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The term “dead zone” or “hypoxia” refers to low-oxygen areas in the world’s lakes and oceans and is so called because very few organisms can survive in hypoxic conditions. Hypoxic zones can occur naturally, but human activities can also lead to the creation of new dead zones or the enhancement of existing ones. What are dead zones, how many are there in the world and how can they be prevented?

What is a Dead Zone?

A dead zone occurs as a result of eutrophication, which happens when a body of water is inundated with too many nutrients, such as phosphorus and nitrogen. At normal levels, an organism called cyanobacteria – or blue-green algae – feeds on these nutrients. With too many nutrients, however, cyanobacteria grow out of control, which can be harmful. 

When the algae die and sink to the bottom of the water bed, they provide a rich food source for bacteria, which when decomposing consume dissolved oxygen from surrounding waters, depleting the supply of marine life. If stratification of the water column (when water masses with different properties form layers that prevent water mixing) occurs, these waters will remain oxygen poor. 

Human activities mainly cause these excess nutrients to be washed into the ocean, which is why dead zones are often located near inhabited coastlines. 

Shallow waters are less likely to stratify than deep waters, and so are less likely to develop hypoxic conditions. This is because shallow waters tend to be well-mixed by winds and tides. Additionally, waters that are shallow and clear enough to allow light to reach the bottom can support primary producers such as phytoplankton, algae and seagrasses that release oxygen during photosynthesis.

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What Causes Eutrophication?

This process has increased because of the rise in intensive agricultural practices, industrial activities and population growth, which all emit large amounts of nitrogen and phosphorus that settle into our air, soil and water. Fossil fuels also release nitrogen into the atmosphere. 

In developed countries, heavy use of animal manure and commercial fertilisers are the main contributors to eutrophication, which runs off from fields into creeks and bays. In developing countries, untreated wastewater from sewage and industry are the main contributors, which is sometimes dumped into rivers, lakes or the ocean. 

Eutrophication’s Impact on the Environment

The eutrophication process has severe environmental impacts.

Phosphorus, nitrogen and other nutrients increase the productivity or fertility of marine ecosystems. Organisms such as phytoplankton, algae and seaweeds grow quickly and excessively on the water’s surface. This rapid development of algae and phytoplankton is called an algal bloom. Algal blooms can create dead zones beneath them, because they prevent light from penetrating the water’s surface. They also prevent oxygen from being absorbed by organisms beneath them. Sunlight is necessary for plants and organisms like phytoplankton and algae, which manufacture their own nutrients from sunlight, water and carbon dioxide.

Algal blooms are sometimes referred to as “red tides” or “brown tides,” depending on the colour of the algae. Cyanobacteria causes red tides. 

Algal blooms are also often cause of human illness. Shellfish, such as oysters, are filter feeders. As they filter water, they absorb microbes associated with algal blooms. Many of these microbes are toxic to people. Algal blooms can also lead to the death of marine mammals and shore birds that rely on the marine ecosystem for food. 

They can also impact aquaculture, or the farming of marine life. One red tide event wiped out 90% of the entire stock of Hong Kong’s fish farms in 1998, resulting in an estimated economic loss of USD$40 million.

Algal blooms usually die soon after they appear because the ecosystem cannot support the huge number of cyanobacteria. The organisms compete with one another for the remaining oxygen and nutrients.

Hypoxia events often follow algal blooms. 

Natural Dead Zones Around the World

Not all dead zones are caused by pollution. The largest dead zone in the world, the lower portion of the Black Sea, occurs naturally. Oxygenated water is found in the upper portion of the sea, where the Black Sea’s waters mix with the Mediterranean Sea that flows through the shallow Bosporus strait.

How Many Dead Zones Are There In the World?

The Chesapeake Bay in the US was one of the first dead zones to be identified in the 1970s. Even though there are a number of programs to improve its water quality and reduce pollution runoff, the bay still has a dead zone whose size varies with the season and weather. 

