This document provides an overview of marine ecosystems. It discusses the key characteristics of marine ecosystems, including the abiotic (temperature, nutrients, salinity) and biotic (organisms, predators) components. It also describes several important marine habitats like oceans, estuaries, and on-shore ecosystems. Estuaries are productive areas where freshwater and saltwater mix that provide nurseries for many species. On-shore ecosystems like beaches, lagoons, and salt marshes experience tidal influences and land-sea interactions.

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MARINE ECOSYSTEMS
by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science
University of Mysore
India

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Introduction:
Life on earth is supported by the
surrounding ecological conditions and the
natural resources.
Continents and oceans are the two major
divisions, comprising of all the life and
also the ecosystems throughout the world.
About 75 % of the earth’ surface is covered
with oceanic waters which has a very rich
wealth of marine life.

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The ocean is a major source of food, energy,
and mineral resources.
Oceans also control the global climate.
If we say that land is dominated by plant
biomass, then we have to say that animal
biomass dominates the oceanic waters.
Water-based living environments are called
as aquatic ecosystems.
Aquatic Ecosystems are classified into two
types as freshwater ecosystems and marine
ecosystems.

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The habitats of the open seas and adjacent
coastal zones are classified under marine
ecosystems. In addition to these, marine
ecosystems also include the salt marshes and
wetlands located along the shores and river
mouths.
Within the coastal zone, several unique
habitats like estuaries, tidal inlets, and
foreshore ecosystems are also included.

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Such zones including estuaries contain the
habitat of both freshwater and salt water.
The subject of Physical geography includes
the study of all these ecosystems.
The following aspects are to be studied
under marine ecosystems:
1. Characteristics of Marine Ecosystems
2. Oceans
3. Estuaries
4. On-shore ecosystems.
5. Coral reefs.

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1. CHARACTERISTICS OF MARINE
ECOSYSTEMS
Marine ecosystem is characterized by the
many biotic and abiotic components.
Biotic components are:
Organisms and their species
Predators, Parasites, Competitors and
Mates.

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The Abiotic Components (Physical and
Chemical) are:
Temperature , Concentration of Nutrients
Sunlight, Turbulence, Salinity and density.
The following are the properties affecting
the life in the seas and oceans:
1. Quality of sea water
2. Skeleton shape and size of organisms
3. Buoyancy
4. Gravity

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5. Temperature of water
6. Density of water
7. Light penetration in water
8. Availability of Nutrients
9. Water turbulence and
10. Hydrostatic pressure.
Marine environments have altogether
different kinds of Physical conditions for life
to survive.

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Majority of marine plants are floating
species and microscopic.
Many marine animals are invertebrates.
They do not have massive skeletons.
For floating and swimming, marine life
require little energy. Water which is a
fundamental constituent of all living
organisms, is available in plenty within
these marine environments.

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Temperature is yet another parameter which
do not vary as drastically as seen on land or
in the air. However, there are certain
properties which are less favourable for life
in the seas and oceans. Plant growth in the
sea is limited by light.
It is a fact that 50% of the total solar
radiation which penetrates into the sea
surface, gets disappeared rapidly within a
few meters of depth.

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Much of the marine environment is in
perpetual darkness. Under such
circumstances, many of the marine life
depends on the availability of essential
nutrients only.
It is also to be noted that the zone just below
the sea surface, experiences the maximum
environmental fluctuation.

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This is the zone where there is more air-
water interactions, much variations in
temperature, salinity and turbulence of
water.
Winds plays a major role on the surface of
the sea.
All environmental parameters show
predominant vertical gradients.

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2. OCEANS
The World Oceans are the huge volumes of
sea water belonging to the hydrosphere.
Oceans cover more than 70 per cent of the
earth's surface. About 97 per cent of all the
water available on the earth are in the
oceans.

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Oceans contain both valuable and much
needed natural resources.
They are the sources of food, energy, and
minerals.
Oceans also serve as waterways for ships to
transport people and carry cargo between
continents. The primary role of oceans and
seas lie in maintaining the global climate
by regulating the air temperature and by
supplying the moisture for rainfall.

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Without oceans, life would not have
originated in our planet.
When compared to land based terrestrial
ecosystems, the marine ecosystems have
certain unique features.
They are:
1. the oceanic environment is very wide,
covering 75% of the earth’s surface.

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2. The oceanic systems are very deep in
which life extends to all depths.
3. The water masses of sea and oceans are
continuous.
4. The water is in continuous movement
in both vertical and horizontal
dimensions.
5. The water is salty with an average salt
content of 35 g/litre.
6. The concentration of dissolved
nutrients is low.

