When Mangroves No Longer Protect the Coastline

Science Daily 1 Feb 2013;

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.

Notes :

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 1,600 km coastline between the mouths of two major rivers is composed of mud from Amazon sediments.
Polders are dried shore areas below sea level, with the water removed (usually artificially).

Journal Reference:

Anthony E. J., Gratiot Nicolas. Coastal engineering and large-scale mangrove destruction in Guyana, South America : averting an environmental catastrophe in the making. Ecological Engineering, 2012, 47, p. 268-273. ISSN 0925-8574 fdi:010057269