8.4 Human Impacts on Wetlands and Mangroves

A wetland is an area where the soil is saturated or covered with water for at least part of the year, and where the plants are adapted to wet conditions. Marshes, swamps and bogs are all wetlands, and they can be freshwater or saltwater.

Mangroves are salt-tolerant trees that grow along tropical and subtropical coastlines, with tangled prop roots that arch down into the mud and water. Mangrove forests grow in places such as southern Florida, Southeast Asia, West Africa and northern Australia.

• Water filtration: slow-moving water lets sediment settle out, and plants and microbes absorb or break down excess nutrients and some pollutants before they reach rivers, lakes or the sea.
• Flood control: wetlands act like sponges, soaking up floodwater and releasing it slowly, which lowers flood peaks downstream.
• Groundwater recharge: water held in wetlands seeps down to refill aquifers.
• Storm protection: mangrove roots and coastal marshes absorb wave energy and storm surge, protecting the land behind them, and hold shorelines against erosion.
• Habitat and nurseries: many commercially important fish and shellfish spend their early lives in wetlands and mangroves, and migratory birds depend on them.
• Carbon storage: waterlogged soils decompose slowly, so wetlands and mangroves store large amounts of carbon.

Wetlands have often been seen as useless swamps. People drain and fill them for farmland, housing, roads and ports. Dams, levees and water diversions cut off the water that feeds them. Runoff from farms and cities brings excess nutrients (causing eutrophication), sediment and toxic chemicals, industrial waste adds more pollutants, and pumping too much groundwater can dry them out. Overfishing in and around wetlands and mangroves also strips out the fish and shellfish that keep their food webs in balance. The lower 48 US states lost roughly half of their original wetlands between colonial times and the 1980s.

Mangroves are cleared for coastal development, tourism and especially shrimp farms (topic 5.16). When they are gone, coasts erode faster, storm damage increases, and fisheries that relied on mangrove nurseries decline. Protecting and restoring wetlands, and laws that require developers to avoid or replace wetlands they destroy, help reduce these losses.

In the United States, filling or dredging most wetlands requires a permit under the Clean Water Act. Since 1989, federal policy has aimed for 'no net loss' of wetlands: developers who destroy a wetland usually must restore, create or protect wetland elsewhere, a practice called mitigation. Critics point out that newly built wetlands often don't function as well as natural ones for years.

Other tools include buying land to preserve it, restoring drained wetlands by removing drainage tiles or breaching old levees, replanting mangroves, and building constructed wetlands to treat runoff and wastewater.