Which of the following would most likely cause an increase in these contributors to dead zones?

Dead zones are areas in bodies of water like the Chesapeake Bay that have little to no oxygen. Fish, crabs, oysters and other aquatic life literally suffocate in these zones.

What Causes Dead Zones in the Chesapeake Bay?

Dead zones are caused by excessive nitrogen and phosphorous pollution from human activities, including:

• Agricultural runoff from farmland that carries nutrients from fertilizers and animal manure into rivers and streams, eventually flowing into the Chesapeake Bay.
• Urban/suburban runoff from developed areas that washes nutrients from fertilizers, septic systems, and other pollutants into local waterways that flow downstream into the Chesapeake Bay.
• Wastewater treatment plants that release treated water—often still containing large amounts of nutrients—into streams and rivers across the watershed that flow into the Chesapeake Bay.
• Air pollution from our cars, factories, gas-powered tools, and power plants that contribute nearly 30 percent of the total nitrogen pollution to the Bay's waterways.

When there are excessive amounts of nitrogen and phosphorus in the water, algae can bloom to harmful levels. Dead zones form when the algae die, sink to the bottom, and are decomposed by bacteria—a process that strips dissolved oxygen from the surrounding water. Dense algal blooms also block sunlight, which prevents underwater grasses from growing. In turn, the animals that depend on these grasses for food and shelter suffer, as well.

Once a dead zone forms, other factors can influence its size and duration. For example, wind can mix oxygen from the surface into deeper water and help break up dead zones. Hot temperatures can make dead zones worse by warming a layer of surface water that locks colder, denser water below where oxygen from the surface can’t mix in. Heavy rainfall increases the amount of pollution washed into waterways.

But while weather can play a role, the best way to reduce algal blooms and dead zones is to stop pollution at its source by implementing the Chesapeake Clean Water Blueprint.

Where Are the Dead Zones in the Chesapeake Bay? 

Dead zones form in both the Bay’s mainstem and its tidal rivers, typically in deeper water near the bottom. The location, extent, and severity of the dead zones can change throughout the year, usually peaking during the summer months. There can also be significant variation from year to year due to changes in weather. For example, a rainy spring can wash more pollution into rivers like the Susquehanna, the Bay’s largest tributary, leading to a bigger dead zone that year.

On average, the Chesapeake Bay dead zone covers between 1.2 cubic miles during the summer months, when the water is warmest and oxygen levels are historically lowest. Long-term trends indicate the Bay's dead zones are getting smaller.

The Virginia Institute of Marine Science (VIMS) announced in its annual Dead Zone Report Card that the 2022 Chesapeake Bay dead zone covered an average of 0.65 cubic miles during the summer, making it the 10th smallest recorded dead zone in the survey's 38-year history.

How Do Dead Zones Affect People Living Around the Chesapeake Bay?

Dead zones are not a direct threat to humans, but they are extremely harmful to fish, crabs, oysters, and other aquatic animals that humans rely on for seafood and livelihoods. For example, researchers at VIMS have hypothesized that the Bay’s dead zone contributes to stress and disease in striped bass, loss of animals from the bottom of the Bay’s food chain, and a reduction in nutrition for predators, as bottom dwellers’ growth is stunted by lack of oxygen.  

In addition, the algal blooms that fuel dead zones can be detrimental to tourism and recreation. Harmful algal blooms can make water unsafe for swimming by generating toxins, release unpleasant odors, and cause fish kills that can wash hundreds of dead fish onto beaches.

It’s unhealthy for the Bay, it’s harmful to local economies, and it’s fixable.

The best way we can reduce algal blooms and dead zones is to implement the Blueprint by putting in place best management practices that control pollution from urban and agricultural lands, as well as wastewater treatment plants and septic systems. This includes things like planting trees as buffers along rivers and streams, improving soil health on farms, reducing the amount of hard surfaces in cities, and upgrading wastewater treatment plant technology.

Together, and only together, can we prevent dead zones and ensure vibrant habitats for aquatic life in the Chesapeake Bay.

