Predicting Responses of Geo-ecological Carbonate Reef Systems to Climate Change: A Conceptual Model and Review

Coral reefs provide critical ecological and geomorphic (e.g. sediment production for reef-fronted shoreline maintenance) services, which interact in complex and dynamic ways. These services are under threat from climate change, requiring dynamic modelling approaches that predict how reef systems will respond to different future climate scenarios. Carbonate budgets, which estimate net reef calcium carbonate production, provide a comprehensive ‘snap-shot’ assessment of reef accretionary potential and reef stability. These budgets, however, were not intended to account for the full suite of processes that maintain coral reef services or to provide predictive capacity on longer timescales (decadal to centennial). To respond to the dual challenges of enhancing carbonate budget assessments and advancing their predictive capacity, we applied a novel model elicitation and review method to create a qualitative geo-ecological carbonate reef system model that links geomorphic, ecological and physical processes. Our approach conceptualizes relationships between net carbonate production, sediment transport and landform stability, and rates knowledge confidence to reveal major knowledge gaps and critical future research pathways. The model provides a blueprint for future coral reef research that aims to quantify net carbonate production and sediment dynamics, improving our capacity to predict responses of reefs and reef-fronted shorelines to future climate change

Reef state and performance as indicators of cumulative impacts on coral reefs

Coral bleaching, cyclones, outbreaks of crown-of-thorns seastar, and reduced water quality (WQ) threaten the health and resilience of coral reefs. The cumulative impacts from multiple acute and chronic stressors on “reef State” (i.e., total coral cover) and “reef Performance” (i.e., the deviation from expected rate of total coral cover increase) have rarely been assessed simultaneously, despite their management relevance. We evaluated the dynamics of coral cover (total and per morphological groups) in the Central and Southern Great Barrier Reef over 25 years, and identified and compared the main environmental drivers of State and Performance at the reef level (i.e. based on total coral cover) and per coral group. Using a combination of 25 environmental metrics that consider both the frequency and magnitude of impacts and their lagged effects, we find that the stressors that correlate with State differed from those correlating with Performance. Importantly, we demonstrate that WQ metrics better predict Performance than State. Further, inter-annual dynamics in WQ (here available for a subset of the data) improved the explanatory power of WQ metrics on Performance over long-term WQ averages. The lagged effects of cumulative acute stressors, and to a lesser extent poor water quality, correlated negatively with the Performance of some but not all coral groups. Tabular Acropora and branching non-Acropora were the most affected by water quality demonstrating that group-specific approaches aid in the interpretation of monitoring data and can be crucial for the detection of the impact of chronic pressures. We highlight the complexity of coral reef dynamics and the need of evaluating Performance metrics in order to prioritise local management interventions

How Much Shallow Coral Habitat Is There on the Great Barrier Reef?

Australia’s Great Barrier Reef (GBR) is a globally unique and precious national resource; however, the geomorphic and benthic composition and the extent of coral habitat per reef are greatly understudied. However, this is critical to understand the spatial extent of disturbance impacts and recovery potential. This study characterizes and quantifies coral habitat based on depth, geomorphic and benthic composition maps of more than 2164 shallow offshore GBR reefs. The mapping approach combined a Sentinel-2 satellite surface reflectance image mosaic and derived depth, wave climate, reef slope and field data in a random-forest machine learning and object-based protocol. Area calculations, for the first time, incorporated the 3D characteristic of the reef surface above 20 m. Geomorphic zonation maps (0–20 m) provided a reef extent estimate of 28,261 km2 (a 31% increase to current estimates), while benthic composition maps (0–10 m) estimated that ~10,600 km2 of reef area (~57% of shallow offshore reef area) was covered by hard substrate suitable for coral growth, the first estimate of potential coral habitat based on substrate availability. Our high-resolution maps provide valuable information for future monitoring and ecological modeling studies and constitute key tools for supporting the management, conservation and restoration efforts of the GBR

Predicting Responses of Geo-ecological Carbonate Reef Systems to Climate Change: A Conceptual Model and Review

[Chapter Abstract] 230Coral reefs provide critical ecological and geomorphic (e.g. sediment production for reef-fronted shoreline maintenance) services, which interact in complex and dynamic ways. These services are under threat from climate change, requiring dynamic modelling approaches that predict how reef systems will respond to different future climate scenarios. Carbonate budgets, which estimate net reef calcium carbonate production, provide a comprehensive ‘snap-shot’ assessment of reef accretionary potential and reef stability. These budgets, however, were not intended to account for the full suite of processes that maintain coral reef services or to provide predictive capacity on longer timescales (decadal to centennial). To respond to the dual challenges of enhancing carbonate budget assessments and advancing their predictive capacity, we applied a novel model elicitation and review method to create a qualitative geo-ecological carbonate reef system model that links geomorphic, ecological and physical processes. Our approach conceptualizes relationships between net carbonate production, sediment transport and landform stability, and rates knowledge confidence to reveal major knowledge gaps and critical future research pathways. The model provides a blueprint for future coral reef research that aims to quantify net carbonate production and sediment dynamics, improving our capacity to predict responses of reefs and reef-fronted shorelines to future climate change.https://nsuworks.nova.edu/occ_facbooks/1116/thumbnail.jp

