Download 1 The scale of life in the ocean ranges from the simplest single

WHAT IS THE SCALE OF OCEAN LIFE AND WHAT STRUCTURES SUPPORT THAT LIFE?
T
he scale of life in the ocean ranges from the simplest single-cell organisms to the
largest animal that has ever known to exist on Earth, the blue whale. This complex
and diverse web of life is intimately linked to the physical and chemical characteristics of the marine environment, such as dissolved oxygen or circulation patterns.
This theme (Life - Scale and Structure) describes the scale of life in the ocean and the various
structures that support that life, including current circulation patterns. It also discusses diversity
and classification of ocean life.
Related Themes:
• The origin of ocean life and changes over time are covered in Life - Process and
Change.
• Biotic and abiotic influences on marine ecology and the oceanic food chain is discussed in Life - Systems and Interactions.
• Information on energies associated with life in the ocean including photosynthesis
and hydrothermal vents is addressed in Life - Energy.
• How ocean factors such as pH are measured is found in Life - Measurements.
• Ocean energy and how the oceans’ surfaces interact with the atmosphere is discussed in Oceans - Energy.
• Water properties such as heat capacity and latent heat of vaporization are examined
in Climate - Scale and Structure.
• The effect of salinity, temperature, and density on the oceans’ vertical structure is
presented in Oceans - Scale and Structure.
• The dynamics of ocean gyres is addressed in Oceans - Systems and Interactions.
Related Activities:
• Describing “Classroom Communities”
• Dichotomous Keys
• Plankton Identification
• Observing Dissolved Air in Water
• Carrots, Celery, Dehydration, and Osmosis
INTRODUCTION
Life on Earth has its origins in the ocean and until very recently in our geologic history there
was no life on land. The complexity of abiotic factors of the marine environment determines the
various ecosystems and communities found in today’s ocean. Favorable physical and chemical
substrates facilitate the explosion of diverse communities. Other non-living factors that influence
the patterns of ocean life include large scale circulation, nearshore slopes, local tides, and storm
events.
An incredible diversity of life exists in the ocean. Plants and animals have many shapes,
sizes, colors, and textures. Dolphins, bass, seaweed, starfish, octopi, coral, whales, barnacles,
sharks, and eels are just a handful of the uncountable organisms that live in the ocean [Fig. 1].
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Figure 1. The diversity of ocean life. A wide variety of plants and animals live in the oceans. Our
oceans’ physical and chemical conditions favor many types of ecosystems.
Life is found on the ocean floor, on continental slopes, floating or swimming in the water. Life
thrives even thousands of meters deep in the ocean, far from the reach of sunlight, surrounding
chimney-like vents that release hot water rich with minerals.
Like life on the land, the diverse forms of life in the ocean are intimately linked by the flow of
energy and the cycling of nutrients. Large fisheries reflect the presence of actively growing microscopic plants. Likewise the oxygen of today’s atmosphere has its source in the photosynthetic
activity of the same small plants.
SCALE OF LIFE IN THE OCEANS
The smallest and the largest forms of life on Earth are found in the ocean. The range of sizes
goes from the tiny bacteria which are less than 1 micron (about one hundred thousandths of an
inch) in size to the blue whale which is 30 meters (100 feet) long. As on land, the number of
individuals is greater for smaller organisms than for larger ones.
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Plankton
Thus the base of marine life is a large complex group of organisms known as plankton. Plankton are not defined by size nor taxonomy, but rather because they are passively carried by water
motion. Plankton’s ability to swim is generally less than the strength of water movement. Planktonic organisms are generally small (less than a 1 mm or 0.039 inch), although larger jellyfish are
also considered plankton. The plankton include a wide variety of organisms such as algae, bacteria, single-celled animals, the larvae of some forms that live on a substrate, and crustaceans.
Plankton can be broadly divided into phytoplankton, the plants or photosynthetic organisms,
zooplankton, the animal component, and bacteria. Phytoplankton carry out photosynthesis and
are the base of the food chain in the ocean. Zooplankton are heterotrophic consumers. Bacteria
decompose organic matter into its abiotic constituents.
