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

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.

Coral reefs are of huge ecological importance and immense socio-economic value: they provide goods and services in the form of tourism, fishing and recreational industries. Internationally 109 countries border coral reef ecosystems and this includes Britain's Overseas Territories within the Indian Ocean and the Caribbean regions. SeaWeb reports that worldwide, "More than 450 million people live within 60 kilometres of coral reefs, with the majority directly or indirectly deriving food and income from them". This research thus focuses on a globally significant, economically important ecosystem, but one which is in critical need of effective management. In this context, our present inability to constrain, at spatial scales that are meaningful to reef management, the timescales over which individual coral reefs have grown, how fast they may grow in the future, and how they may interact with on-going human disturbance, clearly impinges upon the effectiveness of coral reef management. Most importantly, this limits our ability to predict future reef ecological trajectories under changing environmental conditions.

This project will develop a novel regional-scale dataset on past rates of Holocene reef growth, and utilise this as a tool to project the timescales over which individual reefs will move through different evolutionary states. We anticipate that these data will help the user communities to: (i) make better informed choices regarding the prioritization of sites for reef management; and (ii) better understand the role of geomorphology generally (and past reef accretion specifically) as a key control on future reef landform change. In addition to academics working in the immediate research field (reef geomorphology) beneficiaries will include:

(i) Marine ecologists and those interested in the controls on marine-based biodiversity and the interactions between humans and key marine ecosystems;
(ii) Management agencies both locally (such as the Great Barrier Reef Marine Park Authority, GBRMPA) and internationally who desire improved management protocols to help preserve ecosystems for economic gain and habitat conservation;
(iii) Legislatory bodies concerned with the impacts of terrestrial inputs on marine environments, and thus with the regulation of land use-related sediment and nutrient inputs; and
(iv) Organisations such as the IPCC and UNEP who are concerned with predicting near-future ecosystem response to environmental and climatic change.

Different aspects of this study will benefit different parts of the likely user community, but we anticipate the following as the primary impacts that will arise from this project, with potential benefits to the user communities as follows:
(1) New data on submerged reefs and seafloor coral communities: This new data will thus benefit the user community at several levels: (i) at regional levels such that reef managers and legislators will be able to integrate new reef ecological data into assessments of total coral cover and integrate these into future zoning considerations; and (ii) at global scales by providing highly novel data on the occurrence and form of juvenile (new) reef development - this alone should provide key data on the relevance of terrigenous sediment dominated nearshore environments as potential habitats for new reef development.
(2) Development of inner-shelf reef accretion models: This new data will benefit the user communities by providing regional datasets on the interactions between reef growth and ecology (and future ecological potential). An improved understanding of these controls is of most significance given that inner-shelf reef areas are considered 'most at risk' from land-based sediment and nutrient run-off.
(3) Developing projections of future reef geomorphic change: This will provide an opportunity for managers to consider recent and near-future reef accretion potential when designating high priority management targets