Lecture 6- Coral Reefs

Question 1

What is a coral reef?

Answer 1

-An ecological association of corals and other tropical organisms

Question 2

What are coral reefs made of?

Answer 2

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

Question 3

In what latitudes can coral reefs occur?

Answer 3

30 S to 30 N

Question 4

What is the primary network for the coral reef?

Answer 4

-coral -some coralline algae

Question 5

What is a coral reef and coral community?

Answer 5

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

Question 6

How can the reef change the environment?

Answer 6

-by growing vertically and laterally

Question 7

What are coral reefs composed of?

Answer 7

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

Question 8

What is the far reef area?

Answer 8

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

Question 9

What is the reef front?

Answer 9

-the waves start shoaling here, more shallow type corals

Question 10

What is the reef crest?

Answer 10

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

Question 11

What is the reef flat?

Answer 11

-largest part of the reef, flat bit

Question 12

What is the lagoon?(back reef)

Answer 12

-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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

• 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)

Question 15

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

Answer 15

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

Question 16

What sort of coral is there on the reef flat?

Answer 16

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

Question 17

What sort of coral is there on the back reef?

Answer 17

-can become thin and branching

Question 18

What sort of coral is there in the lagoon?

Answer 18

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

Question 19

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

Answer 19

-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.

Question 20

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

Answer 20

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

Question 21

What are some examples of reefs with long reef slopes?

Answer 21

-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.

Question 22

What is the spur and groove zone?

Answer 22

-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

Question 23

How long and deep can grooves be?

Answer 23

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

Question 24

What are spurs?

Answer 24

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

Question 25

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

Answer 25

-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.

Question 26

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

Answer 26

-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

Question 27

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

Answer 27

-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)

Question 28

What are the characteristics of the reef crest?

Answer 28

-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

Question 29

What are the characteristics of the reef flat?

Answer 29

-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

Question 30

What are the characteristics of the lagoon?

Answer 30

-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

Question 31

What are the lagoons in terms of sediment?

Answer 31

-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

Question 32

What are the characteristics of islands?

Answer 32

-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

Question 33

What are the two types of islands dependent on?

Answer 33

-depending on the reef structure, circular or lens shaped

Question 34

What does this picture demonstrate?

Answer 34

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