Quantifying the linkages between water quality and the thermal tolerance of GBR coral reefs
Led by: Dr Line Bay, AIMS
This project will deliver two outcomes: (i) understanding of how water quality (WQ) exposure affects the ability of corals to resist and recover from bleaching, (ii) a modelling framework to identify management options that would mitigate the effects of warming on reefs exposed to WQ pressures. Using SeaSim experiments and field data, we examine which WQ parameters (nutrients/light/turbidity) affect corals’ thermal tolerance, and assess how temperature and WQ exposure histories affected coral bleaching and recovery during the 2016 bleaching event. We will refine the eReefs model to derive WQ management scenarios expected to maximize coral survival in a warming climate.
The GBR experienced the worst coral bleaching event in recorded history in early 2016, with an estimated 25–35 % of corals killed in the northern and Far Norther GBR, along a south-north gradient of human impact on water quality, after weeks of high temperatures and calm conditions. Such heat waves will inevitably become more frequent in the future, even if global warming is contained to <2°C. Hence, coral bleaching and mortality will further increase unless corals become more tolerant to thermal stress.
Water Quality (WQ) management is a major investment focus of the Australian Government and has been proposed as an important mechanism to achieve increased thermal tolerance of corals. Bleaching and mortality of corals generally occur at 4–8 degree heating weeks above local summer temperature. However, considerable variation exists in this relationship among species and reefs, likely because local environmental exposure history including water quality (WQ) alters the stress tolerance of local populations. The negative effects of individual WQ agents on coral bleaching are increasingly appreciated and due to its relevance, the linkage initially proposed by Wooldridge et al. (2009) merits further refinement. For example, we still don’t understand the main agents of WQ that affect bleaching susceptibility and the capacity to recover (e.g., nutrients/nutrient ratios, fine sediments, light, turbidity, pesticides), nor do we understand the main mechanisms. As a consequence, it is not yet clear if, and what level of, WQ improvement would enhance thermal tolerance and coral recovery following a major heat stress event.
How Research Addresses Problem
This project will fill this knowledge gap with a three-pronged approach that includes (i) SeaSim experiments to determine the strength of the relationship between heat tolerance of corals and WQ exposure history, and their recovery trajectories, (ii) analyses of the GBR bleaching and recovery survey data from 2016 in the context of local environmental conditions including WQ, determined by direct observations and models, and (iii) incorporation of field and laboratory data into the eReefs models, to compare scenarios of contrasting WQ management actions, and river load reductions required to elevate the thermal tolerance of corals in regions affected by poor WQ, to keep pace with projected temperature increases.
The research will provide clear advice on the links between WQ and bleaching thresholds, to be used for further developing critical thresholds for reef health indicators (Reef 2050 EHA6), early warning tools under the Reef Health Incident Response System (e.g. bleaching risk EHA26), and refining of WQ targets (e.g. for load reductions via catchment management). The project will address the following questions:
- Over which temperature and light ranges is WQ a determinant of variability in coral colony and reef thermal tolerance?
- Which WQ parameters (nutrients/nutrient ratios, light/turbidity) are most important, in determining coral thermal tolerance, and what are the ecological and physiological mechanisms?
- How does WQ affect the recovery potential of bleached coral colonies and reefs?
- What can we do to mitigate the problem, and up to what point is this effective (i.e., what is the time window for action, in which GBR regions will this be likely to have an impact and what end of river load reduction would be needed to ameliorate bleaching risk under a projected warming of 1.5°C or 2°C)?
A number of physiological models, centering around the corals’ relationship with photosymbionts, have been proposed to explain how WQ parameters may affect the health and thermal tolerance of corals and coral reefs5. Long-term changes in WQ exposure determine the energetic status of corals and the density and type of their photo-symbionts. Consequently, a realistic appraisal of WQ impacts on corals’ thermal tolerance can only be gained if coral host and photo-symbiont traits are simultaneously examined in experiments on long-term acclimatised corals. Our experimental work on bleaching and recovery will focus on corals that are long-term acclimatised to different WQ regimes in two catchments with known WQ issues (Wet Tropics and Burdekin) and will deliver the knowledge required to better forecast bleaching risk. We will also use the survey and oceanographic data collected by AIMS during the 2016 summer, to understand the role of WQ in explaining variation in bleaching and recovery in the Central GBR region.
Adding field and laboratory data of the interactions of WQ exposure and coral thermal tolerance into the eReefs modelling suite will enable us to develop a range of combined WQ and temperature scenarios. First, model outputs will enable us to quantify exposure maps for specific WQ parameters (esp. nutrients, light, suspended sediments). Second, the effectiveness of different catchment management actions on end-of-river loads, GBR WQ conditions, and finally bleaching threshold, will be explored using scenario models. The combination of observational data and scenario models will then be used to compare potential WQ management actions, to better understand mitigation options for future projected warming.
Alignment with NESP Research Priorities
3a) Evaluate the links between water quality and coral bleaching thresholds and how these contribute to management objectives. Water quality parameters evaluated should include those amenable to reductions via catchment management.
Bleaching; Thermal tolerance; Water quality; Ocean warming; Coral reef.
This project is jointly funded through AIMS, CSIRO and the Australian Government’s National Environmental Science Programme.
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