As early as the 18th century, biologists like Darwin and Wallace discovered an astonishing fact: biodiversity on the planet does not follow a random distribution: biodiversity increases at the level of the tropics and decreases at the level of the poles.

To model this distribution of biodiversity, biologists propose the existence of a latitudinal gradient of biodiversity , the GLB.


When we look at organisms suspended in seawater (ie plankton), we discover that this gradient also exists on a large scale at ocean level. So what is the origin of this GLB?


Micro/nano-plastic marine pollution constitutes a major environmental challenge and reliable data continues to be scarce. During the navigation, some other samples of water will be collected to assess their plastic content. Plastic particles will be classified using the new Hartmann et al. method (link) that harmonizes the numerous existing classifications. The data will then be correlated to other relevant parameters (e.g. geographical location of the sampling point, meteorological data, ocean currents, etc.).

The recovered seawater is stored with a chemical agent, glutaraldehyde, which maintains the plankton cells in the state they were in when they were collected.

Another part of the sea water is filtered by three successive filters: the first, 200µm (0.2 mm) eliminates large organisms due to plankton.

The second and third, of 3 (0.003 mm) and 0.2 µm (0.0002 mm), make it possible to preserve fairly small organisms such as bacteria, archaea or eukaryotes.

We will not study plankton viruses, which are often less than 0.2µm in size.


Steps :

First, we will have to confirm that there is indeed a latitudinal gradient of biodiversity in the Atlantic Ocean. To do this, we will take samples of seawater and analyze the composition of micro-organisms and therefore the biodiversity. This work will be done as part of the expedition.

In a second step, it will be a question of finding a correlative variable between biodiversity and the biotic and abiotic environment. We will rely on analyzes of the following parameters: latitude, seawater temperature, pH, salinity, microbial abundance, quantity of chlorophyll a estimated by satellite, as well as the concentration of nitrate, nitrite, phosphate, silicate, and iron. This work will be done as part of an M2R internship.

Limit :

Our approach aims to identify a correlative variable between biodiversity and environment that could explain the existence of a latitudinal gradient of biodiversity. It is not certain that we can produce a model taking into account possible synergies between variables.

A certain number of measurements of the abiotic environment will not be possible from the boat. Thus, it is likely that we will have to use global seawater circulation models to determine the iron concentration in the waters studied, which is particularly difficult to measure, or satellite images to determine primary productivity.

Moreover, we will not study all the biotic and abiotic parameters of seawater but only those that are measurable at our scale.

Student in charge of the project : Baptiste ARNAUD

Supervisor 1 : Dr. Ingrid OBERNOSTERER, Microbial Oceanography Laboratory, Oceanographic Observatory of Banyuls sur Mer

Supervisor 2 : Dr. Christian JEANTHON, ECOMAP marine plankton ecology team, Roscoff biological station