Settlement, maintenance and monitoring of Biorock reefs in Bora Bora (French Polynesia): Focus on the comparison of healing and growth of coral colonies from diverse species and origins

Supervisors: Thomas Goreau, Denis Schneider (Global Coral Reef Alliance)
Over recent decades, the decline of marine ecosystems has become increasingly obvious. The major causes of such deterioration are anthropogenic activities, as a whole. Pollution, climate change, physicochemical modification of seawater, negligence, all of these factors induce a dramatic decrease of the ecological populations in the ocean. In particular, coral reef ecosystems, which are particularly vulnerable to all of these human-induced impacts, are very negatively affected.Nowadays, conservation measures are being developed in order to address this problem. Indeed, Marine Protected Areas are spreading all around the world to conserve the healthy remaining ecosystems and prevent their destruction. However, these measures are not enough as the extent of the damages is already considerable; the restoration of coral reefs already destroyed is needed. The creation of artificial reefs for coral reef restoration marked the beginning of a new era in terms of coral research. The last generation of artificial reefs, so-called Biorock, appeared in 1975 and constitutes a revolution in terms of reef regeneration. Indeed, such structures made of metals, cause the electrodeposition of useful minerals, helping corals to regenerate in an efficient and sustainable way.Based at the Hilton resort, on Bora Bora island (French Polynesia), an experiment has been settled on the concept of Biorock and its effects upon a variety of coral species originating from different families. The project is based on two structures, one under electrolysis and a control one, on which have been settled eight colonies of different coral species, chosen according to their morphotype and their abundance on the barrier reef surrounding the island, with the principal aim of being as careful as possible with any colony. The main objective of this study is to determine the efficiency of Biorock constructions in terms of survival and growth of corals, in comparison with the control structure which has been settled in similar conditions. For this, observations, photograph comparisons and direct coral growth measurements have been regularly done on a total period length of four months. This study is the first of the kind in French Polynesia, region which has been particularly affected by the decline of its coral reefs over the last 20 years. It gathers the first dataset obtained with the Biorock technology for certain coral species such as Pavona cactus and Porites synaraea. The other selected species cover the genera Acropora, Astreopora, Montipora et Pocillopora. The results obtained concerning the coral survival prove the efficiency of the Biorock technology for the whole selected species. In terms of growth, most species, except from the genus Montipora, present a preference for the Biorock structure, with a growth of 1 to 7% higher than on the control structure. This advantage is small if it is compared with results obtained within previous studies based on the Biorock concept. The probable reason of the weakness of these results is the fact that the experiment has been settled in an open-system, with freshly broken fragments of corals used for wire transplantation.This study nevertheless constitutes a new advance for the Biorock concept which still need a lot of research, although it is already widespread, in terms of knowledge concerning the particularities linked to the electrolysis itself and its effects on a large variety of calcifying and non-calcifying organisms.