Biological oceanography

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BIOLOGICAL OCEANOGRAPHY
By
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science,
University of Mysore
Mysore

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Introduction:
Seas and Oceans occupy about 71% of the Earths
surface.
The total volume of water under marine
environment is 1370 million cubic km.
The space available for marine life is 300 times
more than the space available for other aquatic or
terrestrial life on land. It is also believed that the
earliest organisms originated from the ancient
oceans, many million years before.

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The first texts of Oceanography were published by
Charles Wyville Thomson in 1873, entitled as
the “The Depths of the sea”. His initiation made
the Challenger Expeditions of 1872-76 in which
people traveled for 110, 900 km visiting all the
major oceans except Arctic.
Detailed surveys were made with respect to the
physical features, chemistry and biology of oceans.
The Challenger expedition also attempted to
integrate the geology, biology, chemistry and the
physico-chemical phenomena of the oceans.

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It was carried out for about 19 years by a group of
76 scientists.
The first seafloor map was produced with details
of life existing at greater depths. About 715 new
genera and 4417 new species of marine organisms
were described by the great German Biologist
Ernst Haeckel.
Since 1872, different countries conducted many
major biological oceanographic expeditions.

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The study of marine life has seen more and more
such findings in course of time.
Biological oceanography gradually grew into a
major scientific discipline with all these
observations of marine organisms and their
environments.
New tools and techniques were adopted from
sonar methods to satellite borne sensors to study
the oceans.

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These include sonar, underwater sound recording
devices, submarines, scuba diving and sensors for
continuous recording of oceanography
parameters.
In this episode, the following modules are
highlighted:
1. Nature of marine environment
2. Classification of Marine environment
3. Role of abiotic parameters on marine life
4. Marine Flora
5. Marine Fauna

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I. NATURE OF MARINE ENVIRONMENT
Historical Development:
The interest to study biology by Humans started as
early as fourth century BC when Aristotle
described about 180 species of marine animals.
The geographical knowledge of oceans got
improved after the great sea expeditions of the
15th and 16th centuries.
Edward Forbes, a British Naturalist (1815-1854) is
the founding father of Oceanography, who has
systematically studied the marine biota and
benthic marine animals.

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It was he, who specified that different species
occupy different depth zones. His Nephew, James
Ross during 1839-43 collected the benthic animals
as deep as 730m, and gave a lot of information to
others. Marine ecosystem is characterized by the
following biotic and abiotic components:
Biotic components:
Organisms and species ,Predators, Parasites,
Competitors and Mates.

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Abiotic Components (Physical and Chemical)
Temperature , Concentration of Nutrients
Sunlight
Turbulence
Salinity and density.
Temperature decreases and light diminishes with
depth.
Hydrostatic pressure increases with depth.
Nutrients become more concentrated with depth.
About 2% of the total human food consumption
comes from the marine species.

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The economic utilization of marine natural
resources is very high. Hence, biological
oceanography is a rapidly developing field of
marine sciences and Earth sciences.
Marine environments have altogether different
kinds of Physical conditions for life to survive.
Marine species are buoyed up by water and need
not have to store large amount of energy is skeletal
material. Majority of marine plants are floating
species and microscopic.

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Many marine animals are invertebrates. They do
not have massive skeletons.
For floating and swimming the animals require
little energy.
Water which is a fundamental constituent of all
living organisms, is available in plenty within these
marine environments.
Temperature is yet another parameter which do
not vary as drastically as seen on land or in air.

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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 with depth.
Much of the marine environment is in perpectual
darkness. Under such circumstances many of the
marine life depends on the availability of essential
nutrients only. Much of the decaying matter sinks
inside the seas.

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The zone just below the sea surface, experiences
the maximum environmental fluctuation. This is
the zone where there is air-water interactions,
variations in temperature, sality and turbulence of
water from winds.
All environmental parameters show predominant
vertical gradients.

