coral reefs (types/theories/conditions)

Question 1

coral reef

Answer 1

underwater ecosystems characterised by lots of coral, held together by calcium carbonate
- create env suitable for lots of wildlife to live in
- 3 to 6 cm in size
>1000 species
- feed on tiny fish + plankton using tentacles

Question 2

conditions required for growth in a coral reef

Answer 2

temp
- 23-25 degrees ideal
- steady temp between 20 and 30 all year (tropical waters) - therefore found within 30 degrees latitude of equator
- must be above 16 below 33
- 92% inn indo-pacific region
salinity
- only survive in salt water, need greater than 32psu (non fresh water)
- can tolerate v high salt (>42 psu)
(PSU = practical salinity unit)
depth
- less than 25m but not exposed to air
- need light for photosynthesis (98% of food provided by photosynthesis of algae)
- often in shallow offshore waters
- corals die if exposed to air too long
sediment
- clean, clear water with no sediment
- not near river etuaries as water has tm sediment, not saline enough so clogs up feeding and cleaning systems
waves
- strong wave action to produce well oxygenated water
- waves remove any trapped sediment and supply oxygen + plankton as food

Question 3

fringing coral reef

Answer 3

• low and narrow bands of coral, next to coast and attached to shore/land
• coral builds up and out, having attached to submerged land on continental shelf in shallow water
• no lagoon, or v small one between land and reef
• highest and most active coral growth on seaward side (breaking waves provide energy for polyps)
• @ low tide, flat bottom surface exposed
  e.g Fiji/Mauritius/Tahiti

Question 4

barrier coral reef

Answer 4

• also grow parallel to shoreline but further out
• broader and older reef, with wide and deep lagoon which separates from land
• form a ‘barrier’ between lagoon and seas, so impedes navigation
• not usually continuous as easily damaged by storms
• seaward edge highest past
  e.g Australian Great Barrier Reef

Question 5

atoll coral reef

Answer 5

• narrow, circular reefs (rings); may be broken circle
• surround deep and circular central lagoon (from 2km to 90km across)
• begin as fringe reefs surrounding a volcanic island. as volc sinks from tectonic plate movements, reef grows and eventually only reef remains

Question 6

darwins subsidence theory

Answer 6

• fringing reefs grow around new islands, which subside/sea level rises relative to land
• if process is slow, reef will grow upwards to form larger barrier reef, separated from the island by a deeper lagoon
• An atoll forms when the island submerges under the water (disappears). corals continue to grow on outside to keep reef on surface. on inside, where land used to be, quiet water w/ more sedimentation prevails

Question 7

daly glacial theory 1915

Answer 7

• glacial period = fall in SL = abrasion/hydraulic action = wave-cut platforms at new level. coral reefs also died due to cold temps
• end of glacial period = rise in SL = coral reef grew on wave-cut platforms (shallow enough for light etc)
• fringing reefs formed on narrow wave-cut platforms
• barrier reefs formed on broader wave-cut platforms
• atolls formed around wave-eroded island peaks
  critique
• depth of atolls/barriers not uniform
• no evidence of wave-cut platforms