The integrated growth response of coral reefs to environmental forcing: morphometric analysis of coral reefs of the Maldives.

Abstract

Coral reefs are bioherms whose structure comprises a dynamic mixture of geologically inherited and environmentally forced morphologies. The major debate of coral reef history is over the relative importance of antecedent, erosional and recent, constructional processes in controlling the pattern and pace of reef growth. Landscape scale studies of reef morphology enable us to distinguish between these two morphological lineages on modern reefs. This thesis quantifies empirical relationships among spatial patterns of coral reef growth, geomorphology and environmental forcing in the archetypal atoll nation of the Maldives. The main hypothesis is that asymmetric ocean wave forcing interacts with antecedent reef platform structure to produce characteristic growth configurations and predictable reef morphologies during the Holocene (at least). The hypothesis is tested by regressing a set of reef growth morphometrics derived for every single coral reef larger than 1 ha on impinging wave energy for the entire archipelago (n = 2041). The methods involved the classification of eight Landsat-7 Enhanced Thematic Mapper Plus (ETM+) satellite images covering all reefs of the Maldives, and the calculation of morphometric indices using a geographical information system (GIS). The spatial pattern of coral reef growth, as defined by the distributions of distinct reef geomorphologies, was quantified by multiple morphometrics of well-defined geomorphic zones: reef slope, reef crest, coral rubble, sand flats, reef lagoons and reef islands. These features were delineated with an overall accuracy of 81%. The total area all coral reef and lagoon habitats that comprise Maldives is 21,372.72 km2 . A total of 2,041 ±10 distinct coral reef structures larger than 0.01 km2 occupy a vertically-projected surface area of 4,493.85 km2 . Smaller areas of coral reef substratum cover another 19.3 km2 , bringing the total area of coral reef to 4,513.14 ±135.40 km2 . Islands occupy only 5.1% of the total reef area. Spatial gradients in environmental forcing (i.e., southern ocean swell and monsoon wind-wave fields) were characterized and quantified along the same dimensions as the reef geomorphology, and statistically related to the reef morphometrics. Nonparametric Multidimensional Scaling (MDS) and Analysis of Similarities (ANOSIM) procedures identified statistically significant differences among groups of reefs located on atoll rims that were exposed to nine differing hydrodynamic regimes. The widths of rim reef slopes, crests and flats widths were significantly related with incident wave power (r2 > 0.07, p<0.01, n=488), with the largest reef growth zones facing the major monsoon wind direction, and the smallest facing the relatively calm Maldives Inner Sea. The hydrodynamic openness of the 16 complex atolls of the Maldives was quantified by a rim aperture index (range from 0.03 to 0.35). The total area of various reef growth forms in atoll lagoons (i.e., patch reefs, knolls and faros) was significantly positively related with the aperture index (r2 > 0.62, p < 0.001, n=16). The extensive, detailed and accurate data provided by this study for the first time on the exact numbers, sizes, shapes and areas of reef features of the entire Maldivian archipelago demonstrates the value of synoptic technologies to seascape ecology, supports the hypothesis that the spatial patterns of coral reef growth predominantly reflect recent hydrodynamic forcing, and provides a sound basis for predictive modelling and management decision support in a developing nation of 300,000 people living on coral reefs and confronted with rising sea level.