1. Paradox of low nutrients but high productivity of coral reefs.

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Marine Ecology, June 6, 2008
Lecture 12: Coral reefs
I.
II.
Overview of coral reefs
A. Where found?
1.
Limited to ocean regions with surface water temperatures above 20C (68F)
but below ~30C (86F)
2.
Occupy 600,000 sq. miles
B. Among the most highly productive ecosystems in the world.
1.
Amount of photosynthetic tissue on a reef (algae, symbiotic protists) is much
higher than in open ocean.
a)
Important for sustaining reef-associated fish and invertebrate populations
b)
Important in terms of fish and seafood production for humans
C. Significant sink for CO2, due to photosynthesis and CaCO3 deposition.
1.
Reefs help mitigate the effects of increased atmospheric carbon dioxide.
D. Highest diversity per unit area of any marine ecosystem
1.
4-5% of all species on earth; comparable to tropical rain forests.
Focus on Phylum Cnidaria, Class Anthozoa, Order Scleractinia: stony corals
A. Symbiosis of coral and zooxanthellae
1.
Anatomy
a)
Specific coral species harbor specific species of symbionts
b)
Symbionts located in gastrodermis
c)
Note that gastrovascular cavity links polyps, zooxanthellae are present in
these “interpolyp” regions
2.
What do the zooxanthellae provide to the coral?
a)
Products of photosynthesis (carbohydrates, proteins)

98% of a coral’s nutrition comes from the zooxanthellae

Location of zooxanthellae (gastrodermis) allows efficient transfer
of these materials between the zooxanthellae and the polyps.
b)
Oxygen
c)
Aids in calcification (see below)
3.
Calcification
a)
Active transport of calcium from seawater into epidermal cells of coral
occurs; calcium carbonate is then deposited externally

At peak of calcium deposition, a cell must transport 50-100 times
its own volume.
b)
Calcium carbonate deposition is 14X greater in light than in the dark

Highest deposition on sunny days at noon.

Role of zooxanthellae in this process is not well-understood

Zooxanthellae may assist by increasing oxygen and/or pH
4.
What does the coral animal provide to the zooxanthellae?
a)
Nutrients, especially nitrogen and phosphorus-containing compounds
b)
Carbon dioxide
c)
Location in the photic zone
d)
Protection
Page 1 of 6
5.
Polyps feed at night, close up during day for maximum surface
exposure/photosynthesis
a)
zooplankton on which the polyps feed are also vertical migrators, and
come to surface primarily at night.
b)
Feeding mechanisms

Direct capture with cnidocytes

Mucus/cilia method
6.
Review: Role of coral in food web
a)
Primary producers (zooxanthellae)
b)
Primary consumers
c)
Secondary consumers, and up
d)
Detritovores
B. Basic life cycle
1.
Both sexual and asexual reproduction
a)
Planula larvae (lecithotrophic) produced sexually

Mass spawning
b)
Larvae settle and produce colonies via asexual reproduction
2.
Breakage of some coral species can spread reef
3.
Competition between coral colonies
a)
“acontia” = mesenteries from the g.v. cavity: secrete digestive enzymes!
4.
Role of other organisms in reef-building
a)
Gluing the reef together (submarine lithification)

calcareous cement (from bacteria?)

coralline algae

encrusting organisms such as sponges and bryozoans
b)
Filling in pores and cavities with additional calcium carbonate as they
decompose

Coralline algae and other calcium-carbonate containing algae

Molluscs, echinoderms & other CaCO3 containing organisms
C. Reef breakdown
1.
Physical erosion/breakage
2.
Biological erosion: organisms bore into coral/weaken it
a)
algae, fungi, sponges, worms, crustaceans
b)
crown-of-thorns sea stars
c)
parrotfish grazing (especially certain species)
d)
sea urchin grazing
III. Factors affecting reef growth
A. Temperature: both lower and upper limits for reef-building coral
1.
Lower limit: ~20C average temperature
a)
The process of CaCO3 precipitation requires this (endothermic reaction).
2.
Upper limit: ~30C
a)
Coral bleaching will occur above this temperature
B. Depth: Best growth in 25m or less, but can continue down to 50-70m.
1.
Why a depth limit?
C. Salinity: true marine (~32-36 ppt) or even slightly hypersaline
Page 2 of 6
D.
IV.
V.
Sediment: high levels of sediment can be damaging to reef
1.
Blocks light
2.
Suffocates polyps
E. Wave action
1.
Increases oxygen and nutrient exchange, decreases sediments
F.
Nutrients
1.
Paradox of low nutrients but high productivity of coral reefs.
a)
Water in tropics is low in dissolved inorganic nutrients (nitrate,
ammonia, phosphate) but productivity is high, especially compared to
areas of the open ocean with similarly low levels of dissolved nutrients
b)
Hypotheses to explain this include:

Efficient coupling between coral and its symbionts

Nutrients in particulates  consumed by filter & deposit feeders

High flow rate of water over reef  high integrated nutrient
amounts

Nitrogen fixation by cyanobacteria creates a source of nitrate
2.
High nutrients are actually detrimental to the reef because they promote
algal overgrowth.
Reef invertebrate & fish diversity: focus on interrelationships
1.
Eusocial shrimp within sponge,
2.
Attacking Polychaete & protective mechanisms
3.
Glassfish, red-mouthed grouper and lionfish
4.
Harlequin shrimp and sea star prey
5.
Powderblue tangs and convict tangs
6.
Basketstars
Key threats to reefs
A. The scope of the problem
1.
An estimated 30% of reefs worldwide are already severely damaged
2.
Close to 60% may be lost by 2030
B. Coral bleaching: loss of photosynthetic symbionts ("extrusion") or loss of
pigments within the symbiont.
1.
Causes
a)
Natural, seasonal fluctuation of zooxanthellae populations, probably due
to light level.
b)
High temperatures
c)
uv radiation
d)
Low salinity (as with increased runoff)
2.
Potential benefit of expelling the zooxanthellae: replacement of
zooxanthellae with those better adapted to withstand higher uv levels
a)
Several variants of the zooxanthellae, and much genetic variation within
species.
b)
Can have bleached and unbleached corals side by side
c)
But high mortality of coral that has bleached... (argument against benefit)
C. Overfishing: for food and aquarium trade
1.
Change in balance of reef fish, invertebrates can lead to reef decline
Page 3 of 6
Conch harvest  increase in crown-of-thorns. Why?
Fishing causes shift in fish populations

Decline in some herbivorous fish leads to algal overgrowth
c)
Fishing causes an increase in sea urchins (by removing their predators, as
well as removing competitive fish)

Urchins are herbivores, but also erode the reef.
2.
Fishing methods are destructive to the reef
a)
Dynamite blasting for collecting of aquarium fish
b)
Use of cyanide and other poisons

Fish may be killed

Collected fish usually damaged (less than 1:10 make it.)

Also affects the coral itself
Influx of sewage and other pollutants
1.
Nitrate/phosphate influx (human waste & agriculture)
a)
Increases in algal growth

Caribbean-wide, a high proportion of reefs have shifted from coral
to algal dominance
b)
Increases in harmful algal blooms
2.
Influx of sediments (rain forest clearing, agriculture)
a)
Can bury reef structure or reduce photosynthesis
3.
Freshwater runoff
a)
Salinity changes can cause coral bleaching
Large storms [BP, Coral Seas, last segment—also has lobster migration, can go
past lobsters to storm section, 45:23]
1.
Physically damage the coral
a)
Slow growth  slow recovery time
b)
In combination with other factors, can promote shift to algal domination
(Example: Glovers Atoll, Belize)
c)
Can shift composition of corals  faster-growing corals become
dominant.
a)
b)
D.
E.
Study questions
1.
How does the coral reef ecosystem compare to others in terms of its level of productivity?
How does the coral reef ecosystem compare to others in terms of its diversity?
2.
Are coral reefs important to us? Why or why not? You may think of your own reasons as
well as those from lecture.
3.
What is the relationship between coral reefs and global temperature increase? Think both
about how global temperature increase affects coral reefs as well as how coral reefs affect
global temperature increase.
4.
To which Phylum, do stony corals belong? Which common intertidal organisms also
belong to this Phylum?
Page 4 of 6
5.
Name the symbionts within the tissues of the coral. Where, specifically, are these
symbionts located?
6.
What do the zooxanthellae provide to the coral polyp?
7.
What is calcification? Which member of the symbiotic relationship, the coral polyp or the
zooxanthellae, is needed for calcification?
8.
What does the coral animal provide to the zooxanthellae? Be sure to name all relevant
contributions.
9.
At what time of day do polyps actively feed (as opposed to simply receiving organic
molecules from the zooxanthellae)? What is the adaptive value of actively feeding at this
particular time rather than all day long (at least two reasons…)?
10.
How do coral polyps capture food (2 methods)?
11.
How do corals mate? What happens to their offspring?
12.
Describe how a reef is built, including the role of the coral itself as well as the supporting
roles of other reef organisms.
13.
Describe how a reef breaks down, describing the role of organisms as well as abiotic
factors.
14.
List several factors that impact reef growth (i.e. light, etc…), and then explain (where
possible) the mechanism whereby it has this impact.
15.
Coral reefs are found in regions where inorganic nutrients (including nitrates and
phosphates) are relatively low, and yet productivity is high. Provide at least four
hypotheses which attempt to explain this paradox.
16.
Describe several interrelationships of reef organisms (HINT: think about various predatorprey relationships as well as mutualistic relationships in the video. You can describe the
type of organism on the exam if you can’t remember the specific names of the organisms.
17.
What is coral bleaching? Be as specific as possible.
18.
List several proposed causes of coral bleaching.
19.
Besides simply wiping out conchs, what effects could overfishing of conchs have on a
coral reef (Pacific) and why?
Page 5 of 6
20.
Describe several ways that fishing (of fish) can be destructive to the reef. Be sure to
connect to mechanisms of reef growth and breakdown.
21.
How would influx of sewage and other pollutants cause damage to a coral reef? You
should be able to describe at least three distinct types of influxes and their effects on the
reef.
22.
How do major storms impact coral reefs?
23.
What major shift has occurred on Caribbean reefs over the past 30 years? What are the
proposed causes?
Page 6 of 6
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