8.3 Coral reefs Flashcards
What are coral and what are their characteristics?
- Tiny, marine invertebrate animals or polyps belonging to the jellyfish family (soft)- coral is made when the polyps secrete calcium carbonate skeletons called calicles
- about 3 to 6 cm in size
- Structure: Sac-like body + mouth + stinging tentacles
- feed on tiny fish and plankton using their tentacles
- > 1,000 species of coral
- Coral polyps are translucent (clear)
- However, organisms called zooxanthellae (small algae) live inside the tentacles of the polyps. These produce food by photosynthesis (they have a symbiotic relationship) using the CO2 and H20 the polyps produce in respiration which the algae then share with the coral providing them with sugars and O2 which the polyps use to grow and respire(gives coral reefs ‘beautiful’ colours). In return, the algae receive shelter and food from polyp as the Polyp provides Co2 and H20 from respiration for photosynthesis.
- Coral polyps attach themselves to a hard surface, usually rock of seabed – but can be a shipwreck!
- As they grow, they produce calcium carbonate at their base which hardens to form a protective exo-skeleton, joining them all together and fixing them to that one place on the ocean floor
Coral structure diagram
What is a coral reef?
- Where polyp calicles connect – this creates a colony – this grow and act as a single organism
- A reef is a ridge of material at or near the surface of the ocean. Reefs can occur naturally. Natural reefs are made of rocks or the skeletons of small animals called corals. Reefs can also be artificial—created by human beings.
- Over time this limestone reef (a ridge of rocky, stony material at or near surface of ocean made up of millions of coral polyp skeletons) builds up, which is hard and rock-like on the inside and alive on the outside.
- As one generation dies, the next one grows on top of it, so reef grows upwards and outwards. Tropical reefs can grow at rates of under 2.5–60 cm/year and can form huge structures extending for many miles along the coastline over incredibly long periods of time
Coral reef distribution + egs
- Parallel to coastline in linear pattern
- Near coastline – margins of continents & islands
- Coral reefs develop in shallow, warm water, usually near land, and mostly in the tropics; coral prefer temperatures between 21 - 30 °C.
- Not near river estuaries
Examples
- Great Barrier Reef – NE Australia
- Nigaloo Reef NW Australia
- Pacific islands – Fiji, Tahiti, Tonga
- Indian Ocean – Maldives, Seychelles, Sri Lanka, Mauritius
- Caribbean - Atlantic
There are coral reefs off the eastern coast of Africa, off the southern coast of India, in the Red Sea, and off the coasts of northeast and northwest Australia and on to Polynesia. There are also coral reefs off the coast of Florida, USA, to the Caribbean, and down to Brazil.
Conditions required for coral growth
Corals are very sensitive to their environment and thrive only under very specific conditions – these conditions explain their distribution:
- Temperature: 23-25 C. Steady temperatures of between 20°C and 30°C all the year round – i.e. warm tropical waters (preferably 23-25°C) – therefore found within 30o latitude of equator. 92% of total found in Indo-Pacific region. Must be above 16o C and max 33oC.
Not in areas with cold sea currents i.e. west sides of continents e.g. west coast of Peru (Humboldt Current) and west coast of S Africa (Benguela Current)
May be found further north if warm water brought by ocean current e.g. Bermuda – Gulf Stream!
- Salinity: Required. Need salinity of greater than 32psu (i.e. not fresh water). Also can tolerate high salinity levels (>42psu – e.g. Red Sea / Persian Gulf)
PSU = parts per thousand
If you are super keen: (Practical Salinity Unit), which is a unit based on the properties of sea water conductivity. [1 PSU = about 1g / kg]
- Depth: Less than 25m but not exposed to air. Need light for photosynthesis. Shallow seas, between 10m and 50/60m in depth (light needed for algae – photosynthesis – 98% of food provided by photosynthesis of the algae; plankton (also coral food) needs light); most less than 25m depth and ideally 10m deep and a solid surface on which reef can grow– i.e. found in shallow offshore waters on margins of continents & islands- continental shelf.
