Shallow Seas: Seagrass Beds, Kelp Forests, and Reefs
Marine Plants:
A. Seagrass Beds
a. General Characteristics:
Seagrasses are the only flowering plants that live entirely in the sea. They grow best in shallow, sandy
lagoons or enclosed bays, where the water clarity is good. They thrive in the tropics, and are tolerant of
water with different salinity levels.
b. Structure:
Seagrasses have roots (unlike seaweeds),
which they use to absorb nutrients from
within the sediments. This recycles nutrients
that would otherwise be locked up below the
surface. Their underground roots are
wrapped around each other, helping to
stabilize the sand or sediment. This protects
against erosion and encourages the buildup
of sediments. As a result, it helps other
species to gain a foothold in the shifting
sand.
c. Seagrass Bed Communities:
The productivity and complex physical structure of seagrasses attracts a high diversity of species, some of
which can only be found in seagrass beds.
 Variety of seaweeds grow on the leaves or fronds of the seagrasses
 Sedentary animals – such as hydroids, bryozoans, ascidians, live on the grasses
 Some animal species have evolved camouflage and spend their entire lives in the seagrass
 Some use the shelter of the seagrass beds to raise their young
 Seagrasses are also important food for manatees, dugongs, green turtles, and aquatic birds
B. Kelp Forests
a. General Characteristics:
The term “kelp” is generally used to describe the many kinds of brown seaweeds. Kelp forests grow best
in cold, nutrient-rich waters, extending into Polar Regions. The top edge of some kelp beds is visible at
the lowest tides. Kelps grow densely on rocks and rocky slopes down to around 30-70 ft. deep, depending
on the water clarity. In deeper water, there is less light for photosynthesis, so kelps grow more sparsely.
Kelp is some of the world’s fastest growing plants and can grow at 160 ft. deep in extremely clear water.
b. Structure:
Many kelps are tree-like in shape, with a branched structure (called a
holdfast) that attaches it, and a long stem (stipe) with floats (gas
bladder), supporting a palm-like frond. This makes a kelp forest a
multilayered environment in which different organisms live.
c. Kelp Communities:
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Within the kelp forest, waters are calmer and many organisms live in
its shelter. Although kelp habitats support rich marine communities,
only about 10% of kelp is eaten directly by animals. The rest enters
the food chain as detritus or dissolved organic matter.
Small spaces in the holdfast can house hundreds of small animals from predators. Sea urchins and limpets
are commonly found grazing on red seaweeds growing on the kelp in deeper water.
Actively growing kelp fronds are usually coated in slime, which deters most animals from settling on
them. But as growth slows later in the season, the fronds may become covered with bryozoans, hydroids,
and tube worms. These animals reduce the light reaching the fronds, and some kelp shed their fronds to
get rid of unwanted settlers before growing new ones.
The sea floor beneath the kelp may be covered with marine growth, or relatively barren if heavily grazed
by sea urchins.
C. Seagrass Bed and Kelp Forests as Nurseries and Refuges
a. Seagrass beds and kelp forests are important refuges for young fish that need to hide from predators until they
reach maturity. Many fish do not live among seagrasses or kelps as adults, but come into these habitats to
spawn, giving their young a greater chance at survival.
b. Small fish find an abundance of small prey in the form of tiny worms, crustaceans, and mollusks among the
seagrasses and in the sediment beneath. These young fish are often camouflaged in shades of green and brown
to avoid detection.
c. Some herbivorous fish from surrounding reefs come into seagrass beds only at night.
d. Seagrass beds are important nurseries for some commercial invertebrates, including shrimp and cuttlefish.
Coral Reefs
B. General Characteristics: Coral reefs are solid structures built from the remains of small marine organisms
called stony (or hard) corals. Reefs cover about 100,000 square miles of the world’s shallow marine areas. They
grow gradually as the organisms that form their living surfaces multiply, spread, and die, adding their limestone
skeletons to the reef. Coral reefs are among the most complex and beautiful of Earth’s ecosystems, and are home
to a wide variety of animals and other organisms. They are also, however, among the most heavily utilized and
economically valuable areas. Today, the world’s reefs are under pressure from numerous threats to their health.
C. Types of Reefs
Coral reefs fall into three main types:
Fringing Reef
Barrier Reef
Atoll
1. Fringing reefs: These occur adjacent to the land, with little or no separation from the shore. They develop
through upward growth of reef-forming corals on an area of continental shelf.
2. Barrier reefs: These reefs are broader and are separated from land by a lagoon that can be miles wide and
dozens of yards deep. Parts of the reef structure often protrude above sea level as low-lying coral islands.
These develop as wave action deposits coral fragments broken off from the reef itself.
3. Atolls: These are large, ring-shaped reefs, enclosing a central lagoon. Most atolls are found well away from
large landmasses, such as in the South Pacific. Just like in barrier reefs, parts of atolls may protrude above the
surface forming coral islands as well.
There are two other types of reefs as well:
1. patch reefs: These are small structures found within the lagoons of other reef
types
2. bank reefs: These are made up of reef structures that have no obvious link to a
coastline.
