Most of the world's estuaries are younger than farming
When the last ice age ended and seas began climbing roughly 10,000 to 12,000 years ago, oceans flooded valleys carved by rivers and glaciers. Those drowned valleys became today's estuaries, brackish meeting places of river and sea that rank among the most productive and most pressured habitats on Earth.
An estuary is a partly enclosed stretch of coastal water where river water mixes with seawater. Ecologists call it an ecotone, a zone where different ecosystems overlap. The word comes from the Latin aestuarium, a tidal inlet, from aestus, tide. Broader definitions allow for rivers that stop flowing part of the year, mouths that close seasonally and tides that barely register. Scientists classify estuaries by salinity, landform, circulation and layering.
Shape tells their origin. Drowned river valleys, also called coastal plain estuaries, are the most common type in temperate regions; they are wide and shallow, rarely deeper than 30 metres, and include Chesapeake Bay and the Hudson. Bar-built estuaries form where sediment has kept up with rising seas, leaving shallow lagoons behind sand spits or barrier islands with only narrow inlets, and are common in the tropics and subtropics. Fjord estuaries occupy steep U-shaped valleys gouged by Pleistocene glaciers, shallowest at the mouth where moraines or rock sills restrict the flow. A few are tectonic, created where faulting, volcanoes or landslides dropped or cut off land.
Constant mixing creates shifting chemistry and remarkable productivity. Freshwater, marine and land species all interact here in dense food webs. Estuaries act as nurseries for many young fish and shellfish, sustain mangroves, seagrasses and salt marshes, and host migratory birds and endangered species. Their wetlands also filter sediment, nutrients and pollutants carried down from uplands, improving water clarity.
That position leaves them exposed. Many are degraded as land upstream loses soil and trees, livestock overgraze, fish stocks are stripped and marshes are filled in. Runoff rich in nitrogen, sewage and animal waste can trigger eutrophication, and pollutants such as heavy metals and hydrocarbons accumulate in them.
Source: Estuary