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Creatures living on land need to deal with
gravity, dehydration, great temperature
variations, and many other factors which are
normally not encountered in oceans.
The Divisions of Marine environment
included in the study of open sea are:
a) pelagic environment,
b) epipelagic environment,
c) mesopelagic zone,
d) bathypelagic zone,

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e) abyssopelagic zone, and
f) benthic environment- sea bottom.
The distribution of marine life includes the
study of planktons, Phytoplanktons,
Zooplanktons, Nektons and the Benthos.
The Temperature fluctuation is minimum in
oceanic waters.
Arctic waters are colder.
Tropical waters are warmer.

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The variation is seasonal and diurnal.
Ocean animals show a varied response to the
temperatures.
There are two kinds of animals.
1. Stenothermal animals – the ones which
live within 20°C
2. Eurythermal animals – the animals which
can withstand the wide range of temperature
variations.

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The cold water forms also show an increase
their in sizes.
The surface temperature of the oceanic
waters vary from about -2 °C near the North
and South poles to about 30 °C near the
equator.
In the polar regions, the surface sea water
freezes. The western tropical Pacific has the
warmest surface water. Ocean currents
affect the surface temperature.

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Ocean temperature also varies with depth.
In general, the temperature falls as the depth
increases.
Light is very essential for the marine life.
For photosynthesis, heating, radiations and
vision are related to this factor.
It determines the plant and animal life and
depends on the angle of incidence.

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Perpendicular rays go deeper into the
oceans.
Based on the intensity of light the water
body of marine ecosystems can be
differentiated into three zones.
They are
Euphotic zone -extends upto 100 m depth,
illuminated zone production zone , rich with
phytoplankton, primary consumers and
secondary consumers.

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Disphotic zone - lies between 100 and 200
m depth, weakly lighted, secondary
consumers live with a few primary
consumers, Rare plant life.
Aphotic zone - above 200m depth , No
light, no producers, no photosynthesis,
secondary consumers, feed on other animals
also.
Euphotic Zone is the uppermost layer of the
world's oceans.

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Marine Ecology involves the understanding
of all these aspects.
3. ESTUARIES
Estuaries are the coastal areas where the
saline waters of the ocean meet with the
fresh water of the rivers. Estuarine habitats
are usually very productive because of the
accumulation of nutrients from a large
catchment of a river.

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Estuaries offer good fishing grounds as the
water column is shallow and easily
approachable. These are the breeding habitat
for a variety of Shrimp and Prawn species,
oysters and fishes.
They also provide sheltered harbours for
ships and their traffic. Estuarine waters are
used for cooling of water in power
generation plants.

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An estuary is a semi-enclosed coastal body
of water that has a free connection with the
open sea at one side and a river on the other
side.
The circulation of water in any estuary
depends on the amount of freshwater
outflow, the strength of the tidal current and
the amount of vertical mixing.

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Estuarine waters are therefore salty or
brackish in quality. Structurally, they are
deep funnel shaped mouths. They are
complex ecosystem comprising of three
zones as, polyhaline zone(low salt),
mesohaline zone (medium salt) and
oligohaline zone(fully saline).
The marine side is called as lower estuary.
It has a free connection to the sea with
salinities exactly equal to that of the oceanic
waters.

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It is called as Polyhaline zone. The total
dissolved salts of this zone may range from
30 to 35 ppt.
The middle estuary which is subject to
mixing of salt and freshwater is called as the
mesohaline zone. The salinities of this zone
may be around 15 ppt in the waters.
The upper or fluvial estuary which is
characterized by the flow of freshwater is
known as oligohaline zone.

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The level of salinity in such zone may be
around 0 to 5 ppt.
Ecologically, an estuary is an area of
dynamic biogeochemical activity.
Estuaries are critical for the survival of
many species of birds, mammals, fish and
other wildlife.

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Rivers and streams drain into the estuaries.
They bring-in enormous amounts of
nutrients from uplands.
Plants use these nutrients, along with the
sun's radiant energy, carbon dioxide and
water to manufacture food. As long as the
nutrient-rich freshwater flows towards the
estuaries , the estuaries will remain
productive.

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Estuaries may be small or large depending
upon their dimension, size, shape and the
quantity of river staying inside and flowing
inside.
The sediments carried by the streams are
also dropped into the quiet water mass of
estuaries. Continuous sedimentation may
shorten the dimension of estuaries especially
in the upper reaches.

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Estuary is also considered as a Maritime
Nursery. It is a place where young fishes
and a few other organisms are protected
from predators and competitors which may
not be able to enter into the low salinity
waters.
The Biota of Estuaries are classified as
a) Producers
b) Zooplanktons
c) Benthic invertebrates

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d) Fishes
e) Birds and wildlife.
1. the Producers include
Phytoplankton - diatoms, dinoflagellates,
green algae.
Macroalgae - red algae, green algae, brown
algae.
Microalgae - benthic diatoms.
SAV – seagrasses are common in estuaries.

