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Humans are rapidly transforming carbon-storing estuaries into cities

Estuaries, the vital wetland ecosystems and carbon sinks that connect freshwater rivers to saline ocean waters, have been quietly disappearing over the past 35 years.

A new study reveals that worldwide, dams and land reclamation activities have converted a staggering 250,000 acres of estuary — an area roughly 17 times the size of Manhattan — to urban land or agricultural fields.

Importance of estuaries as carbon stores

The majority of this land conversion and estuary loss has occurred in rapidly developing countries, and the findings could help these nations avoid the problems faced by countries that have already lost or degraded their estuaries.

Estuaries are gateways connecting land and sea, providing habitat for wildlife, sequestering carbon, and serving as hubs for transport and shipping.

According to Guan-hong Lee, a geoscientist at Inha University in South Korea who led the study, “Estuary change is really interesting, especially in 20th century, because estuaries have been altered by humans by the construction of estuarine dams and land reclamation. When estuaries are modified by humans, the consequences for land loss are surprisingly huge.”

Price of estuary degradation

People have been molding estuaries to fit their needs for thousands of years, and now, some countries are paying the price.

Estuary degradation and loss can lower water quality, shrink and fragment critical habitats, and remove coastlines’ protection from storms.

Many developed countries, such as the Netherlands and Germany, have already modified or lost large areas of urban estuaries.

These countries could serve as a warning for developing nations, and acting soon to conserve estuaries is an opportunity to protect their environmental and economic benefits.

Measuring carbon footprint and estuary loss

The study, published in the AGU journal Earth’s Future, used Landsat remote sensing data from 1984 to 2019 to identify 2,396 estuaries around the world that were large enough to measure with satellite imagery (those with mouths wider than 90 meters, or 295 feet).

Nearly half (47%) of these large estuaries are in Asia, and the dataset includes estuaries on all major land masses except Antarctica and Greenland. The researchers also identified land-use changes, including land conversion and dam building.

The team then measured the change in estuarine surface area and compared those changes to where land reclamation and dam building had occurred.

For the studied estuaries, between 1984 and 2019, humans converted 1,027 square kilometers (397 square miles, or 250,000 acres) of estuary to urban or agricultural lands in a process called land reclamation.

Land reclamation, which can include drying land and adding sediment to build land, accounted for 20% of estuary loss. Globally, humans altered 44% of the estuaries with dams and/or land reclamation.

Economic development linked to estuary loss

To explore the relationship between estuary gain or loss and economic development, the researchers compared countries’ gross income per capita to land reclamation and estuary area.

They also analyzed historical maps of high-income countries to find evidence of earlier estuary alteration and included 8 case studies of low-, middle-, and high-income countries’ estuary loss.

Middle-income countries lost the most estuarine area during the study period, and almost 90% of all land reclamation (921 square kilometers, or 356 square miles) occurred there, too. “As a country is transitioning to middle-income, they tend to increase development,” Lee said.

High-income countries lost little estuary area over the study period. In most cases, that’s because estuary alteration occurred decades earlier when they were in developing, middle-income statuses.

In those countries today, the focus has moved from development to environmental conservation efforts — attempts to undo the environmental damage that estuarine development caused.

Opportunities for developing countries

The findings highlight the opportunities developing countries have to minimize the negative environmental and economic impacts of degraded estuaries while balancing their own economic and development needs.

Lee emphasized that acting soon to conserve estuaries is an opportunity for developing countries to protect their environmental and economic benefits.

Ecology and economics of Estuaries

Estuaries host an incredible diversity of life, serving as nurseries for countless species of fish, shellfish, and other marine organisms.

The mixing of freshwater and saltwater creates a nutrient-rich environment that supports complex food webs and provides shelter and feeding grounds for a wide array of wildlife, including migratory birds, mammals, and reptiles.

In addition to their ecological significance, estuaries contribute substantially to local and global economies. They serve as natural harbors and ports, facilitating trade and transportation.

Estuaries also support thriving fishing and shellfish industries, as well as recreational activities such as boating, fishing, and wildlife watching, which generate billions of dollars annually.

Preserving estuaries for future generations

Estuaries act as natural buffers against storms, floods, and erosion, protecting coastal communities and infrastructure.

The wetland vegetation and intricate root systems found in estuaries help absorb wave energy, stabilize shorelines, and filter pollutants from the water, maintaining the health and resilience of coastal ecosystems.

To ensure that estuaries continue to thrive and provide their invaluable services, it is essential that we prioritize their conservation and restoration.

This involves implementing sustainable land-use practices, reducing pollution, and promoting public awareness about the importance of these ecosystems.

By working together to protect and preserve estuaries, we can safeguard these vital links between land and sea for generations to come.

Estuaries, carbon, and Earth’s environmental future

In summary, the study by Guan-hong Lee and his team at Inha University in South Korea reveals the alarming extent of global estuary loss due to dams and land reclamation activities, particularly in rapidly developing countries.

The findings underscore the urgent need for developing nations to balance their economic growth with the protection of these vital ecosystems, learning from the experiences of developed countries that have already suffered the consequences of estuary degradation.

By taking proactive measures to conserve estuaries, developing countries can safeguard the environmental and economic benefits these wetlands provide, ensuring a more sustainable future for generations to come.

The full study was published in the journal Earth’s Future.


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