 square kilometres of ocean floor and is growing larger year on year.

How do dead zones impact the planet and people?

Ocean dead zones impact millions of people and animals around the globe. When there is a large dead zone, there is a loss of species, disruptions to food chains and a reduction in food availability.

Over 40% of US seafood is sourced from the Gulf of Mexico. These areas have a significantly reduced population of fish therefore fisheries may move elsewhere, leading to overfishing in surrounding habitats or even just stop fishing all together. Many jobs and business will be lost and not just in the fishing industry. Marine tourism will be heavily impacted if dead zones continue forming and changing the ocean life we visit for. In the Gulf of Mexico, recreational fishing contributed $10 million to the economy in 2009 alone. And this is true for many other countries around the world which have both have large fishing industries and booming tourism.

But the algal blooms don’t just affect oceans either. Pollutants and runoff can also feed into lakes which provide clean drinking water for millions of people as well as affecting fishing, recreation and tourism to those areas. Lake Erie in Cleaveland, Ohio now has annual harmful algal blooms which leave residents without clean drinking water to their homes. In Wisconsin, blue-green algae fills lakes during the summer. Harmful algal blooms are yearly occurrence in the Gulf of Maine.

How can we change it?

By reducing the land based pollution, we can stop dead zones from forming at all. By following marine scientists advice and adhering to laws put in place to limit the pollution found in waterways, dead zones can be reduced. The size of these areas can be controlled by limiting runoff and pollution from the land which all starts with political pressure, governmental change and a significant change in industrialized agriculture. The introduction of buffer zones, areas of deep rooted trees and plants, between fields and waterways could help reduce the volume of runoff and chemicals into streams and river. Establishing sustainable farming practices, such as organic farming, that work alongside nature, with mixed vegetation instead of mono-cultures would see a decrease in the use of synthetic fertilizers along with growing seasonally and locally. Dead zones are also driven by the continuous warming of sea surface temperatures as the ocean circulations rely on the balance between the warm surface water and the cool deep waters, so reducing individual carbon emissions and implementing governmental climate laws are both important parts of the solution.

Want to know more?

Read Dead Zones and Dead Zones, Explained by National Geographic.

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