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II. CLASSIFICATION OF MARINE
ENVIRONMENT
It is necessary to understand the basic ecological
divisions of the ocean (Fig) Marine environments
can be subdivided into two divisions as Pelagic
and Benthic environments.
The word Pelagic means ‘Open sea’ and benthic
means ‘bottom’, Pelagic refers to the ocean water
column from the surface to the greatest depth.

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Benthic refers to the deep ocean floor. In the open
sea, the zone extending from the high water and
low water coastal belt, upto a depth of 200 m
inside the sea is called as Neritic zone.
The Pelagic zone is further subdivided into
a) Epipelagic zone – extends upto 200m from
the ocean surface
b) Mesopelagic zone – extends upto 1000m
below epipelagic zone

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c) Bathypelagic zone – extends from 1200 to
4000m below the ocean surface
d) Abyssopelagic zone – extend beyond 5200 m
and upto 6000m from the surface
e) Hadal Pelagic zone – beyond 6000m and upto
10000m below the ocean surface.
Benthic environments are further classified into
a) Supra littoral zone – on the beach, high water
line
b) Littoral zone – High water to low water line
region

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c) Sub littoral zone – Low water line to 200m
depth on continental shelf.
d) Bathyal zone – 200m to 3000m depth zone
e) Abyssal zone from 2000m to 6000m depth on
continental slope region
And
f) Hadal zone lying beyond 6000m upto the end
of 1000+m depth in the deep oceans.
The Pelagic environment supports the life of
Planktons and Nektons.

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It is seen that the epipelagic zone is the sunlit zone.
Enough amount of sun light penetrates into the
top layer of oceans for plants to carry on
photosynthesis.
The mesopelagic zone is known as twilight zone.
This is a dim zone where little light penetrates,
but not enough light for plants to grow.
The bathypelagic zone is known as the midnight
zone.

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This is the layer between 1000m and 4000m
depth where there is no light penetration.
The abyssal zone is the pitch-black bottom layer of
the oceans. The oceanic water present in this zone
is almost freezing and its pressure is immense.
The Hadal zone is the last deepest zone. This is the
most inhospitable zone of the oceans.
Plants are found only in the sunlit zones where
there is sufficient light for photosynthesis.

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Animals are found at all depths of the oceans
though their numbers are greater near the surface
where food is plentiful. Still over 90% of all
species dwell on the ocean bottom where a single
rock can be home to over ten major groups of
organism like corals, mollusks and sponges.
Almost all marine life depends directly or
indirectly on microscopic algae found only at the
ocean surface. Hence, most of the animals in the
oceans live in the sunlit zone or migrate to it in
search of food.

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Some animals eat only plants.
III. ROLE OF ABIOTIC PARAMETERS ON
MARINE LIFE
The ecology of seas depends on the biotic and
abiotic physical and chemical parameters. Nature
of sea water, properties of sea water, interaction of
sea water with atmosphere, solar radiation and
density differences, wind driven forces and
hydrostatic pressure are the abiotic aspects
controlling the oceanic marine environment.

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The significant roles played by the following
parameters are to be understood:
1. Solar Radiation
2. Temperature
3. Salinity
4. Density
5. Pressure
6. Surface currents

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Solar Radiation:
Sunlight is the most important parameter for
many marine life in the sea. Only a fraction of
sunlight can penetrate thought the surface zone
and help in carrying out the photosynthesis by
plants. The energy is consumed for the conversion
of inorganic matter to organic compounds. Some
amount of radiation is absorbed by water
molecules and converted to heat. This heat
controls the temperature variations of the oceans.

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The depth-wise distribution of plants and animals
is also controlled by the light. There is also a
periodic change in the solar radiation and light
penetration. These help the vision in animals,
their migration and breeding periods within the
sea.
Radiation at the sea surface:
Solar radiation at the sea surface and the vertical
variation in the light intensity are to be measured
in proper units.

