Bioconstructor Guild Analysis to Assess Maldivian Reefs Following Ocean Warming and Coral Bleaching

Extreme climatic anomalies related to global warming have triggered coral bleaching events across most tropical regions. The hot wave of 1998 El Niño caused mass coral mortality throughout the Indian Ocean. The Maldives has been among the most affected countries, with 60-100% coral mortality reported. Hard coral cover, which dropped to less than 10% after the bleaching, returned to pre-bleaching values of around 50% only by 2014.

Between 1997 and 2015, we evaluated the change in cover on Maldivian reef flats of different bioconstructor guilds: i) primary builders are those organisms that build the reef framework and therefore assure reef aggradation; ii) secondary builders provide calcareous material to fill in the frame; iii) binders are encrusters that consolidate the reef edifice; iv) bafflers are soft-bodied algae and colonial invertebrates that, although not actively participating in the bioconstruction, help retaining sediment; v) abiotic attributes (rock, rubble, sand) evidently do not give any contribution to the bioconstruction. A bioconstruction potential index (BCP) was devised using the following formula:

BCP = Σ_iⁿ (s_iC_i%) × 100^-1

where, n is the number of bioconstructor guilds (5, in this case), s_i is an importance score assigned to the i_th guild, and C_i% is the cover of the i_th guild. In this study, the value of s_i has been established at 3 for the primary builders, 2 for the secondary builders, 1 for the binders, 0 for the bafflers, and ‑1 for the abiotic attributes. Therefore, BCP ranges theoretically from 3, in the unrealistic case of 100% cover by primary constructors, to ‑1, when only abiotic attributes are present and no bioconstruction is possible, the reef thus being prone to erosion and drowning.

When applied to the Maldives data, BCP provided clear threshold values to evaluate constructional capacity. Negative values characterised Maldivian reefs between 1999 and 2003-3007. Values between 0 and 1 depict reefs capable of constratal growth only, and were found 2004-2008 and 2010-2013. Values of BCP greater than 1 are indicative of superstratal growth, and were found in 1997-1998 and again after 2012-2014.

Further applications of BCP to other reefs in different regions of the tropical ocean are needed to test how universal this index might be.