Wetlands & Estuaries
For centuries, people drained and filled wetlands, treating them as useless wasteland. The lower 48 US states have lost more than half of their original wetland area since the 1780s. We now understand that wetlands are among the most valuable ecosystems on Earth, providing flood control, water filtration, carbon storage, and habitat for an enormous number of species.
Key idea: wetlands are areas where the soil is saturated with water for part or all of the year. Estuaries are partially enclosed coastal water bodies where fresh river water mixes with salty ocean water.
What Are Wetlands?
A wetland is an area where the soil is saturated with water frequently enough to support water-tolerant vegetation. The water may be permanent or seasonal.
Freshwater wetlands: marshes (grasses and reeds, no trees), swamps (dominated by trees, like cypress swamps), bogs (acidic, fed by rainwater, accumulate peat), and fens (fed by groundwater, less acidic than bogs).Coastal wetlands: salt marshes (grasses in temperate tidal zones) and mangrove forests (salt-tolerant trees in tropical and subtropical tidal zones).Wetlands exist on every continent except Antarctica, from the Everglades to the prairie potholes to the Pantanal in South America.
Ecosystem Services
Flood control: wetlands act as natural sponges, absorbing excess water during storms and releasing it slowly. A single hectare of wetland can store 3,000–15,000 m³ of floodwater.Water filtration: wetland plants and soils trap sediment and absorb some excess nutrients from water passing through, improving water quality downstream, though they cannot filter all pollutants.Carbon storage: wetland soils, especially peat, store large amounts of carbon accumulated over thousands of years. Coastal wetlands (mangroves, salt marshes, seagrasses) store about 3–5 times more carbon per equivalent area than tropical forests, often called blue carbon.Habitat and biodiversity: wetlands support a disproportionately large share of species relative to their area. Many fish, amphibians, birds, and invertebrates depend on wetlands for breeding, feeding, or shelter.Because of these filtering functions, wetlands are sometimes called "nature's kidneys."
Estuaries
An estuary is a partially enclosed coastal body of water where fresh river water mixes with salty ocean water, creating brackish conditions, salinity between fresh and ocean water.
Estuaries are among the most biologically productive ecosystems on Earth. Nutrients from rivers combine with tidal mixing and sunlight to support dense communities of phytoplankton, shellfish, fish, and birds.
Nursery function: many commercially important fish and shellfish species spend their juvenile stages in estuaries, which provide food and shelter from open-ocean predators.
Salinity gradients: salinity varies within an estuary, lowest near the freshwater upper end and highest near the ocean mouth. It also changes with tides, seasons, and rainfall.
Threats & Protection
Draining and filling: historically the primary threat. Wetlands have been drained for agriculture, filled for construction, and channelized for navigation.Pollution: nutrient runoff causes eutrophication in wetlands and estuaries; chemical and sediment pollution degrades habitat quality.Sea level rise: coastal wetlands can be submerged if they cannot migrate landward fast enough. Where development blocks landward migration, coastal wetlands are squeezed between rising seas and hard infrastructure.Protection: the US Clean Water Act provides some wetland protections, and the Ramsar Convention (1971) designates internationally important wetlands. Restoration efforts focus on restoring hydrology, removing fill, and replanting native vegetation.Key Vocabulary
Wetland: Land where soil is saturated with water for part or all of the yearMarsh, swamp, bog: Types of freshwater wetlandsEstuary: A coastal water body where river and ocean water mixBrackish water: Water with salinity between fresh and ocean waterBlue carbon: Carbon stored by coastal wetland ecosystemsNursery function: Estuaries providing shelter for young fish and shellfish