NERC/Emerging controls on coral reef growth potential under low coral cover states

Coral cover on reefs globally has declined markedly over recent decades and is further threaten by climate change. Corals are the main contributors to reef-building and so their decline has major implications for the maintenance of the physical structures of reefs, for future reef-building, and thus for the wide range of ecosystem goods and services their frameworks support. These span coastal protection benefits, food provisioning, and tourism revenue generation, with an estimated global value >US$375 billion/annum. Whilst reefs globally are affected, those across the Western Atlantic (WA), the focus of this research, have been especially severely impacted. Coral cover on WA reefs averaged 50-60% in the 1970’s but now rarely exceeds 15% and is often lower. As coral cover has declined the surfaces of most WA reefs have become dominated by a range of what we term here Non-Coral Dominated Substrates (NCDS). These NCDS, which commonly cover >85% of reef surfaces, mainly comprise mosaics of inter-mixed communities of fleshy, turf and calcareous algae, and substrate-colonising bioeroding taxa. Furthermore, NCDS coverage is likely to increase as coral cover further declines due to thermal stress, disease and poor water quality impacts. A historical research focus on the resilience, fate and physical contribution of corals to reef-building has, however, left us in a situation where we know little about the carbonate producing and eroding processes which define different NCDS types. This is now a significant knowledge gap which is limiting our ability to accurately quantify net carbonate production rates on contemporary reefs and hampering efforts to realistically model future trajectories of reef growth. This project will deliver novel empirical data to address this gap, with the aim of enabling us to better constrain the key emerging controls on reef growth potential under low coral cover states. Through Obj. 1 we will characterise the benthic colonising taxa and biotic and abiotic calcifying and erosional processes defining NCDS types on WA reefs. Obj. 2 will determine seasonal-annual changes in the abiotic and biotic components of NCDS and temporal rates of NCDS surface elevation change. Obj. 3 will quantify carbonate production and erosion rates associated with NCDS under future IPCC-aligned sea-surface temperature (SST) and ocean acidification (OA) scenarios, and Obj. 4 will spatially quantify the implications of NCDS dynamics for either enhanced or suppressed rates of net reef framework accretion at habitat and regional scales. Our future modelling work (Obj. 3) will be undertaken in collaboration with U.Miami/NOAA and leverages >US$200k for experimental facility usage. Collectively, the project will deliver highly novel data on the carbonate production and erosion rates associated with these increasingly spatially dominant substrate types, and data on how these may change in the future. These data can then: 1) be immediately integrated within established open access carbonate budget calculation tools (ReefBudget and CoralNet) that are widely used by scientists, NGOs and managers to quantify reef geo-ecological functions and reef growth potential; and 2) used to address critical gaps associated with non-coral substrate behaviour within reef geo-ecological models, including those aimed at constraining reef growth under sea-level rise and modelling coral restoration scenarios. We hypothesise that overall low net positive accretion rates will define the ensemble of NCDS, but regardless of the outcome, our data will provide critical insights that will enable modellers to generate far more accurate projections of reef futures.