Dead zone (ecology)

Dead zones are areas in the world's oceans and large lakes where there is very little oxygen. When the oxygen level drops too low, many sea creatures cannot survive. These areas often appear near coastlines where lots of plants and animals live.

Human activities, like farming and city waste, add extra nutrients into rivers that flow into the ocean. These nutrients cause lots of algae to grow. When the algae die, they sink to the bottom and use up even more oxygen as they break down. This leaves little oxygen for fish and other sea life.

Scientists have found many dead zones around the world. In 2004, there were 146 known dead zones, and by 2008, that number had grown to 405. Some dead zones are very small, but others can cover huge areas, like one that stretches across 70,000 square kilometers.

Causes

Dead zones in oceans and lakes happen when there isn't enough oxygen for sea creatures to survive. This usually starts when too many nutrients, like nitrogen and phosphorus, get into the water. These nutrients help tiny plants called algae grow very fast, creating what we call an algal bloom.

When these algae die, tiny creatures in the water break them down, using up oxygen in the process. This leaves less oxygen for fish and other sea animals, making it hard for them to live. Both natural events and human activities can cause dead zones. For example, weather patterns, ocean currents, and even climate change can play a role. Humans add to the problem by using fertilizers on farms, which then wash into rivers and lakes, carrying extra nutrients that feed the algae.

Types

Dead zones can be grouped by how long they last. Some stay for a long time, while others appear only for a short while.

• Permanent dead zones are deep areas that almost always have very low oxygen.
• Temporary dead zones last only for hours or days.
• Seasonal dead zones happen every year, usually in the warm summer and autumn months.
• Diel cycling hypoxia is a special kind that only has low oxygen at night.

The time it takes for a water area to get healthy again depends on how bad the oxygen problem is. Areas with very little oxygen and fewer plants and animals need more time to recover than those with just a small oxygen drop.

Effects

The most notable effects of eutrophication are plant blooms, sometimes harmful, loss of biodiversity and lack of oxygen, which can lead to the death of many water creatures.

Because dead zones have very little oxygen, most sea creatures try to leave the area. Fish and other moving animals usually swim away when oxygen levels get too low. Some smaller sea animals might also leave, but they can suffer greatly if the oxygen is very low. In very severe cases, tiny sea creatures can change in ways that harm the balance of life in the water.

Low oxygen levels make it hard for sea creatures to survive. Studies along the Gulf Coast of North America show that low oxygen slows down the growth and reproduction of fish and small sea animals. Creatures that can leave the area usually do so when oxygen gets very low. Those that stay behind may show signs of stress and can die. Some animals can live in low-oxygen areas and have special ways to cope, like using different ways to get energy.

Locations

In the 1970s, areas with low oxygen levels in the water, called dead zones, were first noticed in places like the Chesapeake Bay in the U.S., the Kattegat strait in Scandinavia, the Black Sea, and the northern Adriatic.

Dead zones have since appeared in coastal waters of South America, China, Japan, and New Zealand. A study in 2008 found 405 dead zones around the world.

Baltic Sea

The dead zone in the Baltic Sea has grown from about 5,000 square kilometers to over 60,000 square kilometers in recent years. Causes include the use of fertilizers, large animal farms, burning of fossil fuels, and waste from cities. The Baltic Sea is divided into nine areas, each reacting differently to these changes.

Virginia

Chesapeake Bay

The Chesapeake Bay was one of the first places where dead zones were identified in the 1970s. Efforts to reduce these zones have shown some success, but more work is needed.

Lake Erie

A dead zone exists in the central part of Lake Erie from July to October, growing as large as 10,000 square kilometers. This is caused by too much phosphorus from farms and cities, leading to algae growth and low oxygen levels. This affects fish and the water used for drinking.

Lower St. Lawrence Estuary

A dead zone is found in the Lower St. Lawrence River area, especially at depths over 275 meters, and has been noticed since the 1930s.

Oregon

A low-oxygen area covers the coasts of Oregon and Washington, reaching its largest size in 2006. Strong winds cause algae to grow, leading to low oxygen levels that affect sea creatures and fishing.

Gulf of Mexico 'dead zone'

The largest dead zone in the U.S. happens each summer off the coast of Louisiana in the Gulf of Mexico. The Mississippi River carries nutrients from farms and cities into the Gulf, causing algae to grow and oxygen levels to drop. The size of this dead zone changes each year.

