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Le bassin de Thau
A MINI SEA AND A LIVE OYSTER FACTORY
The Bassin de Thau is a world of its own, where visual and culinary delights are just waiting to greet you. Here’s a bit of useful information to help you plan your trip there, and to give you an idea of how this highly successful 'live oyster factory’ works.
It is a large round blue rock held in place by four green claws: the Mourre hills and Mont Saint Loup to the south-west, and the Massif de la Gardiole and Mont Saint Clair to the north-east. A mini sea twenty kilometre long by around four or five kilometre wide, separated from the Mediterranean by an eleven kilometre sand bar, the lido, which forms Sète beach.
It holds three hundred and forty million cubic metres of water, which is why it gets called a 'bassin' (basin). But it is often thought of as more of an ‘étang’ or small lake, even though its water is far from stagnant. Quite the opposite in fact. It is really a lagoon. Either way, it is the largest and deepest coastal body of water between Argelès and Le Grau du Roi. It has a surface area of 7,500 ha and an average depth of 4.50 metres (15 ft) while the other lakes in the area are rarely deeper than 3 metres (10 ft).
TWO CANALS, FOUR SEAS
The bassin is therefore totally safe for sailing - but just avoid the south-eastern bank along the lido. There the bottom rises sharply from 5 metres (16 ft) to just a few tens of centimetres. The boggy wetlands around there are called ‘gourds’ by the locals and they form important nature reserves.
You can sail down it, so people do. The Bassin de Thau is the saltwater junction between the freshwater Canal du Midi and the Rhône – Sète Canal, and therefore between the four European seas: the Atlantic, the Mediterranean, the North Sea and the Baltic Sea. Just look at the flags flown by the yachts that put their masts back up after crossing the continent on inland waterways – they’ve come here from the British Isles, and the coasts of Belgium, Holland, Germany and Scandinavia. Even landlocked Switzerland has representatives here.
This is a busy global crossroads, complete with summer sailors who sail their padded houseboats like bumper cars! But it’s a safe stretch of water. Even when the wind gets going here, the sea stays calm. Hazards are clearly signposted, and channels are marked. At night, though, be careful not to bump into the shellfish farming facilities which are not marked. A quarter of the lagoon’s surface area is devoted to feeding our culinary habit and the “tables” which they use to grow oysters… which might just find their way onto your own table!
NORTH SEA AND RED SEA
The Bassin de Thau is the cradle of shell-fish farming. The oyster is king here and Bouzigues is his palace. This area is the main supplier of oysters for the whole of France, and the reputation of “Bouzigues” stretches way beyond those borders, to the chagrin of those who won’t eat anything but Atlantic oysters.
What makes it so special? Oysters need salt water but fresher water helps them get a more refined flavour. Most of the Bassin’s water is seawater that is on its way down from the North Sea to the Red Sea, but you just have to try oysters in winter and then in summer to kno
w that its degree of saltiness varies a great deal from season to season, ranging from twenty-seven to forty grams of salt per litre.
In summer, the lagoon is more salty than the sea. With the sun, and in particular the dry mistral wind, one hundred and ten million cubic metres evaporate every year, leaving about 3.3 million tonnes of salt in the remaining water. That’s a lot – a third of the 10 or so million tonnes contained by the entirety of the water in the Bassin.
And a second phenomenon comes into play here as well - the saltier the water is, the “heavier” it becomes. The surface water ‘sinks’ to the bottom where the oysters grow, weighed down by the remaining salt.
And that’s why, in summer, the basin is more salty than the open sea (40g/l as opposed to 35g/l). But in winter, it is a lot less salty (27g/l as opposed to 35g/l) Why’s that?
A DROP IN THE OCEAN
It’s a simple principle that we all know well – a drop of rain in the ocean makes no difference. But in a limited area like the Bassin de Thau, things are different. The 640 litres per square metre (6,400 m3 per hectare) which fall on average each yea
r, mainly between autumn and spring, equals 48 million cubic metres mixing into the basin’s 7,500 hectares, i.e. half the amount that evaporates in summer.
And don’t forget the rain that falls in the lagoon’s surrounding ‘catchment area’. According to IFREMER calculations, the Bassin de Thau’s catchment area is 25,000 ha which we reckon must mean that it collects about 160 million cubic metres of rain. If, as the same organisation suggests, a third of this seeps into the water table or evaporates, 106 million cubic metres will still end up in the lagoon.
Add to that the ten or so million that come from ‘invisible’ underwater springs, like the Gouffre de la Bise located between Balaruc and Bouzigues, a 20°C spring 30 metres below the surface. On the other hand, the Canal du Midi and the Rhône-Sète Canal don’t really make any difference, because the water there barely moves.
So year in year out the Bassin receives half its total volume in ‘new’, fresh water (164 million m3), mainly between October and March, while it loses a third (110 million m3) by evaporation, mainly in the hot, windy summer. These ‘drops’ are significant enough to need to be continually monitored for agricultural or industrial pollution – the quality of the oyster’s favourite environment and our health depend on it.
THREE NARROW FUNNELS
As for the rate at which the water is renewed, the best sources (such as IFREMER, Thau Agglomeration, the ‘Écologistes de l’Euzière’ environmental group,) say that that takes anything from five months to…several years.
Fortunately, these statistical debates don’t really matter. What is the relationship between the bottom of the lagoon near Marseillan and the banks near La Pointe Courte where the current reaches two knots, and what is the link between the recent surface seawater layer, the mixed layer and the more stagnant layer on the bottom? They are continually swapping places and changing direction.
In fact, it is the tide that causes the largest water movements. In Sète, the tidal range is 40 cm on average. With each high tide, the Bassin de Thau fills up via its three narrow funnels - Le Grau de Pisse-Seaume, the Quilles channel and the Sète canals. Of course, over the six and a half hours before the tide turns, the Bassin does not have time to level out. Depending on whether the winds accelerate
or restrain this movement, the lagoon ‘tide’ can vary between one and five centimetres. Not much you reckon?
Back to the calculator: a 1 cm rise over 7,500 ha - that makes... 750,000 m3. A tidal rise of five centimetres gives 3,750,000 m3 which has to run off in six hours, most of which flows under the bridges of La Pointe Courte. At roughly 150 m3 per second – that’s a swimming pool’s worth every second! And that happens four times every 24 hours 50 minutes and 28 seconds (i.e. each lunar cycle).
As for whether this is ‘new’ water or basically the same water moving between the Bassin du Port de Sète and the Bassin des Eaux Blanches – well all we need to do to find that out is add some pigment to it…
Okay, class over, tidy your things away and go for a swim. But not in winter. The temperature here is half that of the open sea (on average 7°C). But in the summer, it can reach 28°C. You’d think you were in the Caribbean!
And make the most of it, because some experts reckon that despite efforts to preserve it, the lido will disappear with the rise in sea levels. The Bassin de Thau will become the Thau Gulf. But then, other experts say the opposite. They claim the lagoon is entering a new period of sedimentation, and is going to silt up…
