Millenium Atoll – Micronesia
Coral bleaching A vivid sign of corals responding to stress which can be induced by: • increased (most commonly) or reduced water temperatures • increased/decreased solar irradiance (photosynthetically active radiation & ultraviolet band light) • changes in water chemistry (in particular acidification) • starvation caused by a decline in zooplankton. • increased sedimentation (can be contributed to silt runoff) • pathogen infections • changes in salinity • low tide air exposure BUT EVERY MASS BLEACHING EVENT HAS BEEN ASSOCIATED WITH ABNORMALLY HIGH WATER TEMPS Last ditch attempt to replace inadequate zooxanthellae Corals may recover but more often die (depends on severity of event)
Sea warming/bleaching (= loss of algalsymbionts) • (zooxanthellae) Most reef-building corals normally contain around 1-5 x 106 zooxanthellae cm-2 of live surface tissue and 2-10 pg of chlorophyll a per zooxanthella. When corals bleach they commonly lose 60-90% of their zooxanthellae and each zooxanthella may lose 50-80% of its photosynthetic pigments (Glynn 1996).
Acidification/Rising sea level • Roughly half of fossil carbon burned winds up in atmosphere, half in oceans • In water, CO2 is converted to carbonic acid • Acid dissolves calcium carbonate, and increases the energetic expense of CaCO3 deposition • Between 1751 and 1994 surface ocean pH is estimated to have decreased from approximately 8.179 to 8.104 (a change of −0.075).
Change in sea surface pH caused by anthropogenicCO2 between the 1700s and the 1990s
the acidity or pH of an aqueous solution is a measure of the concentration of H+ ions in the solution: high [H+] = low pH; low [H+] = high pH • - Dissolved CO2 in the form of carbonic acid H2CO3, may loose up to two protons through the acid equilibria • H2CO3(aq) H+ (aq) + HCO3- (aq) • HCO3-(aq) H+ (aq) + CO32- (aq) When carbonate ions are less available, calcium carbonate (CaCO3) dissolution is more likely to occur, and consequently its formation is less likely to occur.
There are two forms of calcium carbonate that are used by calcifying organisms - calcite and aragonite. Aragonite is used by pteropods and cold water corals, and calcite is used by coccolithophores and foraminifera. • Chalk • “The White Cliffs of Dover”
Shells of Mollusks, arthropods (crabs, lobsters copepods , etc) are also built using calcium carbonate…. But also other non-obvious effects, such as reduction in survival of larval fish in more acidic water. Studies indicate that ocean acidification can impair olfactory discrimination and homing ability of a marine fish such as the clown fish in coral reefs. We are just starting to uncover the range of effects of ocean acidification….
Acidification/Rising sea level:effects on coral reefs • Grow or die (=need for shallow water) • Acid makes growth more expensive (ultimately just too expensive) • Rising seas means growth must be more rapid • While ocean acidification may not appear currently to be killing corals, decreasing seawater pH is slowing development of coral larvae into juvenile colonies.