A coast subject to influence

Though it is not part of the drainage basin of the Amazon, the Guiana Shield region and especially its coastal strip are still influenced by the king of rivers – the Amazon. Even before landing in Cayenne, with your nose stuck against the plane window you cannot help seeing that the rumour about French Guiana’s ‘brown water’ is indeed true. But where does all this suspended material in the water come from? Some of it comes from rivers in French Guiana, it is true, but not all of it. Morphological alterations and the hydro-sedimentary dynamics of the coastal zones are one of the characteristic features of the French Guiana coastline. These phenomena may be seen from the Amazon to the Orinoco and are the consequence of the Amazonian dispersion system.

The two key factors to take into account to understand this system are the geographical situation of the Guiana Shield coasts and of course their proximity to the mouth of the Amazon. The rotation of the planet creates a system of currents on either side of the Equator, flowing from Africa towards America. One of these currents, the South Equatorial current, gives rise to other currents when it reaches the Brazilian coast, including the current flowing north-west towards the Antilles along the coast of the Guianas.

A Guiana current laden with sediment 

The resulting Guiana Current transports up to 20% of the Amazon’s sediment load, and is responsible for the formation of mudflats which give the waters along the Guiana coasts their chocolate colour. It is estimated that 100 million tonnes of sediment are moved by this current each year and form mudflats, whilst nearly 150 million tonnes remain in suspension in the waters off the Guiana coasts. But the Amazon dispersion system is in fact a lot more complex than this. Two other phenomena need to be taken into account: the fact that the North Brazil Current swings back (giving rise to the Guiana Current), and the Intertropical Convergence Zone (ITCZ), the meteorological equator determining the changing of the seasons in French Guiana (cf. A Season in French Guiana no.1).

Nearly all the annual rainfall is between January and June. The ITCZ is in its southerly position and the north-east tradewind predominates. The Guiana Current is strong at this time of the year and flows along the coast in a relatively narrow band. It transports material discharged from the Amazon and creates a swell perpendicular to the coast with waves of over 2 metres, and it also pushes along the mudflats at a speed of about 1000 metres per year.

July to December is the dry season. The south-east tradewind keeps the ITCZ to the north of the equator. During this period the North Brazil Current may be seen to be swinging back on itself. More specifically the main current virtually doubles back on itself between 4° and 6° North. It then discharges about 50% of its material in suspension out in the Atlantic rather than on the Guiana coasts. The Guiana Current weakens and the swell shifts until it is easterly and almost parallel to the coast. The height of the waves, which depends upon the strength of the wind, is normally less than 2 metres.

From July to December the sky is blue, and the sea is almost blue too, but that does nothing to shift people’s preconceptions about French Guiana.

Migrating mudflats

From cap Orange onwards, on the Amapà coast in Brazil, vast mudflats are formed which then move right across the Guiana Shield at a rate of 1km per year. These mudflats can be up to 5 metres tall, 60 kilometres long, and 30 kilometres wide, in depths of up to 15 or so metres in places. They are separated by inter-flat zones of exactly the same size. But why is this mud not washed away by the waves? It would appear that the silt near the surface dampens and stabilises the swell In the unprotected zones between the mudflats they are subject to strong erosion. And so the coastline in French Guiana recedes cyclically.

When a mudflat is formed off the coast it is very difficult for living species to develop there. The mudflats are too soft and therefore offer poor support for ligneous species. What is more it is a salty environment and so toxic for non-marine plant species. When the tide rises the sea regularly covers this environment, and the rest of the time the sun beats down, drying out this sea of mud. What living species could actually want to live in so unwelcoming an environment?

A forest between the land and the sea

Yet there is one plant formation which is adapted to this hostile environment, mangrove forests.

The dominant species in mangrove forests is the mangrove tree. Whilst there are 44 different species in the Indo-Pacific zone there are only 5 species of mangrove tree found in French Guiana. Mangrove seeds from the neighbouring mangrove forests are deposited on the bare mudflats. They rapidly form little white mangrove plants (Laguncularia racemosa), sometimes found with a grass (Spartina brasiliensis). As you move further away from the sea the mangrove plants get taller and another species appears in their shelter, the black mangrove (Avicennia nitida). The black mangroves are soon taller than the white ones (which end up disappearing completely) and grow rapidly (an average of 3 metres per year).