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The emergent plants include Spartina,
Juncus, Batis, Sallornia.
2. Zooplankton - grazing herbivores &
carnivores. They include Copepods, larval
fish, fish eggs, chaetognaths.
Larval invertebrates - oysters, clams, corals,
etc.
jellyfish (small & large)
ctenophores - very abundant in plankton of
estuaries.

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The benthic invertebrates include:
clams, oysters, mussels, snails (mollusks)
polychaete worms
amphipods
decapods - crabs, shrimp, lobsters
The Fishes include the migratory fish which
enter via the inlet are anadromous fish,
striped bass and salmon, spotted seatrout,
and red drum.

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Juvenile fishes predominate in estuaries – at
nursery level.
Estuaries are important feeding areas for
many migrating birds. The estuarine shores
show plovers, skimmers, gulls, oyster
catchers, terns, pelicans, ducks, and ospreys.
Reptiles include sea turtles, which consume
fish, inverts, and seagrasses.

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Mammals include otters, raccoons, nutria,
dolphins and sometimes whales.
4. ON-SHORE ECOSYSTEMS
Marine Ecosystems are distributed as on-
sore and off-shore ecosystems. The on-
shore ecosystems are very typical
ecosystems subjected to the everlasting
action of waves and tidal influences.

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The life of on-shore ecosystems are always
under the dynamic impact of various factors
including human interventions. They are
called as Foreshores.
Foreshores are the lands bordering the seas
and oceans. The geomorphology of
foreshores may vary from place to place and
based on the slope of continental margins.

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Foreshores are at the forefronts of the
continents, facing the action of waves and
tides directly. They are the zones of land-sea
interactions.
The abiotic factors controlling the fauna
and flora of foreshores are:
1. Geomorphic factors- Width, length,
Slope, thickness and Geology of the
coastline.

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2. Hydrographic factors- Water and its
salinity.
3. Light
4. Temperature, Air, Wind and humidity.
5. Tides, Waves and mean sea level.
The notable fore-shore ecosystems are
a) Sandy foreshore,
b) Shingle foreshores,
c ) Lagoons,
d) Muddy foreshores

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e) Salt-marshes and
f) Rocky foreshores.
Foreshores are aesthetically pleasing
environments.
The following are the characteristic features
of the shore life:
1. Littoral forms shelter under rock pools,
boulders and crevices.

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2. Shore animals have a flattened shape for
hiding in narrow spaces.
3. Organisms, subjected to the dehydration
problems (due to their exposure to air) have
protective covers.(thick cuticle).
4. Surface-dwelling animals have strong
shells with orifices.
5. Shore animals lose waters by excretion.
They excrete ammonia as their chief
nitrogenous waste product. This is toxic and
has to be eliminated by dilution.

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6. Shore dwelling animals perform aquatic
respiration, in the littoral fringe and aerial
respiration on exposure to air.
7. Benthic organisms start their life as
floating or swimming forms. Shore creatures
have the risk of losing pelagic eggs and
larvae stages due to drifting currents. Some
eliminate the pelagic stage and emerge to an
adult form.

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8. Physiological and behavioural adaptations
are properly done by these organisms, even
to withstand the twelve hour tidal cycles.
9. A zonation exists in the population of
plants and animals.
A beach is the land bordering the sea and is
defined as an accumulation of sediment laid
by the waves and tides. It usually consists of
sand or gravel.

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An active beach, the area of loose sediment
subject to transport by wind, waves, and
currents, is divided into three regions:
the backshore, the foreshore, and
the offshore.
A beach is characterised by a coastal
upland, which can be a dune, a cliff, a soil
embankment, a fossil berm, or an
engineering structure such as a seawall or a
revetment.

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The geomorphic features of the beach
include berms, scarps, and offshore sand
bars. Beaches are dynamic ecosystems.
Beach sediments may contain shells of
recent to subrecent life fossils, skeletal
remains from coral reefs, sand / silt
transported by waves and materials from the
river input.

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A sand beach will never be static but always
be a dynamic ecosystem. The major factor
is wave action. Large algae or rooted
vegetation can not thrive on shores due to
the lack of stable solid surface.
Lagoons are shallow bodies of brackish or
salt (sea) water partially separated from the
neighboring sea by barriers of sand or
shingle.

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The sea water can flow only through narrow
openings left by the barriers. They become
the coastal ponds or lakes, if they are
completely detached from the sea.
Muddy foreshores are the coastal mud-lands
or mudflats, containing mostly mud
organisms.
They are the feeding grounds for many
wading birds, wildfowl.

