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.