The organisms that float (plankton) or swim (nekton) in the water are known collectively as
the pelagic community. Plankton [Fig. 2] are the foundation of the ocean food chain. They are
eaten by organisms, which are, in turn, consumed by other organisms. Zooplankton can be
classified by whether they spend their whole lives as plankton or not. For example, small crustaceans are planktonic throughout their lifetimes. Barnacles and crabs begin as planktonic larvae
but develop into substrate-dwellers.
Figure 2. Plankton. Examples of phytoplankton (left) and zooplankton (right) seen through a microscope. These were collected off Dana Point, California.
Nekton
At the other end of the pelagic food chain are the active swimming marine organisms, or
nekton. This includes many species of fish and marine mammals. Nekton are often the top predators in marine food chains. However, the distinction between nekton and plankton is not always
clear. Some species, including marlin and tuna, spend the larval stage of their lives as plankton
and become large and active members of the nekton as adults. The vast majority of nekton are
vertebrates (for example, fishes, reptiles, and mammals), mollusks, and crustaceans. The most
numerous group of nekton are fish, with approximately 16,000 species. Nekton are found at all
latitudes and depths of marine waters. Whales, penguins, seals, and icefish abound in polar
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waters. Lantern fish, gulpers, and whalefish are found very deep, well below the depth of sunlight penetration.
The largest animals on Earth, blue whales, are members of the nekton. Blue whales spend
their summers in Arctic and Antarctic waters, and migrate to warmer waters in winter. They can
measure up to 30 meters in length (100 ft), and weigh up to 150 tons [Fig. 3]. The tongue of a blue
whale can weigh 4 tons, as much as an entire elephant!
These huge mammals feed on plankton as do whale sharks, which are the largest fish in the
world (usually 12 to 14 meters, or 40 to 50 feet). The largest carnivores include the toothed whales
(for example, killer whales), great white sharks, tiger sharks, and bluefin tuna.
Nekton form the basis of important fisheries around the world and are an important source
of food for humans. The exploitation of the large schools of anchovy, herring, and sardines generally accounts for one-quarter to one-third of the annual harvest from the ocean. Squid are also
Figure 3. The Blue whale. The largest animal to inhabit Earth is the blue whale. It consumes over five
tons of krill each day and grows to 30m (100 ft) in length.
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economically valuable nekton. Halibut, sole, and cod are bottom-dwelling fish that are commercially important as food for humans. They are generally caught in continental shelf waters.
Benthos
The organisms that live on the sediments of the ocean floor and along shores are known as
benthos. In shallow waters, beds of sea grass provide a rich habitat for polychaete worms, crustaceans, and fish. There are also benthic communities on rocky substrates.
Large algae or seaweeds are abundant in shallow sunlit waters. The mix of algae species
found in any place depends on latitude and also varies greatly with wave exposure and the
activity of grazers. Giant kelp is the fastest-growing plant. It can grow 1 meter (3 feet) each day
and become 30 meters (90 feet) long. They form dense beds or forests on rocky reefs, which are
home to many invertebrates and small fish.
Intertidal
On the surface of and within intertidal sediments, most animal activities are influenced strongly
by the state of the tide. Mollusks, barnacles, and herbivorous sea gastropods are abundant in
intertidal regions. Just below the tidal zones, sponges, ascidians, urchins, and anemones are
very common where the light levels are low and current speeds are high. Firmly or permanently
attached animals are often abundant and diverse in caves and under boulders, where wave action is less.
Coral Reefs
Reef-building coral polyps create a calcareous substrate upon which a diverse array of organisms live. Some of the world’s most complex ecosystems are found on coral reefs. The coral reef
results from a symbiotic relationship between zooxanthellae, a photosynthetic, single-celled organism that lives within the coral and the coral itself. Reef-building corals are found only in
waters warmer than 18ºC (64ºF) and in very clean water. Many tropical islands are composed
entirely of hundreds of meters of coral built atop volcanic rock.
