2.8 Reef Mortality

Climate Change & Bleaching

• sea level rise, changes in storm frequency and intensity, changed ocean circulation & warming waters

• Bleaching summary

  • symbiont photosystem is easily overwhelmed by excess light leading to the production of reactive oxygen species

  • create oxidative stress in the coral tissue, causing coral to expel symbionts to avoid tissue damage

  • previous exposure & symbiont species affects bleaching severity

• Bleaching causes slowed growth, mortality, reduced reproduction, increased susceptibility to disease

Sedimentation

• sediments can smother coral colonies, clogging feeding structures, blocking sunlight, and causing exhaustion by forcing the animal to clean itself off (using mucus)

• caused by deforestation, coastal development, and agriculture

Disease

• black band, white band, white plague etc.

Deoxygenation

• 1-2% decrease in global dissolved oceanic oxygen. Deoxygenation negatively affects disease, bleaching, reproduction & growth rate.

• Sources: UN Report 2019, Hughes et al. 2020, Nelson & Alteri 2019, Report- UN 2019,

Acidification

• Increased CO2 in the atmosphere leads to increased DIC in the ocean, and the consumption of carbonate ions impedes calcification rates, making corals grow more slowly.

• Sources: Fabricus 2011, Johnson et al. 2017, Wall et al. 2017, Schoepf et al. 2013

Bioerosion

• any organism that erodes and weakens the calcareous skeletons of reefbuilding species.

• external - parrot fish, puffer fish, hermit crab, limpet, urchin, chiton

• Internal - algae, fungi, bacteria, sponges, bivalves, barnacles, worms

Reef destruction

• harmful fishing practices, development or other physical destruction of reefs

Algal competition / shifting states

• Decline in herbivores - fewer herbivores like urchins allows the shift to algal steady state

• Overfishing - removal of herbivorous teleosts enables macroalgal growth

  • break down of food web

  • cyanide & dynamite fishing

• predators - crown of thorns sea star,

Pollution

• Sunscreen (Danovaro et al. 2008)

• Excess nutrients - runoff from agriculture (fertilizers) and sewage, increasing N and P which peaks algal growth (phytoplankton blooms -> no oxygen, no light)

• Plastics - microplastics ingested & smothered/light blocked by macro plastics