Download The Sandy Beach Environment

The Sandy Beach Environment
Sandy beaches are composed of sand and loose sediment that is easily shifted and moved
about by wind and water.
Sources of sand
• Erosion from rocks along the shore. Waves pound on the rocky shores, and pieces of
rock break off and fall into the surf. Tides move the rocks back and forth, wearing them
down into pebbles. Over time, the pebbles are ground into sand by rubbing against one
another as they are tossed by the waves.
• Erosion from rocks from mountains
located hundreds of km away. Rivers
and streams wear down the rocks.
Sediments from the rocks are
transported downstream to the ocean,
where they are deposited as sand on the
beach. The erosion of volcanic rock
produced the black sands found on some
beaches.
On beaches where large rivers empty into
sea, sediments carried by the river are
deposited along the shore, producing a
delta. (See Figure) The Nile River, which
flows into the Mediterranean form a delta.
• Erosion from offshore coral reefs. The white and pink sandy beaches are largely
composed of fine sediments eroded from offshore coral reefs.
• Sand may also contain shell or bone fragments, fish scales, and other debris from
marine animals.
Zonation of sandy beach environment
If you were to walk from the upper beach area down to the shore, you would pass through
some of the life zones (described before). Each zone contains a particular group of
organisms that share the habitat. This pattern of marine life, which forms distinct bands, or
life zones, along the shore is called zonation.
The upper beach contains a zone of beach plants that includes trees, shrubs, and grasses.
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The loose sand and cobbles form a highly unstable environment that few species can
tolerate. Consequently these communities have very low biodiversity. However, because
competition here between species is minimal, the few species that can thrive here often
become very abundant, feeding on abundant plankton and food particles washed in by
waves.
Interstitial animals Some of the more abundant organisms in these communities are
interstitial animals - tiny creatures that live in the spaces between sand grains, such as
nematodes.
The Surf Zone
The region of crashing waves called the surf zone, which is white with
foam as a result of air mixing with water as waves pound on the shore.
The surf zone is not a fixed zone. Instead it moves with the tide as it
advances and retreats from the subtidal to the intertidal zones.
Sandy Shores in general are characterized by much lower densities of
organisms than hard substrates. For creatures that live in the surf zone,
life is like always being in a storm. Some small marine animals have
managed to adapt to the turbulence of the surf zone.
£ The mole crab (The right Figure) and the lady crab (Ovalipes
ocellatus) in the intertidal zone and the blue claw crab (Callinectes
sapidus) in the subtidal zone avoids the waves by using its
paddlelike appendages to dig into the sand.
£ The surf clam has thick shell to protect it against wave impact and erosion from
moving sand; it also has a large muscular foot enables it to dig quickly into the sand to
avoid predators.
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The Rocky Coast Environment
Shores made up of solid rock are called rocky coasts. Compared to sandy beaches, rocky
coasts are often very steep and have more densities of organisms.
What kind of environment for living things is provided by a rocky coast?
Rocks provide a surface on which marine organisms can attach themselves. When the tide
is out, you can see seaweeds clinging to the rocks.
Zonation of rocky shores
Similar to that of a sandy beach, the rocky coast shows zones of habitats, with each made
up of different communities of living things.
Causes of zonation
1. The physiological tolerance of desiccation, reduced feeding time, reduced access to
oxygen, and extreme temperature.
2. Competition for resources such as light, food, and living space
3. Predation.
Typically, four major bands, or zones, of life can be observed: the upper intertidal,
mid-intertidal, lower intertidal, and subtidal zones. (See Figure)
1. The Upper Intertidal Zone
The upper intertidal zone, or wave splash zone, is the area above the high-tide mark that
gets moisture from the ocean spray of crashing waves. The moist rocks provide an
environment for the growth of blue-green bacteria, which form a thin film on the rocks.
When the bacteria die, they stain the rocks black, causing a discoloration that is often
mistaken for an oil spill.
The periwinkle snail (Littorina littorea) and limpet feed on algae in the upper intertidal
zone.
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2. The Mid-Intertidal Zone
Below the upper intertidal zone lies the mid-intertidal zone, which is occupied largely by
barnacles, mussels, and seaweeds.
