Exploring the hidden shallows: inner-shelf reef growth and future trajectories of reef geomorphic change

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Regional-scale deteriorations in coral cover and reef architectural complexity, driven by a suite of environmental and climate-change related disturbances, have been documented, with the scale of global reef ecosystem change such that a cessation of reef accretion has been argued by many as the ultimate and imminent trajectory. One of the most pressing and fundamental challenges in coral reef science is thus to project the future for coral reefs under rapidly changing climatic and environmental conditions. Responses will likely vary region by region, and reef by reef, but will ultimately be determined by two basic sets of factors: 1) the ecology (and ecological responses to environmental change) of the key carbonate producers and eroders on a reef, because these interact to determine whether a reef has the potential to add new carbonate to its structure; and 2) the geomorphic evolutionary state and future growth potential of that reef as a function of its past growth history relative to sea level. Whilst there is an expansive and rapidly growing body of data on local and regional spatial scale contemporary ecological processes to inform this debate, there is a remarkable paucity of data on the age, growth history and morphogenetic evolutionary state of the reef structures on which these contemporary processes operate. Indeed, a review of the literature suggests that data on Holocene coral reef accretion rates (as a measure of net vertical reef growth over time) exists for something well below 1% of the World's coral reefs. This represents a major limitation in any attempts to project future rates of reef growth (and thus geomorphic change), and inherently inhibits attempts to integrate data on past rates and timescales of reef growth (at the individual reef scale) into assessments of future ecological states, and thus into management decision-making. For example, if a reef has been at sea level for the past several 100's to 1000's of years, and exists in an essentially senescent evolutionary state (a 'senile' state: sensu Hopley 1982), not only will its current habitat diversity be restricted but its immediate growth potential and its potential for sustained future growth will be severely impaired. The implications of this are clear - that the best informed management plans should integrate knowledge not only of contemporary reef ecology and habitat types but also, as a predictor of future potential geomorphic performance, an understanding of past and potential reef growth rates and of the current geomorphic evolutionary state of a reef. To address this issue, inclusive datasets are needed that can inform our understanding of: 1) when different reefs within individual regions started to grow; 2) how fast they accreted in different settings; 3) which reefs have been most actively accreting in the very recent past; and 4) which reefs, as a function of their current geomorphic state, have the greatest potential for further accretion in the future. The primary goal of this project is thus to address one part of the future reef trajectory challenge - the relevance and role of past geomorphic performance and of current reef evolutionary state as a predictor of future reef accretion potential. This has direct long-term management relevance because contemporary reef morphology is one of the key contributing factors that influences future morphology and thus the characteristics and diversity of reef habitats. Specifically, the project will develop new, spatially inclusive reef accretion and evolutionary state datasets - taking as a case site the inner-shelf regions of Australia's Great Barrier Reef. Whilst regionally focused, the work has global scale relevance because of the implications for understanding the links between reef growth histories, contemporary ecological states and future habitat complexity.