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The notable light units used for biological studies
of oceans are
a) Einstein (E) which measure Photons and
b) Watt (W) which measures the energy of
radiation.
The energy of radiation depends on the
wavelength of the light.
Photosynthesis radiation happens between 400
and 700 mm.

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One Einstein is mole of Photons
(OR) 6.02 x 1023 Photons
One Wm-2 is approximately equal to 4.16 E m-2 S-1
About half of the solar energy is absorbed (or)
scattered by various layers of the atmosphere only
50% of it is reaching the sea surface. In this, some
portion is reflected back to the atmosphere
depending upon the angle of incidence.

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The amount of radiant energy reaching the surface
is a function of the sun angle, the length of the day,
time of the day, time of the year, the latitude, and
the prevailing weather conditions.
The temporal variations of radiation may be
a) Diel variation – change between day and
night
b) Diurnal variation – only during day time
c) Seasonal variation – between seasons,
especially at high altitudes.

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Radiation in the sea:
The properties of water are very unique when
compared to other liquids. Water is normally
transparent to solar radiation.
About 50% of light which penetrates the sea
surface is found to be having the wavelength
longer than 780 nm.
This part is the IR radiation which is absorbed and
converted into heat in the upper surface of the
oceans.

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The visible portion between 400 and 700 nm
penetrates further below the sea and helps many
life to survive.
This portion is known as Photosynthetically Active
Radiation (PAR) as it helps phytoplanktons to
carryout in photosynthesis.
Much of the energy is absorbed and scattered on
further penetration.

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The intensity of light also decreases with reference
to depth.
This is called attenuation of light and is also
referred by a parameter called Extinction
coefficient.
The extinction coefficient depends on the amount
of colored, dissolved organic substances present in
seawater and the chlorophyll pigments of
phytoplanktons.

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Based on the light intensity and relative
penetration of light in the sea, three vertical
ecological zones have been identified as:
a) Euphotic zone
b) Disphotic zone
c) Aphotic zone
The shallowest zone is the euphotic zone just
below the sea surface. Light penetration fully
supports all phytoplanktons to grow and
reproduce.

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The respiration loss is also balanced by a
compensation depth which is the lower boundary
of Euphotic zone.
The dimby lighted zone below the euphotic
layer is the disphotic zone.
The last layer which is under complete darkness
where sunlight can not reach is the aphotic zone.

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Temperature in Ocean
Many marine processes are controlled by the
temperature of water. They may be physical,
chemical and biological processes. Temperature
and salinity of oceanic waters determine the
density. Due to this, the vertical water circulation
and movements are fully controlled by all these
three properties.
Exchange of heat happens between ocean and
atmosphere continuously.

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There is also a wide range of temperature
variation on the sea surface. It exceeds 300C in
tropical oceans, 400C in shallow seas, and as low as
– 1.90C in Polar Regions.
Oceans are cooled by evaporation. A good amount
of heat is transferred during this transformation of
water into water vapor.
Ocean surface temperature fluctuates with
reference to days, months, seasons and years. It
also varies with reference to Polar, tropical,
subtropical and temperate zones of the globe.

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The surface turbulent waters also transfer heat
downwards. Due to this, the uppermost part of
Oceanic water has a relatively raised temperature.
This gets decreased at a depth of 200 to 300m and
upto 1000m.
The water layer showing the steepest temperature
gradient is known as thermocline. The zone
showing the rapid changes in density of water is
known as Pycnocline. Pycnoline acts like a barrier
to vertical water circulations, and also animal
movement.

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At 2000 – 3000, the oceanic water temperature,
never rises above 40C. It also goes upto 00C to 30C
in deeper zones. (Fig).
Salinity of Oceans
Seawater is a Unique water. It contains more
dissolved salts than river, lake and rain waters.
The Salinity is refered interms of total dissolved
inorganic ions and other compounds and gases.
The average Salinity of the oceans is 35.