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3. The Lower Intertidal Zone
Below the bed of mussels lies the lower intertidal zone, which is dominated by seaweeds.
When the tide is low, spaces between the rocks retain water, forming small habitats known
as tide pools. Tide pools are like natural aquarium tanks that contain algae, invertebrates
(such as snails and crabs), and small fish.
4. The Subtidal Zone
Below the lower intertidal zone is the underwater subtidal zone. The rocky coast subtidal
zone has an abundance of life including:
£ Sea urchins feed on the holdfasts of giant kelp.
£ Sea stars cling to the rocks and move from the subtidal zone to the intertidal zone to
feed on the mussels when the tide comes in.
£ A variety of sea anemones, as well as crabs and lobsters, hiding in the rock crevices.
£ Predatory fish that swim in from the open ocean prey on the abundant invertebrates.
The Estuary Environment
Estuaries are partially enclosed coastal regions where fresh water from rivers meets and
mixes with seawater.
Origin of estuaries
Many estuaries were formed at the end of the last Ice Age, about 18,000 years ago, when
glaciers melted and the sea level rose. The ocean invaded low-lying coastal areas, flooding
the mouths of rivers and streams.
Estuaries can be classified by their origin into four types.
•
•
•
•
Drowned ‫ ﻣﻐﻤﻮر‬river valleys or coastal plain estuaries where a river valley has
been flooded by rising sea level.
Fjords (pronounced FEE-yord) where a glacial cut deep valleys which have been
flooded by rising sea level.
Bar-built Here the accumulation of sediments along the coast builds up sand
bars and barrier islands that act as a wall between the ocean and fresh water
from rivers.
Tectonic estuaries created not because sea level rose but because faults ‫ﺻﺪْع‬
َ have
dropped down (sank or subside) a section of the earth’s crust in a coastal area.
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Physical Characteristics of Estuaries
Salinity
The salinity of estuaries fluctuates dramatically both from place to place and from time
to time. The more fresh water that is mixed in, the lower the salinity. Salinity therefore
decreases as one moves upstream (Fig. ).
Salinity also varies with depth in the estuary. The salty seawater is more dense and stays
on the bottom. It flows in along the bottom in what is frequently known as a salt wedge.
Meanwhile, the fresher, less dense water from the river flows out on the surface.
The salt wedge moves back and forth with the daily rhythm of the tides. It moves up the
estuary on the rising tide (Fig. a ), then recedes as the tide falls (Fig. b).
This means that organisms that stay in one place are faced with dramatic fluctuations in
salinity. They are submerged under the salt wedge at high tide and under low salinity
water at low tide.
Salt wedge
Fig.
The salt wedge in a typical estuary moves in and out with the tide. (a) At high tide the crab is
covered by water with a salinity of 35‰. (b) At low tide it is covered by water with a low salinity, between
5‰ and 15‰.
Substrate
The substrate or type of bottom, of most estuaries is soft mud (a combination of silt and
clay), brought into estuaries by rivers and settle out in the relatively quiet waters.
Mud is rich in organic material, respiration by decay bacteria uses up oxygen in the
interstitial water, the water between sediment particles.
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Water cannot easily flow through the fine sediments to replenish the oxygen supply. As a
result, the sediments in estuaries are often devoid of oxygen, or are anoxic, below the
first few centimeters. They have the black color and rotten-egg smell typical of anoxic
sediments, in which hydrogen sulfide (H2S), which is toxic to most organisms,
accumulates. Anoxic sediments are not completely devoid of life.
Anaerobic bacteria, which do not need oxygen to carry out respiration, thrive under
these conditions.
Living in an Estuary
Living in an estuary is not easy, so, relatively few species have successfully adapted to
estuarine conditions. This is because life in an estuary need organisms to adapt to
extremes in salinity, temperature, and other physical factors.
•
•
•
Most estuarine organisms are euryhaline marine species, that is, they tolerate a wide
range of salinities.
Relatively few organisms are stenohaline species, species that tolerate only a narrow
range of salinities, may be either freshwater or marine; most are limited to the upper
or lower ends of the estuary.
There are also some brackish-water species, those that are adapted to live in
intermediate salinities.