Korea

Jinhae Bay

Jinhae Bay in Korea has seasonal low oxygen levels from early June to late September, caused by waste and natural water layers.

Shihwa Bay

Shihwa Bay, a reservoir created in 1994, has poor water quality due to waste from homes and factories. Efforts are being made to improve the water by allowing sea water to mix in.

Energy Independence and Security Act of 2007

The Energy Independence and Security Act of 2007 aimed to produce more renewable fuels, including a lot of ethanol made from corn. This would need much more corn to be grown.

Growing more corn means using more fertilizer, which can wash into rivers and lakes. This extra fertilizer can make parts of the ocean or lakes have less oxygen, harming fish and other sea creatures. Experts say this could make such low-oxygen areas even bigger.

Prediction and Use of AI for Dead Zones

Dead zones are tricky to predict because they depend on many factors like nutrient runoff, rainfall, water temperature, and oxygen levels. Traditional methods use field sampling and satellite observations, but these can miss fast changes or large areas. To improve predictions, scientists are using artificial intelligence (AI) and machine learning. These tools help spot areas at risk of low oxygen before problems start.

AI can look through big sets of environmental data to find patterns that lead to dead zones. It processes information such as nitrogen and phosphorus levels and rainfall to guess where and when low oxygen might happen. This is especially useful when monitoring data is missing or hard to get. Researchers at Louisiana State University have created AI tools to predict nutrient pollution in areas connected to the Gulf of Mexico. These tools estimate how much nutrients run off from rivers and farms, helping scientists see how extra nitrogen and phosphorus create low-oxygen areas. AI also helps study links between land use, rain, and nutrient movement that regular testing might miss.

In big freshwater systems like Lake Erie, machine learning is used to watch for harmful algae blooms and oxygen loss. AI sensors and remote monitoring collect real-time data to predict how strong algae blooms will be. Since algae blooms are linked to dead zones and nutrient pollution, this helps scientists act before low oxygen happens. AI could make environmental management better by giving quicker and more accurate predictions. This way, efforts can focus on the right places. But AI still needs good data and can struggle with incomplete records or changing climate conditions. Research in this area is continuing, offering hope for the future.

Reversal

The return of life to areas with low oxygen mainly depends on how long and how bad the low-oxygen conditions were. When the conditions are not too severe and the oxygen level drops only for a short time, sea animals and plants can come back quickly from nearby areas. But when the low-oxygen conditions last longer and are more severe, it takes much longer for life to return.

Warmer water that does not mix well makes it harder for these areas to recover. This will likely make it more difficult to fix dead zones in the future as the ocean continues to warm up.

Some smaller low-oxygen areas can recover well once the extra nutrients that caused the problem stop coming in. In some cases, even large dead zones can recover after about ten years. For example, the dead zone in the Black Sea, once the largest in the world, mostly disappeared between 1991 and 2001. This happened because fertilizers became too expensive to use after the Soviet Union broke apart and changes happened in Eastern and Central Europe. Fishing has become important there again.

The United Nations has encouraged efforts to clean up dead zones by cutting down big industrial emissions. For example, from 1985 to 2000, the dead zone in the North Sea saw a 37% drop in nitrogen because countries along the Rhine River worked to reduce sewage and industrial waste going into the water. Similar cleanup efforts have happened in the Hudson River and San Francisco Bay.

Biotechnology Application

Scientists are exploring ways to help dead zones recover using special methods called biotechnology. One idea is to use animals that naturally clean water, like oysters and clams, to help remove harmful nutrients from the water. These animals can live in areas with low oxygen and help improve water quality over time.

Another method involves creating special zones where tiny organisms can remove nitrogen from the water. These organisms change harmful nitrogen into a gas that disappears into the air. There are also ways to use natural processes to turn nutrients into solid minerals that sink to the bottom, making the water cleaner. While these methods show promise, more research is needed to see how well they work in real dead zones.

Modelling

Scientists use special math tools to study dead zones, which are areas in oceans and lakes with very little oxygen. These tools help leaders understand how different things, like rain and farming, can affect water. By putting in numbers about nutrients such as nitrogen and phosphorus, the models can guess how much algae might grow and how much oxygen might disappear in the water.