Vulnerability of coral reefs

Presented by Janice Lough

Authors

This presentation is based on Chapter 5 ‘Vulnerability of coral reefs in the tropical Pacific to climate change’ in the book Vulnerability of Tropical Pacific Fisheries and Aquaculture to Climate Change, edited by JD Bell, JE Johnson and AJ Hobday and published by SPC in 2011.

The authors of Chapter 5 are: Ove Hoegh-Guldberg, Serge Andréfouët, Katharina Fabricius, Guillermo Diaz-Pulido, Janice Lough, Paul Marshall and Morgan Pratchett

Moving reefs out of their comfort zone

Source: Hoegh-Guldberg et al. (2007)

Outline Coral Reefs

• Key coastal ecosystem• Many different reef types• Narrow environmental limits• Already shown impacts, e.g. bleaching• Combined effects of disturbances• Less time to recover• Simpler reefs• Healthy reefs will cope better

Many different types of reefs

• 13/22 PICTS have more reef than land area

• Dominant coastal habitat

• Majority are oceanic

• Great diversity of reef types

With different levels of human use

• Support local fisheries

• Differences in local pressures

Location matters

• Fringing continental reefs affected by river runoff

• Isolated oceanic reefs not well connected e.g. larval supplies

• Tropical cyclones >10o from equator

• El Niño/La Niña impacts

Important environmental factors

• Warmest parts of oceans

• Narrow temperature range

• Warm water temperatures

• Shallow well-lit waters

• Low sediment and nutrients

• Right ocean chemistry Ω >3.3

Corals must build skeletons fast enough to withstand natural forces of erosion

Photos: AIMS

A special relationship• Symbiosis at heart of tropical coral reefs• Photosynthetic algae live within coral animal • Corals get enough energy for rapid calcification• Form structurally complex reefs• Home to thousands of other plants and animals

Photo: Michael ten Lohuis Photo: AIMS

• Stressed corals lose algae (and their pigments)• Coral bleaching• Seen more frequently due to warmer temperatures• Corals living only ~1-2oC below upper thermal limit• Too much fresh water also causes bleaching

Relationship breaks down due to stress

Photos: Ray Berkelmans

• 30% extra CO2 entered oceans

• Otherwise greater warming!

• BUT changes ocean chemistry

• Harder to form skeletons and shells

• More erosion

Ocean acidification

Ocean acidification: natural laboratory

Source: Fabricius et al. (2011)

• High CO2 volcanic seeps, PNG

• ‘Winners’ = massive corals

• ‘Losers’ = branching, tabulate corals

• Reduced coral diversity

• Much simpler reef with lower pH

Warmer temperatures• Very high vulnerability• Already seen bleaching, diseases

More acidic ocean• High vulnerability• Weakens reef framework

Stronger storms and heavier rainfall• Moderate vulnerability• More disturbances = less time to recover

Higher sea level• Some corals may keep up• Loss of deeper corals

Management interventions

Source: Anthony & Maynard (2011)

What it means for coral reefs• Already shown vulnerability

• Bleaching and diseases

• Physical destruction

• Weaker skeletons

• Lower salinity

• Connectivity between reefs

• Direct and indirect effects on other reef organisms

Summary key issues

• Rates of change• Combined stressors• Less time to recover between disturbances• Adaptation in decades instead of 1000’s years???• Healthy reefs better able to cope• Consequences for reef-dependent fisheries

Coral reefs will not disappear entirely BUT are likely to be MUCH SIMPLER ECOSYSTEMS