How Wetlands Work: Ecosystems, Carbon Storage, and Biodiversity

What Are Wetlands?

Wetlands are ecosystems defined by the permanent or seasonal saturation of soils with water, creating conditions that support water-tolerant (hydrophytic) vegetation and distinctive wetland soils. They exist at the interface between terrestrial and aquatic systems and include marshes, swamps, bogs, fens, estuaries, floodplains, and mangroves. Though they cover only approximately 6% of Earth's land surface, wetlands perform a disproportionate share of the planet's ecological functions: they filter and purify freshwater, store more carbon per unit area than almost any other ecosystem, buffer coastlines and river valleys from flooding, and support roughly 40% of the world's species at some point in their life cycles. How wetlands work depends on the interplay of hydrology, soils, vegetation, and microbial activity that makes them fundamentally different from either dry land or open water.

Types of Wetlands

Water Filtration and Purification

One of the most ecologically and economically significant functions of wetlands is water purification. As water flows slowly through wetland vegetation and soils, several processes remove contaminants:

• Sedimentation: Reduced flow velocity causes suspended particles — including sediment, heavy metals, and pathogen-bound particles — to settle out of the water column
• Nutrient uptake: Wetland plants and algae absorb dissolved nitrogen and phosphorus from agricultural runoff, preventing downstream eutrophication of lakes and rivers
• Denitrification: Anaerobic bacteria in waterlogged soils convert nitrate (NO₃⁻) back into atmospheric nitrogen gas (N₂), permanently removing excess nitrogen from water
• Adsorption: Wetland soils, rich in organic matter and clay minerals, bind heavy metals such as arsenic, cadmium, and lead
• Pathogen removal: UV exposure, predation by microorganisms, and adsorption onto sediments reduce bacterial contamination

The U.S. Environmental Protection Agency estimates that constructed wetlands can remove 80–90% of nitrogen, 60–80% of phosphorus, and over 90% of suspended solids from wastewater. Many municipalities worldwide use constructed or restored wetlands as cost-effective tertiary water treatment systems.

Carbon Storage: Peatlands and Climate Regulation

Wetlands — particularly peatlands (bogs and fens) — are Earth's most carbon-dense terrestrial ecosystems. Peatlands cover approximately 3% of land but store an estimated 550 gigatons of carbon, roughly equivalent to 75 years of global fossil fuel emissions at current rates. This exceptional storage capacity arises from waterlogged, oxygen-depleted conditions that inhibit microbial decomposition. Dead plant material accumulates as peat at rates of 0.5–1 mm per year, and deep peat deposits can be up to 12,000 years old.

When peatlands are drained for agriculture, peat oxidizes rapidly and releases CO₂ and methane into the atmosphere. Drained peatlands, though covering only about 0.3% of Earth's land area, are responsible for approximately 5% of global anthropogenic CO₂ emissions annually.

Flood Regulation and Coastal Protection

Wetlands act as natural sponges and buffers in the hydrological cycle:

• Floodplain wetlands absorb and store peak river flows during storm events, reducing downstream flood peaks by 10–30% in studied watersheds
• Coastal marshes and mangroves dissipate wave energy, reducing storm surge damage; studies following Hurricane Katrina found that intact coastal marshes reduced surge height by approximately 8–9 cm per kilometer of marsh
• Wetland soils store water during wet seasons and release it slowly during dry periods, helping to maintain base flows in rivers during droughts
• The global economic value of wetland flood protection services has been estimated at over $1 trillion per year

Biodiversity

Wetlands support exceptional species richness. They provide breeding, feeding, nesting, and overwintering habitat for a vast range of organisms. Over 20,000 plant species are adapted to wetland conditions. Wetlands host more than 100,000 freshwater species, including fish, amphibians, reptiles, and invertebrates. Virtually all migratory waterbirds — including ducks, geese, herons, and shorebirds — depend on wetlands for staging areas and wintering grounds along their flyways.

Threats to Wetlands

Wetlands are among the most threatened ecosystems globally. An estimated 35% of the world's wetlands were lost between 1970 and 2015 — a rate three times faster than forest loss. Major drivers of wetland loss include:

• Drainage for agriculture: The single largest cause; floodplain and coastal wetlands are drained and converted to cropland or pasture
• Urban development: Coastal and riverine wetlands are filled for housing, industry, and infrastructure
• Hydrological alteration: Dams, levees, and water diversions interrupt natural flood-pulse dynamics that sustain floodplain wetlands
• Pollution: Excess nutrients from agriculture cause algal blooms and oxygen depletion; pesticides and heavy metals contaminate food chains
• Invasive species: Non-native plants such as Phragmites australis can form monocultures that displace native wetland communities
• Sea-level rise: Accelerating sea-level rise is causing coastal wetlands to drown where sediment accretion rates cannot keep pace

Conservation Frameworks

The Ramsar Convention on Wetlands, signed in 1971 in Ramsar, Iran, is the primary international treaty for wetland conservation. As of 2024, it designates over 2,400 Wetlands of International Importance (Ramsar Sites) covering more than 254 million hectares across 172 contracting parties. National legislation such as Section 404 of the U.S. Clean Water Act regulates the dredging and filling of wetlands, requiring permits and mitigation for unavoidable impacts. Wetland restoration — re-establishing hydrology in drained or degraded areas — has become a major conservation and climate strategy in Europe, North America, and Southeast Asia.