Estuary

Did You Know?

• A more comprehensive definition of an estuary includes fjords, lagoons, river mouths, and tidal creeks.
• The effects of tides on estuaries can show nonlinear effects on water movement with important impacts on the ecosystem and waterflow.

Classification based on geomorphology

Drowned river valleys, also called coastal plain estuaries, form where rising sea level progressively penetrates river valleys, leaving the topography similar to that of a river valley. This type is most common in temperate climates and includes the Severn Estuary in the United Kingdom and the Ems Dollard along the Dutch-German border. These estuaries typically have a large width-to-depth ratio, appear wedge-shaped in cross-section in the inner part, and broaden and deepen seaward, with water depths rarely exceeding 30 m. Examples in the U.S. include the Hudson River, Chesapeake Bay, Delaware Bay, Galveston Bay, and Tampa Bay.

Bar-built estuaries occur where sediment deposition has kept pace with rising sea levels, creating shallow estuaries separated from the sea by sand spits or barrier islands. They are relatively common in tropical and subtropical locations and develop on gently sloping plains along tectonically stable continental edges and marginal sea coasts. These estuaries are extensive along the Atlantic and Gulf coasts of the U.S. where tidal ranges are less than 4 m. Barrier beaches enclosing bar-built estuaries form through several processes: building up of offshore bars by wave action, reworking of river sediment discharge by waves, currents, and wind, engulfment of mainland beach ridges due to sea level rise, and elongation of barrier spits from headland erosion by longshore currents.

Implications of eutrophication on estuaries

In temperate zones, nitrogen is often the primary cause of eutrophication in estuaries. During such events, biogeochemical feedback reduces available silica while increasing nitrogen and phosphorus supplies, creating conditions where harmful algal blooms can persist. The unbalanced nitrogen cycle may drive estuaries toward phosphorus limitation instead of nitrogen limitation, and an unbalanced phosphorus cycle interacts with nitrogen and silica availability to cause severe impacts.

Excess nutrients cause plants and algae to overgrow and eventually decompose, producing significant carbon dioxide and consuming nearly all available oxygen, creating hypoxic conditions and an unbalanced oxygen cycle. The excess carbon can lead to low pH levels and ocean acidification, which is particularly harmful to vulnerable coastal regions like estuaries. Eutrophication negatively affects salt marsh ecosystems, where cordgrass vegetation dominates.

Excess nutrients allow greater above-ground plant growth but less energy is allocated to roots, reducing below-ground biomass and destabilizing marsh banks, increasing erosion rates. In mangrove swamps, increased nitrogen causes increased shoot growth and decreased root growth, making mangrove trees less resilient during drought and potentially leading to their death.

Implications for marine life

Estuaries are highly dynamic systems where temperature, salinity, turbidity, depth, and flow change daily in response to tides, making them highly productive but challenging for year-round survival. Fish communities experience strong seasonal variation: hardy marine residents dominate in winter, while marine and anadromous fishes move in and out during summer to capitalize on high productivity. Estuaries provide critical habitat for species such as Pacific Herring (Clupea pallasii), which lay eggs in estuaries and bays; surfperch, which give birth in estuaries; juvenile flatfish and rockfish, which migrate to estuaries to rear; and anadromous salmonids and lampreys, which use estuaries as migration corridors.

Migratory bird populations, including the black-tailed godwit, also rely on estuaries. Two main challenges of estuarine life are variability in salinity and sedimentation. Many fish and invertebrates use osmoconformation or osmoregulation to control or conform to salt concentration shifts. Many animals burrow to avoid predation and find a more stable sedimentary environment, but large numbers of bacteria in the sediment create high oxygen demand, reducing oxygen levels and often causing partially anoxic conditions, which limited water flow can worsen.

Phytoplankton, primarily diatoms and dinoflagellates, are key primary producers whose productivity depends largely on water turbidity. Detritus from sedimentation is a primary food source for many estuarine organisms, including bacteria.

Human impact

Of the thirty-two largest cities in the world in the early 1990s, twenty-two were located on estuaries. Estuaries are under threat from pollution, overfishing, sewage, coastal settlement, and land clearance, and they are affected by events far upstream, concentrating pollutants and sediments. Land run-off and industrial, agricultural, and domestic waste enter rivers and are discharged into estuaries, introducing contaminants such as plastics, pesticides, furans, dioxins, phenols, and heavy metals that do not disintegrate rapidly in the marine environment.

These toxins accumulate in aquatic life tissues through bioaccumulation and in benthic environments like estuaries and bay muds, forming a geological record of human activities. The elemental composition of biofilm reflects areas impacted by human activities and may shift the ecosystem's basic composition from the bottom up. Estuaries are naturally eutrophic due to land runoff, but human activities add fertilizers, livestock waste, and human waste.

Excess oxygen-depleting chemicals can cause hypoxia and dead zones, reducing water quality and fish and other animal populations. Overfishing has occurred, as in Chesapeake Bay where the once-flourishing oyster population was almost wiped out. Oysters filter pollutants and deposit them harmlessly on the bottom; historically they filtered the estuary's entire water volume every three or four days, but today that process takes almost a year. Some major rivers through deserts had vast estuaries reduced by dams and diversions, such as the Colorado River Delta in Mexico, historically marshlands and forests but now essentially a salt flat.