Types of species living in an idealized estuary in relation to salinity. The width of the bars represents the
relative number of species.
Organisms in estuaries have adapted to salinity fluctuations in various ways.
• Osmoconformers let the salinity of their body fluids vary with that of the water.
• Osmoregulators keep the salt concentration of their body fluids constant.
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The Mud Flat
The part of the estuary that become exposed at low tide and no marsh grasses is called the
tidal mudflat.
Mudflat organisms
Low tides expose organisms to desiccation, wide variations in temperature, and predation,
just as in any other intertidal community. Mudflat organisms must withstand regular
variations in salinity.
•
•
The dominant primary producers on mudflats are diatoms and photosynthetic
bacteria.
The dominant animals on mudflats burrow in the sediment and are known as infauna.
Most of the animals are burrowing deposit and suspension feeders that feed on
detritus - Particles of dead organic matter- in the sediment and water. Most of the
food for these animals is brought in by the rivers and tides.
Deposit Feeders Animals that feed on organic matter that settles in the sediment.
Suspension Feeders Animals, including filter feeders, that feed on particles
suspended in the water column.
•
Protozoans, nematodes, and many other minute animals that compose the meiofauna
also thrive on detritus.
The meiofauna are tiny microscopic animals that live on the bottom among the
sediment particles, often used as a synonym of interstitial fauna.
•
•
Very few mudflat animals can be classified as epifauna, those that either live on the
sediment surface or are attached to a surface as sessile forms.
The most important predators in the mudflat community are fishes and birds. Fishes
invade mudflats at high tide, whereas birds congregate at low tide to feed.
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Salt Marshes
The grassy areas that are usually bordered estuaries in temperate and subarctic regions
and extend inland from the mudflats and are partially flooded at high tide are known as
salt, or tidal, marshes.
•
Salt marshes are dominated by
grasses and other marsh plants. One
type of grass, called cordgrass
(Spartina), is tough, coarse, and
resistant to the killing effects of salt;
special glands in its leaves secrete
salt crystals. Few animals eat
cordgrass because it is tough and has
a high salt content.
• Salt marshes are subject to the same
extremes in salinity, temperature,
and tides that affect mudflats. They
also have a muddy bottom, but it is
held together by the roots of marsh
plants and thus is more stable.
• The calm and nutrient-rich waters of
the salt marsh provide an ideal
“nurseries” for many species of ocean
fish. Young flounders are among these
fish.
• The fish and invertebrates that inhabit
the salt marsh are a food source for the
many bird, reptile, and mammal species that live in and around the marsh. Many
migratory birds also depend on the salt marshes for food.
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The Mangrove Forests
Mangroves are large woody, flowering, terrestrial and highly salinity tolerance trees with
a dense and complex root system, which replace the coastal marsh community in tropical
regions and subtropical intertidal area.
• The tree species that grows in the water of the mangrove forests are
• The red mangrove (Rhizophora mangle) has distinctive roots which extend from
the trunk and branches into the soil. At low tide, the arching roots, which anchor
the mangrove trees in the muddy sand, are visible. At high tide, the water covers
the roots, but the tree trunks and leaves remain above water. (See the Figure)
• Black mangroves (Avicennia germinans)) have pencil-like roots called
"pneumatophores" which grow upward from the underground roots.
• White mangroves (Laguncularia racemosa) have salt glands on the leaf surface
to excrete excess salt.
•
•
•
•
•
Mangroves are characterized by viviparous growth: seeds germinate on the tree,
and young plants fall into the water.
Protect shore from erosion during tropical storms, the roots of the mangrove tree
hold the sand and prevent it from being carried away by waves and currents.
Many marine and land animals live in mangrove forests. Crabs are particularly
common in mangrove forests.
Nesting and feeding sites of birds such as brown pelican (Pelecanus occidentalis)
and the osprey (Pandion haliaetus), which hunt and scavenge on the abundant
marine life.
Important fisheries sources and wood supply for boat and fire
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The coral reef
A coral reef is wave-resistant, rock-like structure that is built by carbonate-secreting
animals belongs to the Cnidara and called the coral polyp. The living part of the reef is
just a thin veneer on the surface.
