Tropical ecosystems

Question 1

History of reefs?

Answer 1

Only solitary coral before 490MYA.
Evolved alongside reef fish (eg: wrasse + parrotfish).
Most modern reefs young (eg: GBR = 0.5MYO).

Question 2

Features of corals?

Answer 2

Cnidarians. Catch food w/ nematocysts in tentacles.
Symbiotic partnership w/ dinoflagellates (1 million per cm2).
Zooxanthellae fix CO2 + provide 90% energy to coral in return for protection + nutrients.
Hermatypic (reef-building) + ahermatypic.

Question 3

Features of coral reefs?

Answer 3

Dominated by Scleractinian corals (stony, calcium carbonate skeleton).
Can be extensive (eg: GBR 2000km long).
Rise out of deep, oligotrophic water (eg: Enewetak Atoll rises from 1300m).

Question 4

Conditions required for corals?

Answer 4

Between 30N + 30S.
Clear water under 50m deep - Not in Amazonian Basin because sediment from Amazon river smothers corals.
Optimum temp: 26-28c (tolerate 18-36c).
Full salinity (33-35ppt).
Hard substrate.
None off Namibia (cold upwellings from Atlantic).
None in Bay of Bengal (seasonal cold upwellings - Himalayan meltwaters).

Question 5

Coral life cycle?

Answer 5

Asexual budding/fragments start new colonies.
Sexual reproduction = long-distance dispersal.
Can be gonochoric (male/female) or hermaphroditic.
Mass spawning linked to lunar cycle.
1-3 day larval stage, then settle.
Metamorphosis = secrete basal plate + acquire zooxanthellae.

Question 6

Types of reef?

Answer 6

Fringing reef: Island pushed up by volcano. Intertidal, nearshore reef forms around.
Barrier reef: Island lowers into mantle, but reef remains 1-10km offshore = lagoons.
Atoll: Island sinks = coral reef ring up to 10km across.

Question 7

Reef growth?

Answer 7

Grow faster in summer than winter = bands = show environmental effects.
Net reef growth can be 2cm/yr, but 3mm more common.
Competition limits individual coral growth.
Growth causes limestone deposits (4200 tonnes/km/yr).
Sea levels fluctuated 10m in last 1000yrs.
Bioerosion (parrotfish, urchins, sponges, bivalves).

Question 8

Coral triangle (Evans et al, 2016).

Answer 8

Biodiversity hotspot. 2000 fish species. Studied 45 Indo-Pacific species.
Bayesian skyline plots calculated relative timing of pop size changes.
Older pops closer to coral triangle than periphery = centre of survival, centre of origin, or both (not centre of accumulation).
Role may have changed over evolutionary time (eg: centre of survival during pliocene + pleistocene - major sea level change).
Importance of corals as refugia (prevent biodiversity loss), eg: habitat loss, ocean acidification, thermal stress.

Question 9

How reefs benefit humans?

Answer 9

$375 billion/yr in ecosystem goods + services.
500 million rely on reefs for food/livelihood.
GBR = A$5.1 billion/yr tourism.
Buffer between land + sea = coastal protection + waste processing.
New medicines (bioprospecting) - treatments for cancer, HIV, cardiovascular disease, ulcers.

Question 10

Threats to reefs?

Answer 10

70% destroyed by 2050.
60% threatened at local level by human activity (95% SE Asia, 75% Atlantic, 65% Indian Ocean).
Sensitive ecosystem - decline could warn for threats to other ecosystems.

Question 11

Seagrass?

Answer 11

Marine angiosperms.
Evolved recently (100MYA).
Low intertidal zone to 10m.
Cover 0.1-0.2% ocean.
Rhizome + root system stabilises soft sediment = coastal protection.
High primary production - up to 1200 shoots per m2.
Shelter species - nocturnal predators + juvenile fish.
Grazed by turtles + dugongs.

Question 12

Mangroves?

Answer 12

Form on intertidal zone of sheltered tropical coasts/estuaries. 
16 families (polyphyletic). 
Dominant = black + red mangroves. 
Trap mud = anoxic sediment.

Question 13

Mangrove adaptations to anoxia?

Answer 13

Lenticles in roots obtain O2.
Aerial roots (eg: Rhizophora) + knee roots (eg: Bruguiera).
Pneumatophores (eg: Avicennia).

Question 14

Mangrove adaptations to salinity?

Answer 14

Roots exclude salt uptake (eg: 90-95% in Avecinnia).
Secrete excess in bark/leaves.
Higher root biomass than terrestrial trees = compensate for poor water uptake.
Leaves at angle to sun = avoid excess temp + compensatory transpiration.

Question 15

Mumby et al (2004).

Answer 15

Global mangrove loss = 35%.
Juvenile fish nurseries = affects coral community structure.
Biomass of commercially important species (eg: yellowtail snapper) doubles when connected to mangroves.
Local extinctions of rainbow parrotfish after mangrove removal (juveniles exclusive to mangroves).
H. sciurus moves from seagrass to mangroves to reef. Absence of mangroves, have to move to reefs = lower survival chance.

Question 16

Stambler.

Answer 16

Coral reefs = oligotrophic (low nutrients), but high primary productivity.
Eutrophication = main Enteroanthropogenic stressor faced by coral reefs.
Increased nutrients causes:
- Increased phytoplankton = limits light availability to corals.
- Increased seaweed growth = coral shaded, O2 circulation impeded, + toxic hydrogen sulphide from decaying algae + anoxic sediment.
- Eg: Gulf of Aquaba - nutrient upwelling caused reef smothering by Enteromorpha seaweed.
- Imbalance of nutrient exchange between zooxanthellae + coral (eg: start to compete for carbon).