The Tara Expeditions Foundation organizes scientific missions to study and understand the impact of climate change and the ecological crisis facing the world’s oceans. These scientific expeditions are led in collaboration with world-renowned scientific institutions and laboratories. Since 2003, ten expeditions have been led across the world. The foundation also strives to increase environmental awareness among the general public and young people, notably through several educational programs (http://oceans.taraexpeditions.org/m/education/). Further, the Tara Expeditions Foundation develops a long-term advocacy plan to mobilize society and encourage politicians to take action towards solutions that we all need for the wellbeing of the planet.
Since May 2016, the schooner Tara has been embarking on a new expedition entitled “Tara Pacific 2016-2018: the biodiversity of coral reefs in face of climate change.” After crossing the poles and all the planet’s oceans, the ship will travel the Pacific Ocean for more than two years with an interdisciplinary scientific team onboard, coordinated by the CNRS (French National Center for Scientific Research) and the Scientific Center of Monaco (CSM). Their objective is to examine in an unprecedented way the biodiversity of coral reefs and their development in face of climate change and anthropic pressures, which means human-caused.
Interactions that challenge scientists
On coral reefs, Scleractinian (“hard-rayed”) corals are the foundation of the ecosystem, creating a 3-dimensional structure that is home to thousands of animal species. However, on the reefs, you can also find coralline algae, crust-forming red algae which follow hard coral as the second-largest reef-building organism. Laetitia Hédouin explains that, “These algae form a limestone slab that acts as cement, welding all the elements of the reef together. However, the importance of coralline algae goes much further—some species seem to attract coral larvae, enabling their recruitment.” If adult colonies are attached to a benthic substrate, i.e. at the bottom of the water, the younger life stages of the coral pass through a bentho-pelagic phase during which the coral larvae are able to stay in the water column from a few days to several weeks until they find the appropriate substrate to settle in and metamorphose.
Lagoons, external slopes of atolls and biodiversity
The second research team is intrigued by the connection between the lagoon and the external slopes. A typical Polynesian atoll formation consists of a coral ring that encircles a lagoon linked to the open sea via a pass. Numerous species of fish take advantage of the lagoon in order to develop. They enter the lagoon during the larval stage and leave it once they become adults. The lagoon also contains many nutrients, unlike the outer ocean environment. The following question was asked in terms of atolls: is there a connection between the presence of a lagoon and the particular varieties, or fish biomass, of the external slopes of the reef? Since the lagoon is the calmest and most accessible location, it is also most impacted by humans. If human activity were to become too congested in the lagoon areas, one might question the effect this would have on external slopes. French Polynesia offers the unique opportunity to study open atolls (traditional type), closed atolls (a lagoon without a pass) and filled atolls (no lagoon) within the same territory.
The atolls of French Polynesia: a multitude of unique islands
These two scientific components of the Tara Pacific Expedition were completed in October and it is still too early to discuss the final results. However, researchers are able to determine some very interesting initial assessments over our atolls. For Laetitia Hédouin, the preliminary observations led to an appreciation of the variability between the different explored atolls. Even though they were not too far from each other, these atolls possess a variety of underwater landscapes in terms of coral cover and an abundance and diversity of coralline algae that is strikingly different for each of them. For the first time, a list of 25 species of coralline algae was able to be compiled during this scientific mission. As far as the Valeriano team, it was able to assess that the three atoll morphologies did not display major differences as far as diversity of species or quantity of fish present. However, significant differences appeared in regards to the composition of species. In general, the atolls without a lagoon have a weaker biomass of larger species in comparison to atolls with a lagoon; especially those with an open lagoon, such as Haraiki and Motutunga. Valeriano’s hypothesis is that communication between the external slope and the lagoon could enable a more productive food chain, resulting in the presence of larger fish. These two missions highlight the identity of each atoll. These rings of sand and coconut trees, which all seem to look alike from the air, show us just how unique each atoll is through these scientific studies.
What is unique about this expedition is that first of all, it entails a transversal approach across a very wide geographical area containing more than 40% of the coral reefs on the planet. This type of approach has never been carried out on such a large scale. According to Serge Planes, CNRS researcher and scientific director of the expedition, “Tara Pacific will explore each reef’s genomic, genetic, viral and bacterial biodiversity in order to compare it to the biodiversity of the water surrounding it. The goal is to get a real sense of the overall diversity of a coral colony.” This research will also bring new insights into the still unknown role that the biological, chemical and physical setting can have on the life of coral colonies and their capacity to adapt to changes in the environment.
According to recent studies, coral larvae do not affix themselves just anywhere on the substrate and they prefer certain areas were coralline algae are present. Maggy Nugues (EPHE/CRIOBE), a specialist in benthic algae and co-director of the mission, reminds us that, “One of the objectives of the Tara Pacific Expedition throughout the Tuamotu Islands is to study these specific connections between coralline algae and coral. This is the first time that such a study has been led on location in French Polynesia. The main idea is to study all the microorganisms associated with coralline algae and to get a general understanding of the process through which the algae enable the binding of coral larvae.”
The scientific team led by Laetitia Hédouin consisted of six scientists with complementary expertise (from bacteria on coralline algae to molecular biology and ecology). For 15 days, they explored six islands in the Tuamotu Archipelago: Faaite, Tahanea, Katiu, Makemo, Marutea and Hikueru. A first team of two divers assessed the abundance and diversity of coralline algae by identifying the species present along a 10 meter-long segment. A second team of three divers collected young coral (coral recruits smaller than 2 cm) in a pre-defined area of 50 cm² and collected five samples of the four most common species of coralline algae. The collected samples were identified on board Tara to determine both the coral genus and the coralline algae species. The interaction was noted—was the coral attached to the coralline algae or within proximity? Some samples were also stored and sent to Genoscope (https://www.genoscope.cns.fr/spip/), the sponsoring analysis laboratory for the Tara Pacific Expedition, in order to identify the microbiome, which refers to all microorganisms living in association with coralline algae and corals. From his end, the mission led by Valeriano assembled 6 researchers who took samples from eight atolls from three morphologies: three with filled lagoons – Akiaki, Nukutavake and Tikei; three with closed lagoons – Vahitahi, Vairaatea, Hiti; and two open lagoons — Haraiki and Motutunga. Every day, the scientists covered 4 transects* 50X5m to 10 meters deep on the external slope of each atoll. They had to identify and count all the species of fish they encountered from the bottom all the way to the surface.
* A transect is a narrow section or corridor where measurements are taken, usually consisting of a surface between 25 or 50 meters long and 2, 4 or 5 meters wide, used to count fish species or marine invertebrates. There is also the transect line which is part of the substrate where everything located under the line is described (for example: Acopora from 0 to 15 cm; Porites from 15 to 50 cm, etc.). This same transect can be transformed into a transect point where instead of continuously re-counting, one point every 50 cm or every meter is taken into account.
Cécile Berthe (Criobe), Laetitia Hédouin (Criobe), Valeriano Parravicini (Criobe) et Elodie Bernallin (Tara)