Characteristics of coral reef
• Formed by biological activity of corals belonging to the phylum Cnidaria (relatives of
jellyfish).
• Cover ~600,000 km² in water >20°C (23-29°C opt.)
• Salinity requirement 32 - 42‰, accordingly, Corals can not survive in fresh or brackish
water.
• All corals are mixotrophs, and polyps feed on zooplankton, preferably at night.
• Corals share a mutualistic relationship (mutually beneficial) with the algae
zooxanthallae (dinoflagellates), which lives within the skin of the polyp and can
comprise up to 75% of the polyp’s body weight.
ƒ Symbionts dinoflagellates provide O2 and organic carbon from photosynthesis;
also enhance CaCO3 deposition by corals.
ƒ Animals provide shelter and nutrients for zooxanthellae.
• Corals can not survive in highly turbid water. Light is required for coral growth due to
symbiontic photosynthetic zooxanthellae; < 25 m water depth.
• Reefs are built of tiny individuals called polyps
ƒ Polyps possess tentacles containing batteries of nematocysts to capture and
paralyze prey.
ƒ Polyps reproduce sexually, producing planktonic larvae (called planula), which
settle again for new colonies; planulae possess already symbionts.
• Corals are classified into two types, hard corals and soft corals (lack hard skeleton).
• Polyps in hard coral secrets calcium carbonate cup-shaped exoskeleton of 1-3 mm
diameter called the corallite, which forms the base of the reef.
• Corals do best in nutrient poor water because they are easily out-competed by
benthonic filter feeders in nutrient-rich water where phytoplankton and seaweed are
abundant.
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• Corals can be either solitary or colonial. Colonies are created by asexual division called
budding, and all polyps remain connected by tissue. Individual colonies can range in
size up to some hundred tons.
Types of Reefs
The reef begins to form when microscopic coral larvae, planulae, settle on a hard
substrate and develop into coral polyps. The coral animals live in colonies, with each tiny
polyp sitting in its own limestone home, which it builds. Each new generation of polyps
lays down a new layer of limestone, causing the reef to expand upward (at about 2.5 cm
per year) and outward. Some massive reefs are more than 40 meters high and more than
1000 km long. Corals can grow right up to the ocean surface but cannot grow out of the
water. They will die if exposed for too long.
As a result of corals growing continuously upward towards the sunlight as sea level rises
and/or land subsides and, coral reefs pass through three stages of development: fringing
reefs, barrier reefs, and atolls. (See the Figure).
1. A fringing reef is the earliest stage of reef development where larvae attach to hard
bottom; as corals grow, a fringing reef is formed shorelines around islands or along
continents.
2. A barrier reef A reef that grows farther offshore is called a barrier reef. In this type,
the land is progressively submerged and the coral grows upward. As a result an expanding
shallow lagoon begins to separate the reef from the shoreline and the reef is called a barrier
reef. The world’s most famous barrier reef is the Great Barrier Reef, which lie between 16
and 160 km off the northeast coast of Australia.
3. Atoll: The last geological stage of sinking volcanic island; the land vanishes below the
sea and because corals keep growing upwards on the outside, the reef forms a ring of
islands, called an atoll, around a shallow lagoon. The calm water and sedimentation
prevents coral growth in the Lagoon (South Pacific)
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Why are coral reefs found only in tropical and subtropical regions?
In these regions, the ocean’s water is warm and clear, and there is plenty of sunlight.
These are the conditions that are needed to promote the growth of symbiotic algae.
Why do so many different organisms inhabit the coral reef?
A reef consists of different types of coral growing together. Each type of coral has its own
unique shape, size, color, and texture. The many kinds and shapes of corals create an
irregular pattern of crevices, depressions, and caves in which organisms can live, hide and
provide many areas for surfaces for attachment.
Coral reefs are fragile. Pieces of coral can be broken off easily; even touching coral can
damage the thin membrane that protects its surface. Unfortunately, many coral reefs
around the world are in danger. This is due to
• Partly to the marine pollution. Development along coasts for example has clouded
offshore waters and caused reefs to die.
• The unrestricted use of reefs for fishing and diving also has contributed to their
degradation.
Figure
(left) The staghorn coral is an example of a hard coral.
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