Corals die if exposed to air for too long so mostly found below low tide mark. Found in areas with not too great a tidal range or large changes in sea level - Sediments: Clean, clear water with no sediments – not near river estuaries as water has too much sediment and is not saline enough and sediment can clog up the feeding structures and cleaning systems of coral. Coral reefs will not form where turbidity (sediment) is high
- Waves: Strong wave action to produce well-oxygenated water. Waves remove any trapped sediment and supply oxygen and microscopic plankton as food to the coral.
Often found in breaking wave zone. Provides oxygen and nutrients.
However, extremely strong storm conditions may be too destructive (e.g. tsunami)
The outer edge, seaward edge of the coral grows highest because this is where waves break and where oxygen and food supplies are most abundant
Coral bleaching
When algae are ejected so coral loses its colour, due to warming temperatures
Fringing reefs
- Reefs that form along a coastline. LOW & NARROW BANDS OF CORAL. ATTACHED TO SHORE/LAND + BUILD OUTWARDS. They grow when coral attaches itself to submerged land on the continental shelf in shallow water, growing upwards to sea level or just below. Often form around tropical islands.
- SHALLOW LAGOON ONLY (if any – there may not be one!) which is only 500m wide - only between reef + land (as surface of coral slightly concave).
- Low and narrow band of coral next to coast at about low tide level. Highest on seaward part (breaking waves here oxygenate water and make food available for polyps which build up the coral
- NOT CONTINUOUS.
- e.g. Fiji / Mauritius
Fringing reefs diagram
Barrier reefs
- These grow parallel to shorelines, but farther out than fringing reefs, usually separated from the land by a WIDE & DEEP lagoon.
- They are called barrier reefs because they form a barrier between the lagoon and the seas, impeding navigation.
- Not usually continuous as easily damaged and broken up by storms.
- Over 0.5km from shore. Polyps don’t live in deep lagoon but there is old coral on sea floor.
- EG Great Barrier Reef. BROADER REEF, OLDER REEF< MORE CONTINUOUS.
Barrier reef diagrams
Coral atolls
- These are rings of coral (ring shaped islands rising out of very deep oceans far from land) that grow on top of old, sunken volcanoes in the ocean.
- CENTRAL LAGOON
- They begin as fringe reefs surrounding a volcanic island; then, as the volcano sinks due to tectonic movements, the reef continues to grow, and eventually only the reef remains.
- Usually a broken circle around an island. Narrow circular reefs surround deep circular lagoon.
- E.g. South Pacific Ocean. Maldives
Coral atolls diagram
Different types of coral reef comparisons
Differences:
(1) Land in centre for barrier + fringing; none for atoll
(2) May be no lagoon for fringing; a lagoon for both barrier and atoll
(3) Barrier wider and older than others
Similarities:
- Must have land to attach to
- Highest point of reef at sea level or just above – on seaward side
- All have steep outer slope where meet ocean side
How do coral reefs form?
- Fringing reefs easy to explain: grew in shallow coastal water found where it is today
- Barrier reefs are arguably fringing reefs that continued to grow out & up from the coastline (for better access to light; oxygen; food). This increased height created a wider lagoon as the outer edge (seaward) increased in height to a greater extent than the rest of the reef since this is where waves break and there is most oxygen and food for coral growth.
- Atolls – more difficult to explain because they arise from deep water and are a long way from coastline; no visible connection to island or land mass and with no visible base which is shallow enough for the original corals on which new corals have built.
Coral reef sea level rise relationship
Charles Darwin came up with a theory in 1842 (now supported by others eg Dana) to suggest that there is evolution of coral reefs and that their formation is linked and related to either changing land levels (tectonic processes) and/or changing sea levels – this can therefore be used in an essay on landforms associated with sea level change.