Scientists have identified 415 dead zones worldwide. Hypoxic areas increased from about 10 documented cases in 1960 to at least 169 in 2007. The majority of the world’s dead zones are along the eastern coast of the US, and the coastlines of the Baltic States, Japan and the Korean Peninsula. 

Notable examples include the Gulf of Mexico and the Baltic Sea. The Gulf of Mexico has a seasonal hypoxic zone that forms every year in late summer. Its size varies from smaller than 5,000  to 22,000 square kilometres. 

The Baltic Sea is home to seven of the world’s 10 largest marine dead zones. Increased runoff from agricultural fertilisers and sewage has exacerbated the eutrophication process. Overfishing of Baltic cod has intensified the problem. Cod eat sprats, a species that eats microscopic zooplankton, which in turn eat algae. Fewer cod and more sprats mean more algae and less oxygen. The spreading dead zones are starting to reach the cod’s deep-water breeding grounds, further endangering the species.

The Baltic Sea has become the first “macro-region” targeted by the EU to combat pollution, dead zones and overfishing. The EU is coordinating the Baltic Sea Strategy with eight EU member countries that border the Baltic Sea: Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland and Sweden.

There are also 233 areas of concern around the world, ie. areas that are at risk of becoming hypoxic. 

What Can Be Done to Prevent Dead Zones?

Dead zones are reversible if their causes are reduced or eliminated. For example, a dead zone in the Black Sea largely disappeared in the 1990s, following the fall of the Soviet Union, when the cost of chemical fertilisers skyrocketed. Further, efforts by countries along the Rhine River to reduce sewage and industrial emissions have reduced nitrogen levels in the North Sea’s dead zone by more than 35%. There are only 13 coastal systems in recovery around the world. 

Simply put, countries around the world must reduce industrial emissions and improve agricultural practices in areas where dead zones are a problem. 

To combat the issue of dead zones, policymakers could consider incentivising inland farmers to move away from the use of harmful chemicals. Conservation compliance programmes should be implemented, benefiting farmers who engage in healthy soil and water management practices, such as placing buffers or dams to protect streams adjacent to agricultural land, and scaling up the use of perennial plants that can survive for several years and minimise soil erosion. In exchange, farmers can be allowed discounts on services and lowered taxes. States could alternatively analyse smaller watersheds within the wider basin area that carries harmful chemicals, focusing policy on the most polluted rivers and streams. By understanding which individual bodies of water carry the highest concentrations of toxic runoff to the shore, regulators can be more fiscally and temporally efficient in enacting policy changes.

You might also like: The ‘Evil Twin’ of Global Warming: What is Ocean Acidification?

Featured image by: Seann McAuliffe

There are plenty of novel solutions to be had in tackling the climate crisis. One of them is kelp forests, which have been found to be a vital solution in the restoration and protection of the environment.

What are Kelp Forests?

Kelp is the name given to large brown algae seaweeds, which often accumulate into dense groups known as ‘kelp forests’. These extensive underwater habitats range along 25% of the world’s coastlines, and are one of the most productive and biodiverse ecosystems on Earth. 

Although kelp forests are often neglected and overlooked, they are vital tools of environment restoration, helping to combat climate breakdown by purifying water and providing shelter and food to various marine wildlife, including seahorses, cuttlefish, seals, crustaceans and dolphins. A study found that kelp forests in Southern California alone have helped to support over 200 ‘commercially important’ species of algae, invertebrates, fishes and marine mammals

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Kelp grow at an extraordinary rate, up to 2ft in a single day; seaweeds can grow more than 30 times faster than land-based plants. As a result, kelp forests sequester vast amounts of carbon as they grow. Research has found that per acre, kelp forests can absorb up to 20 times more carbon dioxide from the atmosphere when compared to land-based forests. Globally, these underwater kelp forests absorb roughly 600 million tonnes of carbon per year, which is almost twice the annual carbon emissions of the UK. As carbon is drawn into the kelp, they oxygenate the water, fostering marine environments that can more easily flourish, especially being faced with warming ocean temperatures. 

