Ocean acidification represents one of the most serious long-term threats to coral reef ecosystems and will continue through this century, irrespective of progress in reducing emissions due to the amount of carbon dioxide already in the atmosphere.
Ocean acidification reduces the availability of carbonate ions that are required by many organisms – such as corals and mollusks – to build skeletons and shells. For coral reefs, the most concerning implication of ocean acidification is its effects on coral growth, coralline algae and rates of chemical erosion of reef substrate, which can have significant impacts on the dependent fish communities.
Currently, the best guidance for managing for ocean acidification involves prioritizing management towards protecting natural refugia and managing local stressors on reefs. Management strategies that protect these ‘natural refugia’ from other stresses may help reefs cope with predicted changes in climate and ocean chemistry. Characteristics that may indicate habitats less vulnerable to ocean acidification include: ref
- Carbonate rich areas, such as raised reefs and limestone islands, extensive reef flats, patch reef/coral head complexes, and carbonate sediment deposits
- High-diversity reef complexes that are well flushed by oceanic water. Influxes of fresh oceanic water bring higher total alkalinity and saturation states that support reef and shell building.
- Seagrass beds located near corals. These may provide a short-term local buffering effect for adjacent coral reefs because seagrasses can reduce dissolved CO2. ref
Design MPAs that consider OA
As the science develops, it is important for managers to design select examples of coral reef areas in a variety of ocean chemistry and oceanographic regimes (e.g., high and low pH and aragonite saturation state; areas with high and low variability of these parameters) for inclusion in MPAs. This will increase the likelihood of protecting corals acclimated to a variety of pH conditions and spreads the risk of any coral species’ survival being compromised by ocean acidification.
Reducing stressors that exacerbate ocean acidification conditions — Managers can support the resilience of reefs by reducing other stressors that affect marine ecosystems (e.g., declining water quality, coastal pollution, and overfishing of important species and functional groups, such as herbivores. Such efforts are likely to help marine organisms focus their resources on growth, calcification, and reproduction rather than on repairing damage. ref Specifically, reducing land-based sources of pollution (nutrient runoff and sedimentation) has been identified as an important approach to address acidification in coastal waters because nutrients like phosphorus and nitrogen and land-based carbon inputs can increase the acidity of coastal and oceanic waters. ref A key strategy to address ocean acidification is reinforcing existing environmental laws (e.g., U.S. Clean Water Act) to: limit runoff and associated pollutants, control coastal erosion, and enforce emission limits for pollutants such as nitrogen oxide and sulfur oxide. ref
Exploring and applying innovative interventions that reduce effects of ocean acidification where feasible — Direct interventions must be explored that potentially mitigate the impacts of ocean acidification. These approaches may include geo-engineering approaches and bio-buffering.ref However, the geographic scale, time frame, and economic and environmental costs and benefits of these interventions must be explored further before they can be implemented.
Encouraging research priorities to incorporate ocean acidification into conservation planning and management — Managers have an important role to play in encouraging research that has direct conservation application by partnering with research institutions or supporting such partnerships. Such priorities include: 1) establishing an ocean carbon chemistry baseline; 2) establishing ecological baselines; 3) determining species/habitat/community sensitivity to ocean acidification; 4) projecting changes in seawater carbonate chemistry; and 5) identifying potentially synergistic effects of multiple stressors. ref It will also be important to encourage socioeconomic research on the impacts of ocean acidification, the projected timing of impacts, and the ways to increase adaptability and resilience of socioeconomic systems in the face of changing ocean chemistry. ref
Reducing the effects of ocean acidification — Implementing national or global policies to drastically reduce global carbon emissions is the most critical step towards reducing the effects of ocean acidification. The slides below include highlights from two Declarations that address ocean acidification: the Honolulu Declaration and the Monaco Declaration.
In response to the urgent challenge of ocean acidification, The Nature Conservancy convened a workshop for ocean experts in Honolulu, Hawaii, in August 2008. The workshop included coral reef managers, oceanographers, climate experts, and marine scientists. The goal of the workshop was to develop the foundation for adaptation strategies that marine managers can implement to address the impacts of ocean acidification.
The final workshop report, called The Honolulu Declaration on Ocean Acidification and Reef Management, outlines a suite of policy and management practices that will guide the initial and urgent steps required to give coral reefs the best chance of coping with ocean acidification. ref
The Declaration stresses that two major strategies must be implemented urgently and concurrently to mitigate the impacts of climate change and to safeguard the value of coral reef systems:
- Limit fossil fuel emissions
- Build the resilience of tropical marine ecosystems and communities to maximize their ability to resist and recover from climate change impacts
The following management recommendations were identified:
- Incorporate reefs of low vulnerability or susceptibility to ocean acidification into MPA zoning plans during development or routine review.
- Incorporate into MPA management plans specific adaptation strategies and actions to address climate-change threats (ocean acidification and warming and sea-level rise), including monitoring of their effectiveness.
- Regularly review coral reef management plans to incorporate the latest research and scientific findings into a proactive and adaptive approach to address ocean acidification impacts.
- Develop, test, and, where appropriate, apply interventions to reduce the effects of ocean acidification on high-priority areas and species, for example by reducing impacts from local disturbances.
- Develop, test, and implement innovative interventions to reduce damage to reefs weakened by ocean acidification, and to promote the replenishment of reef communities impoverished by loss of coral species to the combined impacts of climate change, including elevated seawater temperatures and sea-level rise.
- Integrate coral reef management with land-use and coastal zone planning and practices to reduce pollutant inputs (notably, ammonium compounds, nitrogen and sulphur oxides) that increase the acidity of local waters.
Marine scientists who met in Monaco in October 2008 released a strong statement on January 30, 2009 about ocean acidification accelerating due to increasing carbon emissions caused by human-induced climate change. The Declaration calls on Governments to take urgent action to reduce carbon emissions. Over 150 marine scientists from 26 countries signed the Monaco Declaration warning that “ocean acidification could affect marine food webs and lead to substantial changes in commercial fish stocks, threatening protein supply and food security for millions of people as well as the multi-billion dollar fishing industry.” ref
The Declaration urges policymakers to launch four types of initiatives:
- To help improve understanding of impacts of ocean acidification by promoting research
- To help build links between economists and scientists that are needed to evaluate the socioeconomic extent of impacts and costs for action versus inaction
- To help improve communication between policymakers and scientists so that a) new policies are based on current findings and b) scientific studies can be widened to include the most policy-relevant questions
- To prevent severe damages from ocean acidification by developing ambitious, urgent plans to cut emissions drastically