coral reefs
-in reality, few reefs are dominated solely by corals,
hence the term “coral” reef is often a misnomer.
-corals are largest, most showy, obvious, easiest to
recognize on many reefs; however, few people realize
that red algae generally exceed corals in importance as
reef-building organisms.
-therefore, instead of term coral reefs,
tropical reefs or biotic reefs is preferred.
-without certain spp. of calcareous coralline
red algae, most reefs would not exist.
These coralline red algae are harder than
most corals, and this durability enables
them to withstand the tremendous power
of the huge breakers on oceanic islands.
-within such extreme high-energy systems,
coralline algae form an algal ridge that
absorbs wave energy and hence protects
the more delicate corals, fleshy algae,
sponges, & other organisms that inhabit
the protected lagoons and back-reef
habitats.
-cement, holding everything together, is what makes it all work - and
the red calcareous red algae are the principal cementing agents of
nearly all tropical reefs.
-many reefs, especially in the Pacific, are almost entirely formed by
calcified plants
-there is no
significant fossil
record of the
evolutionary history
of the marine red
algae, except for the
order Corallinales
which extends back
as far as the
Jurassic.
also algal nodules
(rhodoliths, maerl)
-Red Algae or Rhodophyta is by
far the largest & most diversified of tropical reef plants
-largely restricted to hard-bottom
-predominantly macroscopic in all tropical reefs
-equally abundant in Atlantic, Pacific, Indian oceans
The red color is due to the presence of phycoerythrin which reflects
red light and absorbs blue light.
- color varies according to the ratio of phycoerythrin to
phycocyanin & they may appear green or bluish from the
chlorophyll and other masking pigments.
-because blue light penetrates water to a greater depth than light of
longer wavelengths, these pigments allow red algae to photosynthesize & live at somewhat greater depths than most other algae.
-occupy entire range of depths inhabitable by photosynthetic
organisms, from high-intertidal to depth as great as 300 M (San
Salvador I. Bahamas, greatest depth for known plant life)
Biologically active compounds produced by reef dwelling
organisms possess antimicrobial and antiviral activity. These
compounds may be important sources for natural product based
drugs and medicines
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The reef flat is a rigorous environment. The
organisms that live here must be adapted to
withstand intense ultraviolet radiation, high
salinities, and elevated water temperatures.
Although some areas of the reef flat are barren
pavement, pockets of branching corals do occur
where there is sufficient water flow to prevent overheating or desiccation.
Beach
Reef flat
Algal Ridge
Sub-Terrace
Terrace
Reef Slope
Encrusting coralline algae flourish where waves break on the reef, forming an
elevated bank known as the algal ridge. As water from the waves returns
seaward, its load of suspended sand and gravel scours a series of narrow
ridges. The alternating ridges and valleys are known as spurs and grooves.
Corals growing on top of the spurs are stoutly formed and compressed in
shape to withstand the extreme force of breaking waves.
On many reefs there is a terrace below the algal
ridge. In this zone we often find stout fire corals
which are able to withstand the wave surge and
strong currents which characterize this area.
Here surgeon fishes and parrotfishes rove
over the submarine terrace grazing on
benthic algae
As we move down the slope from the
submarine terrace, we find increasing coral
cover and fishes that prefer areas of rich coral
growth, like this regal angelfish
Some outer reefs have flattened terraces that interrupt the gradient
of the reef slope. It is here, protected from the wave surge overhead
yet still exposed to abundant sunlight, that we find lush gardens of
corals with expansive body forms designed to maximize their
exposure to sunlight.
Descending the slope, we find corals
continue to expand horizontally in
shape in order to capture as much
sunlight as possible.
Branching forms found in shallow
water are largely replaced by plate like
forms below a depth of sixty meters.
-reef-building (=hermatypic) corals are of the order Scleractinia in
the class Anthozoa of the phylum Cnidaria. ~ 6,000 species
-
-contain large quantities of unicellular microalgae called
zooxanthellae (dinoflagellates), living symbiotically in the
gastrodermis of reef-building corals.
-nutrients supplied by the zooxanthellae make it possible for the corals
to grow and reproduce quickly enough to create reefs. Zooxanthellae
provide the corals with food in the form of photosynthetic products. In
turn, the coral provides protection and access to light for the
zooxanthellae.
-the role of macroalgae is mainly photosynthetic as well as
providing bulk materials,
similar to bricks or
blocks in construction.
Symbiodinium
Coral reefs --Corals need warm water, between 18°C & 30°C. If the water is colder, the
reefs are poorly developed or nonexistent. Most coral reefs lie between the latitudes of 30
degrees N & S where sea temperatures are warmest.
Corals need access to sunlight (the algae living in their cells need light for photosynthesis) so
the water where they grow needs to be shallow and clear. Too much sediment in the water
can smother the coral polyps and too much fresh water kills them. That is why coral reefs
don't grow close to the mouths of rivers.
Orange = atolls / Red = fringing reefs / Green = barrier reefs
Coral reefs are divided into 3 basic types: fringing reefs, barrier
reefs and atolls.
Fringing reefs grow in shallow waters and border the coast
closely or are separated from it by a narrow stretch of water.
Fringing reefs consist of several zones that are characterized by
their depth, the structure of the reef, and its plant and animal
communities. These regions include the reef crest (the part of
the reef the waves break over), the fore reef (the region of
medium energy), and the spur and groove or buttress zone (the
region of coral growth which includes rows of corals with sandy
canyons or passages between each row)
Barrier reefs are reefs that are separated from land by a lagoon.
These reefs grow parallel to the coast and are large and continuous.
Barrier reefs also include regions of coral formation that include the
zones found in fringing reefs along with patch reefs (small reefs),
back reefs (the shoreward side of the reef), as well as bank reefs (reefs
that occur on deep bottom irregularities). Coral reefs also include
reef flats, the reef crest, and a coral terrace (a slope of sand with
isolated coral peaks). These features are followed by another coral
terrace and a vertical drop into deeper waters.
Great Barrier Reef in Australia.
Belize, C.Am.
- third type of coral reefs are atolls, annular reefs that develop at or
near the surface of the sea when islands that are surrounded by reefs
subside. Atolls separate a central lagoon and are circular or subcircular.
- two types of atolls: deep sea atolls that rise from deep sea and those
found on the continental shelf.
- mostly limited to the oceanic basins of the Pacific
Reef morphology -- Reefs are often referred to as the "rainforests of the
oceans". Coral reefs host an extraordinary variety of marine plants
and animals (perhaps up to 2 million) including 1/4 of all marine fish
species. The wide variety of habitats depends a lot on the
morphology of the coral reef. Differences in temperatures, light,
exposure to waves and tides, currents and the amount of food
available result in different habitats and niches.
A) Mud flats close to shore with tide pools - B) Mangroves - C)
Seagrass bed - D) Patch reefs on inner reef slope - E) Inner lagoon F) bottom with mud - G) Pinnacle - H) Outer lagoon - I) Fine sand - K)
Acropora corals - L) Algae ridge - M) Sand and rubble - N) Gorgonians
and black corals - O) Cave or overhang
Coastal bays: Mangroves often live along the edge of bays and river
mouths. The water is often turbid with silty bottoms. Mangroves
provide shelter with their root system and are a nursing ground for
many species of coral reef fishes.
Lagoons: Depths my vary from less than a meter to 90m. If there is
good circulation of water there may be many patch reefs or just large
stands of Acropora corals growing there. There may be seagrass
beds or mud flats. Lagoons can trap many fish varieties as tide
recedes.
A = coarse sedimentation B = fine sedimentation C = daily variation of
temperatures D = number of different species