A kelp forest once stretched along 40km of the West Sussex coastline and 4km out to sea, all the way from Selsey to Brighton. In the 1980s, divers often recorded kelp as being ‘abundant’ or ‘common’ in over 50% of dive sites (from Selsey to Eastbourne.) Additionally, in 1987, a Worthing Borough Council report revealed that historic kelp beds covered 177km², with 10km² being described as ‘very dense.’ Unfortunately, within the last few years, these kelp forests have almost completely disappeared, with only a few pockets remaining. By the late 2010s, these forests now cover an area of just 6.28km², a 96.4% decrease since 1987. Reasons for this decline include changing fishing practices and the dumping of sediment close to shore which blocks light and limits the kelp’s ability to grow. 

As Sir David Attenborough states, “The loss of the Sussex kelp forests over the past 40 years is a tragedy. We’ve lost critical habitat that is key for nursery grounds, for water quality and for storing carbon.” 

Fortunately, a campaign to restore the kelp forest is in the works. The Sussex Inshore Fisheries and Conservation Authority (IFCA) has proposed a new bylaw restricting trawling along the Sussex coastline year-round. This decision was made after the Help Our Kelp campaign received overwhelming support. Featuring a documentary voiced by Attenborough, the film showcases Sussex’s kelp forest decline, and the importance of its conservation.

Led by Sussex Wildlife Trust, Blue Marine Foundation and the Marine Conservation Society, the aim of the bylaw is to give the kelp enough breathing space to recover. Since trawling vessels have repeatedly torn kelp from the sea bed, thus preventing its natural regeneration, regulating them is a vital first step towards the kelp re-wilding initiative. 

Dr Ian Hendy, Head of Science at Blue Marine Foundation says, “There is still a chance to bring back the kelp forests. By re-wilding the kelp forests back to their natural habitat, the oceans will come alive with a diverse abundance of marine wildlife, impacts of climate breakdown will be reduced and local fisheries will improve.” 

Sarah Ward, Living Seas Officer at Sussex Wildlife Trust spoke to The Telegraph about this subject, saying, “kelp forests can absorb and lock up carbon just as effectively as woodland, if not more so, and we’re able to create this habitat on a scale that simply couldn’t be replicated on land. This will be a huge step forward in addressing the escalating climate crisis.” 

The Sussex campaign has come during a rise in global awareness of the value of seaweeds. Findings have suggested the use of seaweed in animal feed could have the potential to decrease methane emissions from cattle. Studies have documented how they contain many essential nutrients, ranging from omega-3 fatty acids, amino acids, vitamins, minerals and bioactive compounds. However, challenges of incorporating seaweed wholescale into the general human diet remain.

With the bylaw being the first of its kind put into place to help cut greenhouse gas, this campaign is a major milestone in alleviating global warming. Charles Clover, executive director of the Blue Marine Foundation, voiced his support, “This is an initiative that tackles climate change and overfishing impacts all at once, the first of its kind in the UK. This is exactly what we need to be doing in marine habitats all over the world.” Hopefully, other countries can take note of Sussex’s conservation efforts and also follow suit. 

Featured image by: California Sea Grant

71% of Earth’s surface is covered in water, yet more is known about the surface of the moon than the entirety of the oceans. Reliance on the world’s oceans feeds into almost every aspect of human lives, including daily weather, food, the movement of goods and most importantly, the oxygen that humans breathe. Blooms of oceanic diatoms, small single-celled algae, produce up to 80% of the world’s oxygen, but are sensitive to changing ocean conditions. The depletion of oxygen from the ocean is called deoxygenation, but what exactly does this mean?

