Threats to Estuaries

Population Growth in Coastal Watersheds

Coastal counties in the United States directly situated on the shoreline account for less than 10% of land (excluding Alaska). However, in 2010, 39% of the nation’s population resided in these coastal counties. This is an increase of 40% since 1970, and the number is only expected to increase in coming years. It has been difficult to control and manage the increased growth that has resulted from this population boom. Inadequate environmental safeguards have led to the elimination of millions of acres of habitat, including vital estuaries and wetlands.

Dredging, Draining, Bulldozing, and Paving

Thousands of acres of estuary habitat, including salt marshes, seagrass meadows, and mangroves, are altered or destroyed every year. Some of the activities that cause this destruction include dredging, draining, bulldozing, and paving.

Dredging. Courtesy of NOAA.

Dredging is the removal of sediment and other natural materials from the bottoms of bodies of water, such as lakes and rivers, in order to create open waterways for the passage of boats and ships. This process prevents the natural buildup of sediment in channels and harbors and causes sediment particles to flow into an estuary, making the waters murky and unhealthy. Other effects include a reduced amount of nutrients flowing from marshes, an alteration in tidal patterns, and the contamination of water, making the environment unsuitable for plant and animal life. Though dredging is essential to the process of waterway transportation, its effects are detrimental to coastal estuarine habitats.

While dredging to remove sediment can have negative impacts, accelerated sedimentation as a result of human-caused erosion is also an issue. Humans can increase the rate of erosion through practices like deforestation and agriculture. This can lead to an increased rate of sedimentation that smothers marine life and upsets the balance of estuaries.

Up until a few decades ago, estuaries were often drained and filled in to create new spaces for agriculture, shipping ports, or urban areas. The destruction of these estuaries caused a major loss to coastal environmental health.

Bulldozing and paving over estuarine waterways is proving to be one of the most destructive activities for these ecosystems. With our growing population, coastal areas have become a hotspot for residential areas. In order to accommodate these population increases, estuaries and waterways are being paved over, causing massive ecosystem damage.

Oil and Gas Drilling

The drilling for oil and gas, especially in the Gulf of Mexico, is becoming an increasingly concerning problem for our nation’s estuaries. The BP oil spill in 2010 was one of the most catastrophic human-caused disasters in history. According to the National Wildlife Federation, as many as 8,000 birds, sea turtles, mammals, and other species were found injured or dead in the first six months of the oil spill. Nearly a decade later, oil from this spill is still causing detrimental effects to the salt marshes of the Gulf Coast. While there has been some progress, the more heavily impacted salt marshes will still take many years to recover. Previous oil spills have even affected the invertebrates, like (example), of salt marshes for over 4 decades.

Nutrient Pollution

A close look at algae blooms. Courtesy of EPA.

Storm water runoff carries a multitude of contaminants, including phosphates and nitrates, from sewage, animal waste, and fertilizers into nearby streams. These contaminants and chemicals drain into the estuary, polluting bays and degrading habitats. In urban harbors especially, polluted runoff into the estuary creates “hot spots” of toxic contamination where nothing can survive.

Excess nutrients can lead to eutrophication, a phenomenon caused by excess nutrients in water systems resulting in massive algal blooms, or the growth of large amounts of algae that cover the water’s surface. As algae die, the process of decomposition depletes oxygen levels within the water. The water can become low in oxygen (hypoxic), or completely depleted of oxygen (anoxic). These hypoxic and anoxic areas are often known as “dead zones,” because organisms either die or are forced to leave the area. As of June 2017, the world’s biggest dead zone is in the Gulf of Mexico. Measuring roughly 8,776 square miles, this dead zone is approximately the size of New Jersey.

Eutrophication can pose additional problems, as well. The massive algae blooms can last for months at a time and block light from other organisms beneath the surface, making it even harder for living things to survive. Additionally, some algae blooms are toxic, posing a risk to both the organisms in the estuary and any humans who may consume shellfish or come into contact with the algae.

Industrial Pollution

Industrial pollution is another threat facing estuaries. Toxic substances, including chemicals and heavy metals, can enter estuaries through industrial discharges and stormwater runoff. Many of these substances are poisonous, carcinogenic (cancer-causing), or otherwise dangerous. As they are consumed by plants and animals, they accumulate in the tissues of living things. As the toxins travel up the food chain, they become more harmful for organisms at the top of the food chain, including humans. These toxins cause stress on the ecosystem, lowering its resilience and productivity.


Oysters. Courtesy of NOAA.

Overharvesting refers to catching a species, like fish, oysters or crabs, faster than they can naturally replenish. Removing any one species can alter the entire ecosystem and have devastating effects. For example, the oyster population of the Chesapeake Bay was nearly wiped out due to overharvesting. Because oysters serve as water filters within the Bay, many other organisms were also put at risk as harmful pollutants remained in the water.

Through the management of oyster harvests and the establishment of sanctuaries, the Chesapeake Bay’s oyster populations are on the road to recovery. This progress is vital for the health of the Bay, since NOAA has found that the areas with higher oyster densities have clearer and healthier water. Additionally, oyster reefs can bring millions of dollars into local economies through ecosystem services.

Reduced Freshwater Inflows

A freshwater inflow is the water that flows from streams and rivers into estuaries, mixing with the marine waters. Estuaries cannot function without these freshwater inflows, as they are needed to maintain their structure and function. However, human activity has resulted in the reduction of these much needed inflows.

Dams, which block the natural routes of streams and rivers, reduce the amount of freshwater that can reach an estuary. The construction of dams for hydroelectric power accounts for significant and ongoing loss of habitat in the watersheds of many of our nation’s estuaries. Freshwater inflow is also being affected as humans extract water upstream and from aquifers to satisfy society’s water needs.

The reduction of freshwater inflows reduces the amount of nutrients that reach the estuary, which some estuarine organisms rely on. Fewer fish returning to the estuary wreaks havoc among the many living organisms in the food web that depend on healthy populations of fish, thus also impacting the fishing industry that many people rely on for food and jobs. Decreased freshwater also causes the salinity of an estuary to increase, making life in the estuary unhealthy for some species.

Invasive Species

Brazilian Peppertree. Courtesy of the US Department of Agriculture.

Invasive species are plants and animals that have been introduced to habitats outside their native ecosystems. Many invasive species are introduced to estuaries through the ballast water of ships; this is the water ships hold to remain balanced. When this happens, organisms can be sucked into the tanks in one area, and redeposited in another, introducing non-native species to new locations.

Invasive species compete with and prey upon native species, reducing their populations and sometimes driving them to extinction. They can reshape entire ecosystems by destroying habitats and altering the relationships between predator and prey, resulting in environmental and economic loss. Invasive species are also known to spread quickly because they have no natural enemies in their new habitats. This means that the problem usually grows worse and worse!

Some examples of invasive species that are problematic for estuaries are purple loosestrife, oyster drills, Chinese mitten crabs, and Brazilian pepper trees.

Coastal Land Loss

Shifts in climate and human manipulation of stream channels cause the loss of tens of thousands of acres of estuary habitat every year. Annually, Louisiana estuaries lose about 25,000 acres to land loss and subsidence (actual sinking of the land into the water). This is also a problem in the Chesapeake Bay.

Climate Change

Information on the impact of climate change on estuaries can be found on our climate change page.

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