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When surface water gets evaporated salinity is
increased in the ocean surface. The Salinity gets
lowered due to rainfall, river water inflow and
after snow melts.
Vertical variation of Salinity is an important
limiting factor for marine life. The layer at which a
rapid change in Salinity occurs is known as
Halocline. Salinity varies with reference to
seasons and locations.
Marine life has unique physiological mechanisms
to cope up with salinity.

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Osmoregulation is one of the essential
mechanisms.
The marine life is also classified based on their
tolerance level of salinity.
The species which can tolerate a wide range of
salinity are called Euryhaline and those which can
tolerate a narrow range of salinity are called as
Stenohaline species.

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Density in Ocean Water
The density of seawater depends mainly on
salinity and temperature and to some extent by
hydrostatic pressure.
When salinity increases, density also increases.
Movement of water masses in the oceans are
controlled by all these 3 parameters.
Horizontal water movement- wind, temperature
and Salinity
Vertical movement of water- Temperature, Salinity
and density.

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The normal density of Seawater at the surface level
is very low.
When it increases, the water mass sinks down
below and reaches the appropriate strata of
matching density zone.
The sinking of water is called downwelling and
upward movement of water is called as upwelling.
Advection is the term used for horizontal and
vertical movements.

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Pressure:
Hydrostatic Pressure of oceanic water is
determined by the equation Sgh, where
g is the acceleration due to gravity,
S is the density of water and
H is the thickness of water column.
Pressure increases due to weight of overlying
mass. It is expressed in newtons per Sq. metre. At
10m depth, the pressure will be 105 Nm-2. This is
equal to 1 bar (or) 1 atom.

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Hence, the depth wise variation in pressure is
calculated based on this range.
Marine life existing in the deep oceans are
subjected to very high pressures.
The pressures may go upto 1000 atom in deep
ocean basins.
Some animals travel up and down for several
hundred metres and experience this change. Both
pelagic and benthic species do inhabit these
changes.

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They are called as eurybathic species. The marine
life which cannot tolerate such pressure variations
are called as stenobathic species.
Surface Currents
Ocean water surface currents are generated by
winds.
These water movements influence biological
productivity and nutrient availability. Due to this,
the geographical distribution of pelagic and
benthic marine species are also varied.

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Ocean surface currents are fully controlled by
global wind systems. Their directions are
modified by the earth’s rotation.
IV. MARINE FLORA
Phytoplanktons and Zooplanktons are the two
types of planktons existing in the shallow depth
layer of the oceans. Most of the phytoplanktons
are unicellular alge.

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They also include diatoms, green algae, yellow-
green algae, blue green algae, red algae,
silicoflagellates and dinoflagellates.
Cryptomonads, Prasinomonads, Chloromonads
and Chrysomonads are the other marine
phytoplanktons of the seas and oceans.
These are present throughout the lighted regions
of oceans.
Phytoplaktons are the major primary producers in
the pelagic realm. The rate of primary production
in plant mass is called as Primary Productivity.

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The total weight of all organisms in a given area
(or) volume is known as the biomass.
Physical controls of Primary Production
The properties controlling primary production in
oceanic shallow waters are:
1. Light
2. Physical forces
3. Abundance of nutrients
4. Temperature
It also varies with reference to seasons and
location.

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There are two general types of plants found in the
ocean.
They are those having roots that are attached to
the ocean bottom and those that are not having
roots which simply drift about with water.
The rooted plants are only found in shallow water
because of the availability of sunlight for
photosynthesis.

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The most abundant plants in the ocean are the
phyto planktons.
These are usually single-celled, minute floating
plants that drift throughout the surface of the
oceans.
The word Plankton means wandering or drifting in
Greek. Planktons are floating plants or organisms.
There may be Phytoplanktons denoting plants and
Zooplankton denoting the animals.

