<-previous | Geol 117 Home | Lectures | Review | next ->
are very diverse and productive marine ecosystems in shallow, tropical seas. Coral reefs are the habitat of thousands of species of invertebrates, vertebrates and algae.
The coral animal is the dominant reef-builders. Corals are
members of the phylum Cnidaria, the same as jellyfish and sea
anemones. Corals are colonial organisms. An individual coral consists
of a polyp and tentacles with sting cells (nematocysts). Corals
gather food by paralyzing small fish and zooplankton (i.e., predators);
they also use their tentacles to collect suspended food particles
in sea water (i.e., filter feeders). Coprals reproduce by asexual
"budding" and by the release of planktonic eggs and
larva that are fertilized sexually. Individual corals secrete
a supporting "cup" of CaCO3 that provides the structural
framework of the reef. [Calcareous algae, foraminifera, molluscs,
echinoderms also contribute to reef construction]. Corals can
grow at rates of up to 10 cm/yr. There are a variety of growth
forms, including massive, columnar, branching, etc.
Environmental requirements of growing coral reefs:
oT: >18°C (23-25° is optimum), thus corals grow only
between 30° N and S latitude
oS: relatively high
oDepth: < 50-150 m
oClear water (no suspended sediment)
Symbiotic zooxanthellae in coral tissue. Zooxanthellae are
photosynthetic dinoflagellates that give living coral their characteristic
color. Because they are photosynthetic, the presence of symbiotic
zooxanthallae limits the depth of active coral growth to the photic
zone. How does symbiosis between corals and zooxanthallae work?
Corals provide for zooxanthellae:
A constant, protected environment
An abundant source of CO2 and nutrients (P, N) from coral metabolic
wastes
Zooxanthellae provide for corals:
A source of oxygen (O2)
A source of food (although zooxanthallae are not ingested by corals)
Stimulate CaCO3 secretion
Types of coral reefs
Fringing reef -- borders shoreline
Barrier reef -- further offshore, separated by lagoon
Atolls -- ring-shaped
These types represent a sequence in the development of coral reefs,
as originally proposed by Charles Darwin.
Zonation on coral reefs. Horizontal and vertical zones are
determined by factors including depth and light intensity, wave
action, temperature and salinity of water, and biological factors.
Living base of a reef begins at about 150 m depth, although there
are few species of coral at this depth.
Outer slope extends over the range 15-150 m. In this environment,
there is adequate sunlight for corals and algae to grow. It is
also below the depth of wave action, so its reasonably well protected.
Buttress zone extends from low tide to 15 m. Wave energy is absorbed
and dissipated in this zone. In addition, channels in the buttress
zone allow sediments on the crest and flat to be drained to the
outer slope and greater depths.
Reef crest is exposed at high tide. It is continuously pounded
by waves.
Reef flat and lagoon is a quiet-water, protected environment.
It is the home to a large number of coral species. It is the habitat
for thousands of species from almost every other animal phylum:
fish, crustaceans, echinoderms, molluscs, etc.
Environmental stress on coral reefs can result from natural causes, such as predation by star fish on the Great Barrier Reef, Australia, and from human activities (fishing, recreation, etc.). Coral bleaching is a clear indication of a reef under stress. Bleaching results from corals expelling their symbiotic zooxanthellae -- the corals loose color and turn white. The cause(s) of coral bleaching is not clearly understood. But it is most common in anomalously warm waters >32C. The increasing occurrence of colar bleaching may be an indicator of global warming.
PRIMARY PRODUCTIVITY IN THE OCEANS
Processes
"Primary Production" = synthesis of organic matter by autotrophs
** Processes:
1. Photosynthesis . . (by phytoplankton is most important)
sunlight
CO2 + H2O -----------------> [CH2O] + O2
chlorophyll
2. Chemosynthesis (e.g., by sulfur bacteria in hydrothermal vent systems):
H2S + O2 ---------------> S0, SO42- + chem. energy
chem. energy
CO2 + H2O ---------------> [CH2O] + O2
3. Biosynthesis . . of other essential organic molecules in autotrophs:
energy
[CH2O] + nutrients (P, N, S) --------> proteins, fats and oils,
DNA and RNA
Resulting average chemical composition of phytoplankton material
("Redfield ratio"):
[CH2O]106[NH3]16[H3PO4]
C106H263O110N16P1
<-previous | Geol 117 Home | Lectures | Review | next ->