The health of our oceans has been steadily declining over the past century due to pollution, overfishing and a warming climate. An increase in global temperatures has led to ocean temperatures responding in concert. Global surface sea level temperatures have been increasing at an average rate of 0.13 °F per decade from 1901 to 2015. While this may not seem like a large change, the warming of ocean temperatures carries serious consequences for the planet. Warm water is unable to hold as much gas as cold water, resulting in lower oxygen levels in the ocean- deoxygenation- and reduced uptake of carbon dioxide from the atmosphere.  

You might also like: Hong Kong’s Shoreline Reclamation: An Interview with Prof. Cindy Lam, Department of Ocean Science at HKUST

Diatom bloom in Barents Sea, 2011. (Source: NASA Observatory)

Effects of Ocean Deoxygenation

At present, the ocean is a net sink for carbon dioxide, meaning it absorbs more carbon dioxide than it produces, helping to counterbalance atmospheric greenhouse gas levels. In fact, the Earth’s oceans have absorbed around one third of the carbon dioxide produced by humans since the beginning of the Industrial Revolution. Yet, as surface waters warm they become more buoyant, preventing them from mixing with cold water found deeper in the ocean. This impacts the amount of carbon dioxide the ocean is able to absorb from the atmosphere as the mixing of surface and deeper waters allows the newly-absorbed carbon to sink and be replaced by cooler bottom water, which can absorb more carbon dioxide. As this mixing slows, uptake of atmospheric carbon dioxide will decrease as the warm, gas-saturated surface water will be unable to continue absorbing carbon dioxide at the same rate, leading to higher concentrations in the atmosphere and increased oceanic warming.

In addition to warming ocean temperatures, nutrient pollution of coastal environments through fertiliser run-off, sewage, animal waste and nitrogen deposition from the burning of fossil fuels promotes excessive algal growth in ocean surface water. After the algae die and their cells sink, bacteria breaks down the cells for energy in a process that consumes oxygen and produces carbon dioxide, leading to a further reduction in water oxygen levels and an increase in carbon dioxide. This process, known as eutrophication, in combination with warming ocean temperatures, results in an overall decrease in oceanic oxygen levels and in some cases the production of hypoxic zones, or ‘Dead Zones’: areas where dissolved oxygen levels are too low to support life. These Dead Zones impact on food web structures, biodiversity and potential fish yields.  

Further, low-oxygen regions have the potential to release nitrous oxide and methane, two powerful greenhouse gasses, through their production in deoxygenated deep ocean waters that may then reach surface waters and be released to the atmosphere.  The Gulf of Oman, the Baltic Sea and the Gulf of Mexico are the three largest recurring Dead Zones in the world, with the Gulf of Oman totalling an area larger than that of Scotland, while another study has reported a growth of 55 000 km² of the Baltic Sea Dead Zone over the last century. 

A new report by the International Union for Conservation of Nature (IUCN) reports an overall 2% decline in the world’s ocean oxygen levels since the 1950s, with a further predicted fall of 3-4% by 2100. This ocean deoxygenation will have serious consequences for biodiversity, commercial fishing and tourism in coastal regions. Large marine animals that require considerable amounts of oxygen, or hypoxia-sensitive animals, such as tuna, swordfish, and most other fish we eat, will be forced to surface waters or other areas of the ocean in search of higher oxygen levels, which may lead either to their overfishing or a severe disruption to commercial fishing. In contrast, hypoxia-tolerant species, such as microbes, jelly fish and some squid, will likely dominate most other species, leading to unsafe water and beach qualities, consequently influencing tourism worldwide. 

Ocean Deoxygenation: Everyone’s Problem

Ocean warming and deoxygenation is a problem that will have repercussions for everyone on the planet. Actions need to be implemented now for the ocean to have a chance to recover. Carbon dioxide emissions must be urgently cut in order to mitigate ocean warming, as scientists predict recovery rates on the time scale of centuries under ‘business as usual’ emissions. Regulations to reduce nutrient run-off from both agriculture and sewage also need to be implemented to decrease the over-enrichment of coastal waters which leads to eutrophication.  

Featured image by: Peta Hopkins

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