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Mud is characterized by its fine texture,
deposited under turbulence of sea water.
Mud has an adhesive quality.
Salt-marshes are the zones lying near to
shores periodically flooded by saltwater.
They are also called as tidal marshes, in
which the water level fluctuates tidally or
non-tidally. Due to the recurring failure of
monsoon, salt marshes extend further land
wards.

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The soil includes heavy to sandy clay, silty
sand or silty peat.
Life in this areas are subjected to tidal
changes and salinity variations. The
herbivores feed on standing marsh plants
and transform energy seasonally.

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5. CORAL REEFS
Coral reefs are major geological structures
built by biological activity.
`Corals' can be seen along some of the
beaches, looking as a piece of rock-like
material.

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The corals are the skeletons of many tiny sea
animals living side by side. Each one of
these animals is known as a coral " polyp".
A coral polyp is a soft, almost transparent
animal which builds its own skeleton outside
its body. Since it has no backbone, it is an
invertebrate.
Corals which produce a hard skeleton are
called stony corals.

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They have special cells on the outskirts of
their bodies which produce a limestone
substance. This makes them hard, like a rock
.The polyps sit in the cup like portions of the
skeletons.
Corals belong to a large group of
invertebrates which includes jelly fish, sea
anemones and hydroids.

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The types of corals are
A . STONY CORALS :
1. Stony corals
2. Finger corals
3. Fungus coral
4. Brain coral
5. Tube coral
6. Rose coral
7. Staghorn coral
8. Lettuce-leaf coral

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B. SOFT CORALS :
1. Sea fan
2. Bushy soft coral
3. Sea feather
4. Black coral.
The live hard corals can be divided into four
major forms:
a. Branching
b. massive
c. Tabulate / flat
d. Others.

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The coral growth needs a clean and warm
waters. The best temperature for coral
growth is 18°C.
In general, reefs form where the following
set of conditions are met:
Warm water, not cooling below 18°C
during the year.
Wave-swept, high energy coast.
Hard substrate.
Normal marine salinity water.

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Absence of abundant clastic detritus.
Shallow water (0 - 30 meters depth -
photic zone)
The ecosystem characteristics are varied
from that of the others.
1. Corals reefs are made up of much more
than corals and their skeletal remains. The
other organisms living in reefs, deposit
calcium carbonate (limestone) materials.

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2. Reefs provide the home, (food and
shelter) for a wider variety of plants and
animals. The spaces, sinks and cracks in the
hard coral mass, make suitable shelter and
feeding grounds for the marine worms,
urchins, sponges, sea fans, molluscs, crabs,
shrimps, sea anemones and fish.
3. The energy which cycles through the reef,
keeps all plants and animals alive: The
energy comes from the sun.

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The reef environment supports the
planktons. Plants are abundant: in reefs.
They provide the basic food for the entire
ecosystems.
There are many reef animals which eat the
plants and are eaten by other animals. They
are small, like crustaceans and worms.
There are many larger animals like sponges.
There are zooplanktons living at the base.

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Coral reefs can be classified based on their
Morphology, evolution, size, depth of
surrounding water, and relationship to
nearby land. Morphology is the major
criteria considered.
The major types of coral reefs are as follows
1. Shore Reefs - or - Fringing Reefs
2. Barrier Reefs
3. Atolls - as

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a. Deep sea atolls
b. Shelf atolls.
MINOR TYPES :
The minor reefs which could be seen are
1. Table reefs
2. The Faro
3. Micro atolls.
4. Knolls
5. Patch reefs and many more.

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Importance of Corals:
1. Coral seas are typical ecosystems, most
of them are shallow marine and a few are
deep marine
2. Coral ecosystems encompass a good
platform for varieties of natural resources
3. Coral seas are good spots for
ecotourism
4. Coral life are unique and exhibit a
natural museum of wonderful marine life,

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which include typical and beautiful plants
and animals
5. Corals are also used as ornamental
stones when they are naturally detached
from the basement
6. Coral ecosystems are good zones for
improving marine habitat.
7. Study of coral ecosystems provide
enormous data for biosphere- geosphere
interaction modeling

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8. Corals are storehouses of organic
calcium carbonate. These are good
compounds for pharmaceutical industries
9. Coral lagoons in some parts of the
globe are considered to be good fishing
zones. Eg. The atolls of the Republic of
Maldives are good examples.
10. It takes several decades to grow a few
corals. The existing corals are to be
protected and the species are to be saved.

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Ecology deals with organisms, populations,
communities, ecosystems and the biosphere.
Marine ecology deals with the study of the
environment and life in marine waters. It
involves the study of marine organisms and
their habitat.
Their global distribution are studied in
different branches of Ecology,
environmental geology, marine geology, and
geography.