Vulnerability of coral reefs
Janice Lough
Moving reefs out of comfort zone
Hoegh-Guldberg et al 2007
Outline Coral Reefs
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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
(e.g. ~Fiji 40%)
• 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
• warm water temperatures
• shallow well-lit waters
• low sediment and nutrients
• right ocean chemistry Ω >3.3
• warmest parts of oceans
• narrow temperature range
Corals must build skeletons fast enough to
withstand natural forces of erosion
waves
tropical cyclones
sunshine
predators
coral eaters
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
Relationship breaks down due to stress
• 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
Healthy - unbleached
Stressed - bleached
Recently dead
Ocean acidification
• 30% extra CO2 entered oceans
• otherwise greater warming!
• BUT changes ocean chemistry
• harder to form skeletons &
shells
• more erosion
Ocean acidification: natural laboratory
• high CO2 volcanic seeps, PNG
• “winners” = massive corals
• “losers” = branching, tabulate corals
• reduced coral diversity
• much simpler reef with lower pH
Normal pH = now
Mid pH = 2050
Lower pH = 2100
Fabricius et al 2011
Warmer temperatures
• very high vulnerability
• already seen bleaching, diseases
More acidic ocean
• high vulnerability
• weaken 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
Opportunities for management
interventions
Anthony & Maynard 2011
Reef status: Fiji
• value of monitoring – appears “stable” condition
• tropical cyclones, bleaching, COTs
• recovery after disturbance
• localised pollution/overuse
• 34% of reefs classed at “low threat”
Morris & Mackay (2008) Status
of coral reefs of the world 2008
What it means for coral reefs
• already shown vulnerability
• bleaching and diseases
• physical destruction
• weaker skeletons
• lower salinity
• connectivity between reefs
• direct & indirect effects on other reef organisms
Summary key issues
• rates of change
• combined stressors
• less time to recover between disturbances
• can adaptation occur in decades rather than 1000’s years?
• healthy reefs better able to cope
• consequences for reef-dependent fisheries
Coral reefs will not disappear entirely BUT
likely to be
MUCH SIMPLER ECOSYSTEMS
Thank you
j.lough@aims.gov.au