Estuary mangrove forests, which are not subject to mudflat movement, are richer environments and are dominated by the red mangrove (Rhizophora mangle). As you carry on moving away from the sea other species commonly found in marshy forests appear, such as moutouchi (Pterocarpus officinalis), manil (Symphonia globulifera), acai palm (Euterpe oleracea), Guiana chestnut (Pachira aquatica), and so on.

But how do the mangrove trees manage to survive and develop in so difficult an environment that is half-sea, half-land? The first difficulty is in remaining upright on soft ground. Confronted with this problem the black mangrove has developed runner or ‘cable’ roots that can be several metres long. These are combined with little vertical roots, called anchor roots, which fix the tree in the soil. As for the red mangrove, it has a large number of arch roots or ‘stilt’ roots increasing the number of points of contact with the soil. These roots have the particularity of growing from branches as the tree grows.

A second major difficulty is the saltiness of the water in which the mangrove trees grow. They have thus developed filter systems. Their roots have salt glands which reduce the intake of sodium irons. This ingenious system is supplemented by a second one on black mangroves, which have glands beneath the leaf epidermis through which they secrete salt, resulting in the presence of clearly visible little salt crystals on their leaves.

Mapping the Amazonian coastlines with PROCLAM

Sedimentary discharge from the Amazon and the strong coastal dynamics it generates along the coasts of the Guiana Shield mean that the coastal ecosystems are highly unstable. What is more, the ever greater level of human pressure on the coast constitutes a further threat to the outstanding biodiversity* of these particularly sensitive environments.

In this context the PROCLAM project (Programme to Map the Amazonian Coastlines, or Programme de Cartographie des Littoraux Amazoniens) is seeking to assess the state of coastal environments by building up a continuous map running across borders from Saint-Laurent du Maroni as far as Sao Luis du Maranhao in Brazil, over 1500 kilometres of coastline in all.

This Franco-Brazilian cooperation project, mainly funded by the European Union, is being led by the IRD in Cayenne in cooperation with research laboratories in Brazilian universities in the States of Amapà (IEPA) and Parà (UFPA, MPEG). The aim of this joint project is not only to share the experience, knowledge, and skills each of the partners has in this field, but also to monitor ongoing changes on either side of the border within one common framework.

The originality of this project resides in its use of a large set of SPOT 5 satellite images for the entire study area, with a precision of ten metres on the ground. These are received by the SEAS-Guyane reception station. The decision to use satellite images is partly due to the scale of the study area to be mapped, as well as to the fact that certain coastal regions are inaccessible, especially in the State of Amapà. What is more, using satellite images means that rapid, regular updates may be provided for the entire map, given the speed of geomorphological change and alterations to the landscapes in the Amazonian coastal regions. Specific objectives are to observe and measure the movement of the mudflats, the evolution of the mangrove forests, and the receding of the coastline. Field missions are of course essential so as to validate the observations, but the aim is to arrive at a common method for monitoring the Amazonian coastal environment, primarily using satellite data.

The PROCLAM project will provide the basis for building up a cross-border observatory for the Amazonian coastal environment, with the aim of providing better understanding of coastal dynamics.

Better knowledge should make it possible to encourage better management of the territories, with an eye not only to preserving the natural environment but also to protecting societies from risks such as erosion.

Modifications over greater time scales

In addition to these relatively rapid changes, the coast of French Guiana has undergone major variations through the ages. In the Quaternary period 20,000 years ago the sea level was about 100 metres lower. Sediment from the Amazon followed a different course then. The waters of the Guiana coasts were clear and even suitable for the development of coral formations, traces of which may now be found along the edge of the continental plateau about 100 kilometres out to sea, and at a depth of about 100 metres.

In comparison to this we are living in a period when sea levels are relatively high. At a time when rising sea levels around the planet would appear to be inevitable due to global warming in particular, any lessons which may be learnt from observation of these changes are very precious, and can help anticipate the inevitably major consequences climate change will have on the Guiana coasts.