What is a dead zone?
Dead zones are low-oxygen areas of the world’s oceans or large lakes. These hypoxic areas have too little oxygen to supports marine life. Although the hypoxic zones are a natural phenomenon, many of the dead zones today are created or enhanced by human activities, and are increasing in shallow coastal and estuarine areas.
Dead zones created by nutrient pollution in bays, lakes, and coastal waters are the most common and problematic since they receive excess nutrients from upstream sources. Excess nitrogen and phosphorus lead to algae overgrowth in a short period of time. This overgrowth of algae consumes large amounts of oxygen and blocks sunlight from reaching underwater plants. Eventually, the algae will die, sink to the bottom of the water body, and the oxygen in the water is used for bacterial decomposition. Creating an oxygen sink and inhabitable conditions for aquatic life.
More harmful algae blooms can be wide-spanning and produce chemicals or toxins. Toxic blooms commonly occur in lakes, reservoirs, rivers, ponds, bays, and coastal waters. In most cases, cyanobacteria, also known as blue-green algae, is the cause of harmful algae blooms. The toxins produced by cyanobacteria can harm both human health and aquatic life.
How can dead zones form?
Eutrophication is the main driver of dead zone formation. As a result of human activities, nitrogen levels are almost doubled, and phosphorus levels are tripled compared to the natural values of these substances that flow into the environment. This has been attributed to the increased use of nitrogen and phosphorous fertilizers, nitrogen fixation by leguminous crops, and atmospheric deposition of oxidized nitrogen from the combustion of fossil fuels (Dybas, 2005). Additionally, some of the nutrient sources found in coastal waters are lawn fertilizers, agricultural manure, sewage output, and stormwater. The amount of nutrients found in nature was limited, however, as human activities have increased, nutrient pollution has lead to massive algal blooms and, ultimately, dead zones. Harmful algal blooms can lead to fish kills, contaminated drinking water, shellfish poisoning, and the death of marine mammals and shore birds.
Another factor contributing to the formation of dead zones is water column stratification due to the difference in water density. For instance, in the Gulf of Mexico, eutrophication initiates a massive phytoplankton growth on the water’s surface. The size of this plankton population surpasses the natural capacity of consumers to graze it down to a balanced level. After their relatively short lifespan, the plankton die and sink to the bottom waters, where bacteria decomposition occurs. During the summer months, the water column is stratified from the limited mixing from wind and wave energy. Additional environmental factors (i.e. temperature and salinity) create stratified layers of water from top to bottom. Freshwater flowing from rivers and the warmed surface water have low density which forms a layer above the cool, dense seawater near the bottom. This stratification leaves the bottom layer isolated from the regular resupply of oxygen from the atmosphere. Organisms capable of swimming have the ability to escape the dead zone, but sessile fauna experience stress or die (NOAA, n.d.).
Categorizing Eutrophic Systems: Where are dead zones?
Scientists have identified 415 dead zones worldwide. Over the years, there has been a staggering increase in the number of dead zones at a global level. In 1960 there were about 10 documented cases, and in less than 50 years, the number dramatically increased to 169 in 2007. A majority of the dead zones are located along the eastern coast of the United States and the coastlines of the Baltic States, Japan, and the Korean Peninsula.
Considering the dramatic increase in dead zones, scientists have prioritzed coastal systems experiencing any symptoms of eutrophication. Namely, a coastal system that exhibits the effects of eutrophication is considered an area of concern. These areas are at the most significant risk of developing hypoxia. There are 233 areas of concern along the western coast of Central and South America and the coastlines of Great Britain and Australia.
Despite the increasing amount of dead zones in our waters, there are systems in recovery from hypoxia. For example, the Black Sea is a system that once experienced yearly hypoxic events, but is now in a state of recovery and improvement. Similarly, Boston Harbor in the United States and the Mersey Estuary in the United Kingdom have improved water quality.
The Largest Dead Zone in the World
The number of dead zones and their size and exact location varies each year. The overall area of dead zones across the world is estimated to be at least 1,544,263 square miles, an area equal to the size of the European Union (Loyd-Smith & Immig, 2018). The largest dead zones are the Gulf of Oman – 63,700 square miles, the Baltic Sea – 27,027 square miles, and the Gulf of Mexico – 6,952 square miles (Carstensen & Conley, 2019; NOAA, 2019; Queste et al., 2018).
The Impact of Dead Zones
In addition to the environmental impact, dead zones have a negative effect on the economy. For fishermen who rely on the ocean to provide a livelihood, dead zones mean they have to travel greater distances from shores to find areas where fish congregate. This is impossible for small boats, and there is added cost for fuel and staff members. According to NOAA estimations, dead zones cost the U.S. seafood and tourism industries approximately $82 million annually.
What are the solutions?
Reducing nutrient pollution and keeping fertilizers on land and out of coastal water is the primary goal of lowering dead zones. And the best way to accomplish that is through cooperation at the international level. The ecosystems in the world’s oceans are fragile. Increasing hypoxia and dead zones, warming oceans, and rising acidification create multiple stressors to marine ecosystems.
Sources:
Carstensen, J. & Conley, D. J. (2019). Baltic Sea hypoxia takes many shapes and sizes. Bulletin of Limnology and Oceanography, 28(4), 125-129. Retrieved from doi:10.1002/lob.10350.
Dybas, C.L. (2005). Dead zones spreading in world oceans. BioScience, 55(7), 552–557. Retrieved from https://doi.org/10.1641/0006-3568(2005)055[0552:DZSIWO]2.0.CO;2
Lloyd-Smith, M. & Immig, J. (2018). Ocean pollutants guide: Toxic threats to human health and marine life. IPEN. Retrieved from https://ipen.org/sites/default/files/documents/ipen-ocean-pollutants-v2_1-en-web.pdf
NOAA. (2019). Large ‘Dead Zone’ measured in Gulf of Mexico. National Oceanic and Atmospheric Administration. Retrieved from https://www.noaa.gov/media-release/large-dead-zone-measured-in-gulf-of-mexico
NOAA. (2011). Congressional interest in harmful algae and dead zone bill prompts hearing. National Centers for Coastal Ocean Science. Retrieved from https://coastalscience.noaa.gov/news/cscor-provides-testimony-to-congress-in-support-of-harmful-algae-and-hypoxia-law/
NOAA. (n.d.). Operational Gulf of Mexico hypoxia monitoring. National Centers for Coastal Ocean Science. Retrieved from https://coastalscience.noaa.gov/project/operational-gulf-of-mexico-hypoxia-monitoring/
Queste, B. Y., et al. (2018). Physical controls on ocean distribution and denitrification potential in the north west Arabian Sea. Geophysical Research Letters, 45(9), 4143-4152. Retrieved from doi:10.1029/2017GL076666.