419 - When mangroves no longer protect the coastline
The mangroves of Guyana, in South America, are gradually disappearing. Contrary to the coastline of its near neighbour, French Guiana, which is still relatively protected, that of Guyana has been largely developed. In order to develop agriculture and aquaculture, earth dikes were built, destroying the greater part of the mangrove forest.
A study( 1) conducted by IRD researchers and the University of Aix-Marseille shows that the reduced protection provided by mangroves against the swell will lead to the large-scale erosion of 370 km of the country's coastline. Only one ecosystem restoration programme will help contain this phenomenon.
Good to know
Mangrove forests are specific to the tidal areas of tropical coastlines, comprised of mangroves that grow in the water (for further information, see Mangroves, a filter for heavy metals).
They cover three quarters of the tropical coastline, i.e. nearly 200,000 km².
The worldwide mangrove forest area has been reduced by 30% over 20 years, mainly due to the rising sea level.
Guyana , formerly British Guiana, is the third smallest and one of the poorest countries in South America. It is also the only English-speaking country in the continent.
The coastal plain, which represents 5% of the country's surface area, is home to more than 90% of its 770,000 inhabitants.
The mangrove forests in the Guyanas (French Guiana, Surinam and Guyana), which spread across the Orinoco and Amazon deltas, are among the most extensive in the world. This particular ecosystem, between the earth and the sea, plays a major role in protecting the particularly unstable muddy coastline( 2) against erosion. However, most of the Guyana mangroves have been destroyed to develop the coastal plain. The retreating mangrove wall will result in large-scale coastal erosion, threatening populations and their economic activities, as demonstrated in a study conducted by researchers from IRD and the University of Aix-Marseille.
Gaining ground on the sea
Although the French Guiana coastline remains protected by human developments for the time being, that of Guyana is already highly disrupted. The entire coastal strip is now inhabited*. In order to conquer this space and subsequently develop the aquaculture and agriculture - mainly rice cultivation - the coastal swamp areas have been transformed into polders( 3). To this end, dikes have been built, reducing the 1km mangrove strip to just a few dozen metres wide.
Less protective dikes
More than three quarters of Guyana's 450 km of coastline along the Atlantic are currently diked up. Coastal stability now depends on these earthen dikes.
However, these dikes do not provide the same level of protection as mangroves against the swell, which is the main cause of erosion. Moreover, they would not withstand the strength of the waves if the mangroves were to disappear completely. Yet, they prevent the sedimentation of mud coming from the Amazon, which enables forest regeneration.
Coastal stability in danger
Scientists have identified the main geomorphological processes at work across the entire Guyana coast.
Thus, they could assess the high risk of destabilisation of the coastline due to the reduction in mangroves. Rocky dikes are currently being built in order to protect economic activities, such as agriculture. However, such works are prohibitively expensive. The only means of protection consists of rebuilding the mangroves.
This assessment will enable the Guyana government to specify the measures for action that should be implemented to help the mangroves recolonise the coastline. French Guiana, of which the coastal area in turn suffers growing demographic pressure, must also draw lessons from these works so as not to encounter the same problems as its neighbour in the medium term.
- This assessment was carried out for and with the support of the Guyana government, in collaboration with MWH and funded by the Europaid programme.
- The 1,600km coastline between the mouths of two major rivers is comprised of mud from Amazon sediments.
- Polders are dried shore areas below sea level, with the water removed (usually artificially).
*Did you know?
Georgetown, the capital of Guyana, is located below sea level.
By Gaëlle Courcoux, DIC