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Planktons are mostly microscopic dimension.
Nektons are free-swimming animals.
Some of the Phytoplanktons are passively
transported by the currents in the sea.
Fish, squid and marine mammals are the major
Nektons, of oceans.
Pelagic organisms are classified into various types
based
Virio Plankton – 0.02-0.2mm – Femto plankton
Bacterio Plankton - 0.2 -2.0 mm – Pico plankton

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Myco plankton - 2.0-20.0 mm – Nano plankton
Phyto plankton - 20-200 mm – Micro plankton
Protozoo plankton - 20-200 mm – Micro
plankton.
Metazoo plankton - 0.2 -20 mm Meso plankton
- 2.0 to 20 cm Macro plankton
- 20-200 cm – Mega plankton
A bucket of seawater might hold a million
microscopic diatoms which are relatives of sea
weeds encased in glassy boxes.

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To grow, phytoplankton need nutrients from the
seawater and lots of sunlight.
The large quantities of diatoms and phytoplankton
give a color to the sea water.
V. MARINE FAUNA
Marine animals are divided into 3 groups:
1. Zooplankton
2. Nekton and
3. Benthos
Zooplankton are drifting animals and are usually
small but grow to fairly large size.

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Eg: Jellyfish
The zoo plankton population also includes some
members like fish eggs or larval forms of
organisms which may grow up and leave the plank
tonic community to join the nekton or bethos.
Nektons are the free swimmers and the largest
portion of familiar population of animals found in
the oceans.

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Common fishes, octopus, whales, eels and squid
are all examples of nekton.
Whales, Sea mammals, dolphin and porpoise
codfish/trout
The third type of sea animals spend their entire life
on or in the ocean bottom. This group of marine
animals is called benthos.
Lobsters, starfish, various norms, snails, oysters,
etc.

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All marine life need food.
Plants make their own food but animals obtain
food from their environment. A food chain
represents the transfer of body-building
substances and energy when one organism eats
another. Diatoms form the first link in the marine
food chain.
In the ocean, there are innumerable individual
food chains overlapping and intersecting to form
complex food webs.

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Most marine creatures eat a variety of foods. The
rich diversity of life in the sea forms a delicately
balanced.
Network of predators and prey.
All organisms are dependent on one another for
survival.
Marine Zooplanktons animals range from
microscopic unicellular organisms to jellyfish size
which are several metres in diameter.

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They are of two kinds as
a) Holoplankton (Permanent Planktons-Spend
their entire life cycle in water column) and
b) Meroplankton (Temporary residents of
Plankton community)
There are about 5000 species of holoplanktons
existing in seawaters.
Many notable ones are
Forminifers, Radiolarians, Ciliates, Tintinids
Jellyfish, Cnidaria & Ctenophores.

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Arrow worms and some heteropods, Pteropods
Copepods, Ostrocods, Amphipods, Euphansiids,
Mysids, Decapods and Salps.
The Meroplanktons are miniature adults spending
a few minutes to several months and years in the
upper layers.
Most of them are larvae of benthic forms. They
include Snail religers, starfish, sea urchins,
barnacles and crabs.

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Planktons show vertical migration within every 24
hours.
This is called as diel vertical migration.
This occurs in many epipelagic and mesopelagic
species.
Marine Zooplanktons show three kinds of patterns
as:
a) Nocturnal migration - after sunset
b) Twilight migration - rising up at sunrise
c) Reverse migration- surface rise during the
day and descent to depths at nights.

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There are also seasonal vertical migrations
happening in the shallow waters for marine life.
Zoogeographic studies help understanding the
distributions of living organisms and the
physiological or ecological reasons behind these
mobilities.
Long–term observations show that plankton
abundance and species composition may change
with reference to time, climatic variations,
stratification and nutrient availability.

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Properties affecting marine life are:
1. Quality of sea water
2. Skeleton shape and size of organisms
3. Buoyancy
4. Gravity
5. Temperature of ocean water
6. Density
7. Light
8. Availability of Nutrients
9. Water turbulence
10. Hydrostatic pressure.

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