The eddy: second cousin of the gyre


10 May 2010

This evening the curves of Reunion appeared! Massive mountain silhouettes in the dark night, and lights from houses at the water’s edge. Patience though! We still have to wait a little before we can set foot on the island and discover it in the full light of day.

We are currently heading, with the wind at our backs and the sails stretched taut, towards our second plankton sampling point since departing from Mauritius.

Colomban de Vargas, a specialist in planktonic ecosystems at the CNRS laboratory in Roscoff, has rejoined Tara for the third time as senior scientist. He has highlighted two sampling points which correspond to « eddies ». This English word refers to the small, transient whirlpools which form at the edge of large ocean currents. « Gyres », explains Colomban « are huge whirlpools. They are oceanographic structures stable enough to last either for years or a whole season, whereas eddies are much more volatile.”

A major ocean current runs along the East coast of Madagascar forming a retroflexion towards the south. A number of small whirlpools have been expelled from this flow of water at the point where it meets the Mascarene Plateau, which is where we are right now. According to our satellite maps there are about ten of them.

« Essentially, eddies are columns of water where the salinity, density, and chlorophyll level are different from the mass of water around them » explains Colomban. « We will be taking samples from the centre of two eddies this week. »

In an attempt to alleviate somewhat the workload of the scientific team (16 hours of non-stop sampling!), the shift has been broken up into two parts. We arrive at the sampling site the evening before and the scientists install the over-night nets in order to capture the nocturnal migration of zooplankton. They then go to bed for a short night’s sleep and resume operations the following morning: pumping bottles of water samples, collected at different depths and filtrations.

Mission accomplished for the first eddy, northeast of Reunion… with a surprising reading: the depth at which the concentration of chlorophyll reached its maximum was 120 metres! A record since the start of this expedition. At this locale there must be colonies of plankton which do not like the excessive light at these latitudes. The ocean is very clear and the sun’s rays can penetrate down to significant depths.

This evening we will try to locate the centre of another whirlpool, to the south-east of the island, where the surface chlorophyll level is higher.

A few more hours of courage, brave scientists, and we can regain our spirits and strength on the promised land of Reunion!

Sacha Bollet