Causes & Effects
Causes of Dead Zones
Oxygen is essential for life on land and in the water, and deoxygenation occurs when oxygen is used more quickly than it can be replenished. Widespread oxygen depletion is a human-caused issue that has been growing worse as the climate warms. Hypoxia is a term used to describe low-oxygen conditions, and anoxia describes the absence of oxygen.
Modern land use, industry and transportation practices directly contribute to both deoxygenation and the climate crisis. The predominant cause of deoxygenation is storm water runoff that is polluted with fertilizers - primarily from agriculture, sewage, and industry.
Polluted runoff contributes phosphorus and nitrogen to rivers, lakes, estuaries, and near-shore marine areas.
These nutrients are essential for plant life, but when water bodies are over-fertilized, it leads to explosive growth of algae – called algal blooms.
When the algal blooms die, the decomposition process consumes oxygen more quickly than it can be replenished by photosynthesis or mixing of the water column, creating a hypoxic area.
Climate change is exacerbating this issue - intense precipitation delivers more nutrient-rich pollution to water bodies, and warmer water increases the severity and extent of algal blooms.
The physical properties of water also play a role in deoxygenation. Warmer water holds less oxygen, and it is also is less dense than cold water. The warmer water floats on the surface, preventing mixing of the water column, depriving deeper waters of oxygen. This type of stratification is impacting parts of the open ocean as well as near-shore areas.
Effects of Dead Zones
The effects of hypoxia can be devastating for aquatic life:
Immobile animals like shellfish cannot survive.
Mobile animals may be able to swim away, but exposure to hypoxia been shown to cause developmental, growth and reproductive abnormalities in fish.
Exposure to low-oxygen may make animals more susceptible to other stressors.
Since estuaries and coastal areas are important breeding locations, vulnerable young fishes may not survive exposure to hypoxia, destabilizing fish stocks.
Reduced fish stocks have an impact on human and wildlife diets, as well as global and local economies.
Dead zones reduce habitat availability, forcing some animals to leave preferred locations and move to areas where they may be exposed to predators, or have fewer available prey. Hypoxia disrupts food webs and causes imbalances in ecosystems, leading to cascading impacts that can have repercussions far beyond the hypoxic area. For example, low oxygen conditions can lead to chemical reactions that release pollutants stored in sediments, mobilizing them into the food web. Anoxia can lead to the formation and release of nitrous oxide, a powerful greenhouse gas that contributes to climate change, creating a dangerous feedback loop.
Dead Zones in Study Areas
Baltic Sea
The Baltic Sea is a semi-enclosed, brackish water body that is surrounded by nine countries. There is limited circulation and mixing of water in the Baltic, so oxygen availability in bottom waters are naturally limited. For decades, the Baltic has received runoff polluted with nutrients from sewage treatment plants, agriculture and industry from surrounding countries. Today, despite progress in reducing nutrient inputs, the Baltic contains one of the world’s largest human-induced dead zones. One effect of hypoxia in the Baltic is its impact on codfish. Atlantic cod is one of the iconic fisheries in the Baltic, and populations have been declining due to a suite of threats, including over fishing and hypoxia.
Gulf of Mexico
The Gulf of Mexico is a semi-enclosed basin located on the southeastern coast of North America. Bounded by the US and Mexico, the Gulf receives runoff from the Mississippi/Atchafalaya River Basin, which drains approximately 41% of the contiguous US. Each summer, the largest hypoxic zone in the US is located in the northern Gulf of Mexico. Polluted runoff and the influx of freshwater from the Mississippi River contribute to the problem. The less dense freshwater remains above the denser saline seawater, and prevents mixing of oxygen-rich surface water with oxygen-poor bottom water. The Atlantic croaker has suffered reproductive effects, and loss of growth as a result of hypoxia exposure.
Lake Erie
Lake Erie is the warmest, shallowest and southernmost of the five Great Lakes in North America. Its watershed is also the most populated. Lake Erie is exposed to stresses from agriculture and urbanization, and receives the highest loads of phosphorus of all the Great Lakes. In addition to phosphorus pollution, invasive species such as the zebra and quagga mussels worsen the problem by consuming edible algae, allowing the toxic algae to flourish. In 2014, a harmful algal bloom forced the City of Toledo, Ohio to suspend the use of drinking water from Lake Erie. Hypoxia has a negative effect on prey fish populations, which could impact top predators like walleye and yellow perch, imperiling a world-class fishery.