Because they are transitional areas between the land and the sea, and between freshwater and saltwater environments, estuaries can be seriously impacted by any number of human, or anthropogenic, activities.
The greatest threat to estuaries is, by far, their large-scale conversion by draining, filling, damming, or dredging. These activities result in the immediate destruction and loss of estuarine habitats. Until the last few decades, many estuary habitats in North America were drained and converted into agricultural areas; others were filled to create shipping ports and expand urban areas. In the United States, 38 percent of the wetlands associated with coastal areas have been lost to these types of activities. In some areas, the estuarine habitat loss is as high as 60%.
Of the remaining estuaries around the world, many are seriously degraded by pollution. People have historically viewed estuaries and waterways as places to discard the unwanted by-products of civilization. Pollution is probably the most important threat to water quality in estuaries. Poor water quality affects most estuarine organisms, including commercially important fish and shellfish.
The pollutants that have the greatest impact on the health of estuaries include toxic substances like chemicals and heavy metals, nutrient pollution (or eutrophication), and pathogens such as bacteria or viruses.
Another, less widely discussed human-caused disturbance is the introduction of non-native or invasive species into estuarine environments.
Toxic substances are chemicals and metals that can cause serious illness or death. They may be poisonous, carcinogenic (cancer-causing), or harmful in other ways to living things. Pesticides, automobile fluids like antifreeze, oil or grease, and metals such as mercury or lead have all been found to pollute estuaries. These substances can enter an estuary through industrial discharges, yard runoff, streets, agricultural lands, and storm drains.
Once consumed by plants and animals, some toxic substances can accumulate in these organisms' tissues. This process is called biomagnification. The insecticide DDT, and the metal mercury, are known to progressively accumulate or build up in the tissues of organisms as they make their way from the bottom of the food web (algae, shrimp, oysters, fish) to the top (osprey, eagles, bears, people).
Sometimes, toxic substances become attached to sediments (sand or mud) that flow down rivers and get deposited in estuaries. Toxic substances that enter the estuary this way often contaminate bottom-dwelling animals like oysters or clams, making them a serious health risk to people who eat them.
Nitrates and phosphates are nutrients that plants need to grow. In small amounts they are beneficial to many ecosystems. In excessive amounts, however, nutrients cause a type of pollution called eutrophication. Eutrophication stimulates an explosive growth of algae (algal blooms) that depletes the water of oxygen when the the algae die and are eaten by bacteria. Estuarine waters may become hypoxic (oxygen poor) or anoxic (completely depleted of oxygen) from algal blooms. While hypoxia may cause animals in estuaries to become physically stressed, anoxic conditions can kill them.
Eutrophication may also trigger toxic algal blooms like red tides, brown tides, and the growth of Pfiesteria. Pfiesteria is a single-celled organism that can release very powerful toxins into the water, causing bleeding sores on fish, and even killing them. Although consuming fish affected by this toxin is not harmful to humans, exposure to waters where Pfisteria blooms occur can cause serious health problems.
Eutrophication is often devastating to animals and plants in estuaries as well as the economies of communities surrounding estuaries. Toxic algal blooms disrupt tourism due to foul odors and unsightly views, and poisoned fish and shellfish adversely affect recreational and commercial fisheries.
Nutrient pollution is the single largest pollution problem affecting coastal waters of the United States. Most excess nutrients come from discharges of sewage treatment plants and septic tanks, stormwater runoff from overfertilized lawns, golf courses, and agricultural fields. Over 60 percent of the coastal rivers and bays in the United States are moderately to severely affected by nutrient pollution.
Pathogens are disease-causing organisms. They include bacteria, viruses, and other parasites. Pathogens pose a major health threat to people who swim, fish, or boat in estuaries, as well as to filter-feeding animals, like oysters, mussels and clams. These animals concentrate the pathogens in their tissues, making them dangerous for humans to eat.
Pathogens can come from many sources, including sewage treatment plants, leaky septic systems, pet, livestock, or wildlife wastes, and combined sewage overflows (CSOs). CSOs are probably the largest contributor of bacteria and viruses in most estuaries. They carry the combined sewage from residential, industrial, and commercial wastes in the form of sewage solids, metals, oils, grease, and bacteria. During heavy rains, CSOs combine with storm water and overwhelm sewage treatment plants. The result is that untreated or partially treated waste flows directly into the estuary.
Contamination by pathogens can result in the temporary or permanent closure of beaches and shellfishing areas. In some cases, health officials may warn citizens that they should restrict the amount of fish and shellfish that they eat.
Invasive species, or invasives, are plants and animals that have found their way into areas outside their normal geographic range. In many cases, humans have transported them to their new homes. Invasive species have been called a type of biological pollution. Unlike pesticides or sewage, invasive species do not dissipate over time. With no natural enemies in their new habitat, invasive species often grow, reproduce, and spread quickly.
Non-native species are often introduced to estuaries in the ballast water of ships. When ships are empty, they take in water to help keep them balanced. When cargo is loaded onto the ships, they release the ballast water. In addition to water, aquatic organisms are sucked into the ships' ballast tanks. When ships take on water in one part of the world and release it in another, aquatic plants and animals are transported along with the water and introduced into foreign estuaries. The San Francisco Bay estuary is probably the most invaded estuary in the world. Over 230 non-native species now live there; so many, in fact, that they now dominate the ecosystem. Over 160 invasive species are now found in the Chesapeake Bay in Maryland, and their numbers are growing.
Invasives often cause ecological damage and economic losses where they are introduced. Competing with native species for food, or preying upon native species, invasive species have drastically reduced the populations of native species and have, in some cases, caused their extinction. Purple loosestrife, for example, was introduced to the United States from Europe as an ornamental plant in the early 1800s. Today it has invaded estuaries in 48 states, crowding out 44 species of native plants. Controlling purple loosestrife costs about $45 million a year.