Lecture 6- Coral Reefs Flashcards Preview

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Flashcards in Lecture 6- Coral Reefs Deck (34):

What is a coral reef?

-An ecological association of corals and other tropical organisms


What are coral reefs made of?

-corals secrete calcium carbonate= chemical that hardens the exoskeleton= that is responsible for the reef structure


In what latitudes can coral reefs occur?

30 S to 30 N


What is the primary network for the coral reef?

-coral -some coralline algae


What is a coral reef and coral community?

-reef= the landform, the rocky area -community= the wider interaction of the landform and the organisms living there (fish, predators etc)


How can the reef change the environment?

-by growing vertically and laterally


What are coral reefs composed of?

-variety of sediments, formed by the corals, have different species, they die and sediment can be transported


What is the far reef area?

-deeper area of the ocean, hard to penetrate the deep water, protected from wave breaking


What is the reef front?

-the waves start shoaling here, more shallow type corals


What is the reef crest?

-highest point on the reef, this is where the waves break , high wave energy,


What is the reef flat?

-largest part of the reef, flat bit


What is the lagoon?(back reef)

-basin or central area, low energy, only type of circulation are from waves that get over the reef crest -the reef is a circle sort of


Is there a relationship between the wave energy and the type of coral growing in that area?

-yes, affects its morphology -even the same species will look very different depending on where it is on the reef


What sort of coral is there on the lower reef slope (far reef)?

- not that much light= BRANCHING coral, not much wave energy so can be thinner -tries to get as much light as possible so almost flat sometimes (like a pancake)


What sort of coral is there on the upper reef slope?

-main change in coral is that it is more dense= response to increase in wave energy -shorter


What sort of coral is there on the reef flat?

-waves breaking, shallow water,= coral is compact, grows large laterally fue to lot of light


What sort of coral is there on the back reef?

-can become thin and branching


What sort of coral is there in the lagoon?

-= more branching and thinner -more light available and less wave energy


Where is the reef outer slope and what are its characteristics?

-Beneath the limit of waves and light -Generally very steep, extending from below the limit of reef growth often to the sea floor -Differences between reefs depend on length of the slope, which in turn is affected by the depth of sea floor the reefs are developed on.


What is the upper limit of the reef outer slope characterised by and how do these form?

over hangs and caverns -these are related to multiple sea level cycles and evolution of reef systems --when sea lower and reef is exposed because it is made of calcium carbonate= then wind and rain fall and interacts with water= slightly acidic= weathers down the rock= karstified, then sea becomes higher level= then these caves are flooded -coral re-establishes itself -this cycle occurs over long time periods 100ka -the morphology of reefs today is a result of this form habiatats


What are some examples of reefs with long reef slopes?

-Great Barrier Reef has some stepped outer slopes relating to lower sea level reefs ----e.g. Bikini Atoll outer slope descends to 3.6 ␣ 4.5 km at 46 and 36o respectively -----e.g. Mururoa Atoll outer slope averages 45o from 20 ␣ 500 m depth, then decreases to 30-35o in north & 15-25o in south. Difference related to wind direction.


What is the spur and groove zone?

-Reefward edge of outer slope often has grooves shallowing in a reefal direction -These grooves link into the active spur & groove zone -Major feature in moving water & sediment from the reef crest into deep water -drain water into the ocean --groove= troph


How long and deep can grooves be?

-often 15- 30 m long, 1 -3 m wide, 2- 4 m deep (Bikini Atoll)


What are spurs?

-Spurs occur between grooves and are where wave breaking is concentrated


How is the coralline growth distributed in the groove and spur zone?

-Have greater coralline algal growth on top (if shallow) and coral (if deeper), floor of groove tends to be bare with loose sediment. Benthic growth on sides but less dense than top of spurs.


What is the model 1 of the spur and groove zone evolution: erosional?

-Erosion: Karst antecedent hypothesis Morphology primarily the result of corals growing on already existing relief. -forms by breakdown of rock over time -forms when sea levels are lower and the rock is exposed to wind etc. creates these spurs and grooves, they remain even when flooded and corals grow on it after


What is the model 2 of the spur and groove zone evolution: constructional?

-Reef growth is a feedback (cycle), and form is entirely a product of modern processes -Spurs progressively prograde seawards (as well as vertically) -At their reefal end, often the grooves are over grown with coralline algae and can form caves (or in some cases blow holes on the reef crest)


What are the characteristics of the reef crest?

-High energy top of spur & groove zone -Transition zone from direct wave breaking to wave bores travelling across the reef -Generally low coral cover, esp. in high energy areas, & dominated by coralline algae -Often have a distinct algal ridge formed -on top of the spur and groove zone -can experience build up of sand -vegetation can colonize this point -wave is broken on the crest and some is reflected to the sea and some to the lagoon, reef crest= is a buffer


What are the characteristics of the reef flat?

-Flattest, widest part of reef. e.g. Rodriguez Island, Indian Ocean up to 2km wide -Generally exposed at spring low tides, therefore little coral growth, though can occur in pools -Dominated by algae (green & red types) -Morphology often striated, lying perpendicular to the reef crest -Striation related to wave flow across the reef crest & flat (have a feedback between currents & benthic communities -coral doesn't like being exposed o the air, can only be in the pools mostly


What are the characteristics of the lagoon?

-Occurs behind the reef Form as deep basin up to 70 m deep on atolls, or shallower channels behind fringing reefs -Often have luxuriant branching coral growth as protected from waves, but high amount of current flushing


What are the lagoons in terms of sediment?

-Areas of sediment deposition (sediment sink= collecting sediment over time, contributed by winds -so it will become more shallow over time -takes a long time) -Sediment either a mix of reef crest material, or for deep lagoons have concentric bands of in situ materials =in situ= already there


What are the characteristics of islands?

-Are a mix of sediment derived from the surrounding ecological communities -Form at nodal points on the reef -Very dynamic landforms -corals breakdown and provide sediment for the islands


What are the two types of islands dependent on?

-depending on the reef structure, circular or lens shaped


What does this picture demonstrate?

-series of beach survey, see the outline changes over time