D. Reef Formation
a. Stony Corals: The individual animals that make up corals are called
polyps. The polyps of the main group of reef-building corals are stony
corals, which secrete limestone. As stony corals grow, they build on top of
the hard corals below them. Individual stone corals can grow up to a few
inches per year. The polyps form colonies that create community skeletons in
a variety of shapes. To survive, corals have a mutualistic symbiotic relationship with tiny organisms called
zooxanthellae. These are photosynthetic organisms that live within the tissue of each coral polyp, and
provide much of the food necessary for the growth of corals, and explain why coral reefs thrive in sunlit
waters. Zooxanthellae also produce the coral’s color.
b. Contributions of other organisms: Other organisms that add their skeletal remains to the reef include
mollusks and echinoderms. Grazing and boring organisms also contribute by breaking coral skeletons into
sand, which fills gaps in the developing reef. Algae and other encrusting organisms help bind the sand and
coral fragments together. Most reefs do not grow continuously but experience spurts of growth interspersed
with quieter periods, which are sometimes associated with recovery from storm damage.
E. Distribution of Reefs
a. Stony corals can grow only in clear, sunlit, shallow water
where the temperature is at least 64o F, and preferably 7784o F. They grow best where the average salinity of the
water is 36 ppt and there is little wave action or
sedimentation from river runoff. These conditions occur
only in some tropical and subtropical areas. The highest
concentration of coral reefs is found in the Indo-Pacific
region, which stretches from the Red Sea to the central
Pacific. There is a smaller concentration of reefs around
the Caribbean Seas. In addition to warm-water reefs, there
are also reefs that grow in cold water and do not depend on
sunlight, but we will discuss those later.
b. The conditions needed for the growth of warm-water coral reefs are found mainly within tropical
areas of the Indian, Pacific, and Atlantic oceans. The reefs are chiefly in the western parts of these
oceans, where the waters are warmer than in the eastern areas.
F. Parts of a Reef
Distinct zones exist on coral reefs, each with characteristic levels of light intensity, wave action, and
other parameters. Each zone’s characteristics determine the organisms that live there.
1. The reef slope, or fore-reef, is the part that faces the sea. The upper parts of the reef slope are
dominated by branching coral colonies and intermediate depths by massive forms. These are the
areas of the reef with the greatest diversity of species.
2. At the top of the reef slope is the reef crest. This takes the brunt of the wave action and is subject to
high light levels.
3. Shoreward of the reef crest is the reef flat, a shallow, relatively flat expanse of limestone, sand, and
coral fragments that may become exposed at high tide. The number of corals decreases toward the
shore.
4. Barrier reefs and atolls have a final zone, the lagoon area.
Reef Slope
G. Importance of Reefs
Coral reefs are valuable for many reasons:
1. They provide a protective barrier around islands and coasts, preventing them from eroding away
with waves and storms.
2. Highly productive, creating more living biomass than any other marine ecosystem and providing an
important food source for many coastal peoples.
3. They support more species per square unit area than any other marine environment. In addition to
known coral-reef species, scientists estimate that there may be several million undiscovered species
of organisms living in and around coral reefs.
4. The biodiversity of reefs may be vital in finding new medicines for the 21st century. Many reef
organisms contain biochemically potent substances that are being studied as possible cures for
arthritis, cancer, and other diseases.
5. Reefs contribute to local economies through tourism, attracting snorkelers, divers, fishermen, and
others who come to see and appreciate their beauty.
H. Human Impacts on Reefs
Many types of stress can damage reefs and are doing so on a massive scale. Coral reefs can recover
from periodic natural traumas, but if they are subjected to multiple sustained stresses, they perish. It
has recently been estimated that two-thirds of the world’s warm-water reefs are at risk of
disappearing in the near future.
Much of the harm is caused by human activity, including:
 Coastal pollution
 Destructive fishing practices
 Uncontrolled development of
 Natural disturbances include:
coasts
 Tropical storms
 Diving tourism
 Mass die-offs of animals that
help to maintain reef health
 Collection of corals and reef
organisms for the aquarium and
 Coral bleaching
jewelry trades
 Uncontrolled mining of reefs for
building materials
Coral bleaching – when the corals become stressed, they expel the zooxanthellae. This causes them
to loose their color, and become whitened or “bleached”. This is the color of the coral’s calcium
carbonate skeleton. Coral bleaching is associated with increased temperature in surface waters, and
may have a link to global warming. Stressors such as pollution can cause bleaching as well. In
extreme cases, corals may die as a result of this.
Coral poisoning - One of the most destructive fishing practices, killing corals over wide areas of
reef. Coral poisoning involves the use of poison to help humans catch tropical fish for the aquarium
trade. This is practiced in parts of Southeast Asia such as the Philippines. A diver swimming around
a reef uses a solution of sodium cyanide. The cyanide is used to immobilize selected reef fish,
making them easier to catch with a hand-held net. The poison, however, kills all the living corals that
it comes in contact with, taking a toll on the health of the reef.
I. Coral Reef Diversity
In addition to reef-building corals, the warm, sunny waters of a reef are populated by a huge variety of
other animals and seaweeds. The richest and healthiest reefs are home to thousands of species of fish
and other marine vertebrates, such as turtles, while all the major groups of invertebrate animals are also
represented. These include sponges, worms, anemones, and non-reef-building corals (such as sea fans),
crustaceans, mollusks, and echinoderms. Every part of a reef is used by some animal as a hiding place
and shelter. All the organisms in the reef are part of a complex web of relationships. Many organisms
are also involved in mutualistic partnerships with other organisms, in which both species benefit.