Chapter 4 PREDICTING RESPONSES OF GEO-ECOLOGICAL CARBONATE REEF SYSTEMS TO CLIMATE CHANGE: A CONCEPTUAL MODEL AND REVIEW

oceanography, climate change, reefs, marine science, marine conservation, marine researc

Dispersal of Symbiodinium by the stoplight parrotfish Sparisoma viride

Environmental reservoirs of zooxanthellae are essential for coral larvae settlement; understanding where they occur and how they are maintained is important for coral reef ecology. This study investigated the dispersal of Symbiodinium spp. by the stoplight parrotfish Sparisoma viride, which had high mean densities of viable and cultivable Symbiodinium (3207–8900 cells ml−1) in faeces. Clades A, B and G were detected using amplified chloroplast ribosomal sequences (cp23S-HVR), and corresponded with diet preferences of fish and the environmental Symbiodinium diversity of the region. Cells are constantly dispersed in the water column and deposited in the substrate at a local level (86 ± 17.8 m2), demonstrating that parrotfishes are vectors for short-distance dispersal of zooxanthellae. Such dispersal could constitute a key role in the maintenance of environmental Symbiodinium reservoirs

Factors affecting tolerance to herbivory in a calcifying alga on coral reefs

Herbivores greatly influence the productivity of algae but their impact can depend on the nuances of the timing, location, and intensity of herbivory. While plants can escape herbivory in spatial refugia, small-scale variations in habitat quality play a critical role in plant tolerance to herbivory and might generate complex trade-offs. On coral reefs, overstory branching corals provide a refuge from fish herbivory but also provide refugia for many small fish that excrete nutrients. Therefore, algae living in this habitat might also benefit from higher nutrient delivery. However, because coral branches occlude sunlight, algal growth rates might be impaired despite experiencing elevated nutrients and lower herbivory. In lab-based experiments, light, nutrients, and simulated herbivory were manipulated in summer and winter to investigate how these processes interact to influence the tolerance of herbivory in the calcifying green algae Halimeda, an important producer of reef carbonate sediments worldwide. Halimeda heteromorpha which is commonly found associated with branching corals tolerated tissue damage by increasing rates of segment production. Greater tolerance was observed at levels of light resembling those experienced under the coral’s canopy. Nutrient additions increased compensatory segment growth in winter but not summer. Levels of tolerance were seasonal and nutrient dependent. Results show that small-scale variations in habitat quality may influence tolerance to herbivory in Halimeda. This suggests that if coral habitats are degraded or lost and oceans continue to warm, a likely negative impact on Halimeda populations and its contribution to carbonate sediments could be expected if high levels of herbivory are maintained

Dynamics of carbonate sediment production by Halimeda: implications for reef carbonate budgets

Reef carbonate production and sediment generation are key processes for coral reef development and shoreline protection. The calcified green alga Halimeda is a major contributor of calcareous sediments, but rates of production and herbivory upon Halimeda are driven by biotic and environmental factors. Consequently, estimating rates of calcium carbonate (CaCO3) production and transformation into sediment requires the integration of Halimeda gains and losses across habitats and seasons, which is rarely considered in carbonate budgets. Using seasonal rates of recruitment, growth, senescence and herbivory derived from observations and manipulative experiments, we developed an individual-based model to quantify the annual cycle of Halimeda carbonate and sediment production at Heron Island, Great Barrier Reef. Halimeda population dynamics were simulated both within and outside branching Acropora canopies, which provide refuge from herbivory. Shelter from herbivory allowed larger Halimeda thalli to grow, leading to higher rates of carbonate accumulation (3.9 and 0.9 kg CaCO3 m(-2) yr(-1) within and outside Acropora canopies, respectively) and sediment production (2.5 versus 1.0 kg CaCO3 m(-2) yr(-1), respectively). Overall, 37 % of the annual carbonate production was transformed into sediments through senescence (84 %) and fish herbivory (16%), with important variations among seasons and habitats. Our model underlines that algal rates of carbonate production are likely to be underestimated if herbivory is not integrated into the carbonate budget, and reveals an important indirect pathway by which structurally complex coral habitats contribute to reef carbonate budgets, suggesting that coral losses due to climate change may lead to further declines in reef sediment production

Interpreting coral reef monitoring data: a guide for improved management decisions

Coral reef monitoring programmes exist in all regions of the world, recording reef attributes such as coral cover, fish biomass and macroalgal cover. Given the cost of such monitoring programs, and the degraded state of many of the world's reefs, understanding how reef monitoring data can be used to shape management decisions for coral reefs is a high priority. However, there is no general guide to understanding the ecological implications of the data in a format that can trigger a management response. We attempt to provide such a guide for interpreting the temporal trends in 41 coral reef monitoring attributes, recorded by seven of the largest reef monitoring programmes. We show that only a small subset of these attributes is required to identify the stressors that have impacted a reef (i.e. provide a diagnosis), as well as to estimate the likely recovery potential (prognosis). Two of the most useful indicators, turf algal canopy height and coral colony growth rate are not commonly measured, and we strongly recommend their inclusion in reef monitoring. The diagnosis and prognosis system that we have developed may help guide management actions and provides a foundation for further development as biological and ecological insights continue to grow