What YOU Can Do to Prevent Dead Zones

Restoring the Bay and preventing future dead zones go hand in hand. Here are actions you can take to fix the dead zones in the Chesapeake Bay:

The Gulf of Mexico dead zone is an area of hypoxic (link to USGS definition) (less than 2 ppm dissolved oxygen) waters at the mouth of the Mississippi River. Its area varies in size, but can cover up to 6,000-7,000 square miles. The zone occurs between the inner and mid-continental shelf in the northern Gulf of Mexico, beginning at the Mississippi River delta and extending westward to the upper Texas coast.

The largest dead zone ever recorded in the Gulf of Mexico was reported by National Geographic News in August 2017: New Jersey-Size Dead Zone Is Largest Ever in Gulf of Mexico

The Gulf of Mexico dead zone varies in size annually, but may extend from the Louisiana/Alabama coast to the westernmost Texas coast. Photo courtesy of NOAA.

Where Are the Dead Zones?

Dead zones can be found worldwide(link to NASA dead zone page). The Gulf of Mexico dead zone is one of the largest in the world. Marine dead zones can be found in the Baltic Sea, Black Sea, off the coast of Oregon, and in the Chesapeake Bay. Dead zones may also be found in lakes, such as Lake Erie.

What Causes the Dead Zone?

The dead zone is caused by nutrient enrichment from the Mississippi River, particularly nitrogen and phosphorous. Watersheds within the Mississippi River Basin drain much of the United States, from Montana to Pennsylvania and extending southward along the Mississippi River. Most of the nitrogen input comes from major farming states in the Mississippi River Valley, including Minnesota, Iowa, Illinois, Wisconsin, Missouri, Tennessee, Arkansas, Mississippi, and Louisiana. Nitrogen and phosphorous enter the river through upstream runoff of fertilizers, soil erosion, animal wastes, and sewage. In a natural system, these nutrients aren't significant factors in algae growth because they are depleted in the soil by plants. However, with anthropogenically increased nitrogen and phosphorus input, algae growth is no longer limited. Consequently, algal blooms develop, the food chain is altered, and dissolved oxygen in the area is depleted. The size of the dead zone fluctuates seasonally, as it is exacerbated by farming practices. It is also affected by weather events such as flooding and hurricanes.

What Are the Effects?

Nutrient overloading and algal blooms lead to eutrophication (link to USGS definition), which has been shown to reduce benthic (link to definition) biomass and biodiversity. Hypoxic water supports fewer organisms and has been linked to massive fish kills in the Black Sea and Gulf of Mexico.

The Gulf of Mexico is a major source area for the seafood industry. The Gulf supplies 72% of U.S. harvested shrimp, 66% of harvested oysters, and 16% of commercial fish (Potash and Phosphate Institutes of the U.S. and Canada, 1999). Consequently, if the hypoxic zone continues or worsens, fishermen and coastal state economies will be greatly impacted.

When production increases in an ecosystem, organic matter, such as algal cells and fecal pellets, increases. This situation can lead to hypoxia when decaying bottom organic matter depletes oxygen and water stratification blocks oxygen replenishment. Upwelling oxygen-rich water or destruction of the stratification can alleviate this problem. Diagram courtesy of the U.S. Environmental Protection Agency.

What Can be Done to Remediate the Problem?

The key to minimizing the Gulf dead zone is to address it at the source. Solutions include:

• Using fewer fertilizers and adjusting the timing of fertilizer applications to limit runoff of excess nutrients from farmland
• Control of animal wastes so that they are not allowed to enter into waterways
• Monitoring of septic systems and sewage treatment facilities to reduce discharge of nutrients to surface water and groundwater
• Careful industrial practices such as limiting the discharge of nutrients, organic matter, and chemicals from manufacturing facilities

The government is also funding efforts to restore wetlands along the Gulf coast to naturally filter the water before it enters the Gulf.

Which statement explains the most likely impact Caulerpa has had on the biodiversity in the Mediterranean Sea?

What does the data suggest about the effect the oil spill had on pink salmon?

Which statement describes the most likely impact if species A is removed from the ecosystem?

How much of the energy was provided to secondary consumers?