ABIOTIC FACTORS THAT AFFECT OCEAN LIFE
Properties of Water
The water molecule consists of one atom of oxygen and two atoms of hydrogen. Its chemical
formula is H2O, or dihydrogen oxide. The hydrogen atoms are attached to the oxygen atom through
covalent bonds, a form of chemical bond where atoms share some of their outermost electrons.
Water is a roughly V-shaped molecule, with the oxygen atom at the apex. The V-shape leads to a
distribution of electrons such that the molecule behaves as if it had a negative side and a positive
side. As a polar molecule, water is very good at dissolving salts and minerals.
The heat capacity of water is very high. This means that it takes a great deal of heat to change
the temperature of a given amount of water. As a result, sudden or extreme temperature changes
in ocean water cannot occur--temperatures in the ocean rarely exceed 30ºC (86ºF), and rarely
drop below 2ºC (35.6ºF)--and the amount of heat transferred by ocean currents is large. This
large heat capacity has enormous consequences, not just for the ocean but for the climate of the
planet.
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Table 1. Properties of water.
Water ice is less dense than liquid water because ice has an open crystal structure. This is one
of many unique properties of the water molecule; most liquids occupy less volume when they
freeze. Water ice is less dense than liquid water because it has an open crystal structure. In fact,
pure water attains maximum density at a temperature of 4ºC (39.2ºF). The melting and boiling
points of water are significantly higher than the melting and boiling points of chemically similar
substances. If water behaved like most similar substances, it would be a gas at average Earth
surface temperatures and pressure [Table 1].
Salinity
Salinity is a measure of how much salt is dissolved in a given quantity of water. The salinity
of near-surface ocean water varies from place to place, but typically lies between 3.4 and 3.7%.
Salinity is often expressed as 34 to 37‰, where ‰ means parts per thousand, as shown in Figure
4. Dissolved salts affect the physical properties of water in several important ways. For example, salt water freezes at a lower temperature than fresh water. The vapor pressure of water
decreases with increasing salinity, thereby reducing the rate of evaporation. Water molecules
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Figure 4. Oceanic surface salinities map. Note that highest surface salinities occur in the
middle of ocean basins where the evaporation rate is high and rainfall rate is low. Low salinity
areas are often near sources of fresh water such as major rivers and melting ice.
move through semi-permeable membranes, such as cell walls, from regions of lower salinity to those
of higher salinity. The pressure required to stop the flow of water through a membrane is called
the osmotic pressure. Osmotic pressure increases with increasing salinity. This is a very important
factor in the metabolic processes of some forms of marine life.
Various physical processes can increase or decrease the salinity of seawater. Evaporation at
the surface of the ocean leaves surface water more saline because salt molecules, being relatively
heavy, do not easily rise into the air. The crystal structure of ice cannot hold much salt, therefore
when water freezes, the surrounding liquid water becomes more saline. Seawater salinity decreases when fresh water enters the ocean. Fresh water comes into the ocean primarily as rain
and snow, and runoff from rivers and melting ice. Ocean water mixing by currents can either
increase or decrease the salinity of seawater.
Ocean-atmosphere Exchange
Some fraction of the oxygen, nitrogen, and other gases in the atmosphere dissolves in water
at the ocean surface. The exchange between atmosphere and ocean is aided by wind-mixing and
wave breaking. At a given temperature and pressure, water can only dissolve a limited amount
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of any given substance. When this limiting value is reached, the water is said to be saturated in
that substance.
Most atmospheric gases are saturated throughout the ocean depths, with the important exceptions of oxygen and carbon dioxide. The concentration of these gases in the deep ocean is
controlled by biological processes. Carbon dioxide is consumed by plants through photosynthesis, which produces oxygen. Oxygen is, in turn, consumed by both plants and animals in respiration. This is why fish in an aquarium need a bubbler or aerator to maintain sufficient oxygen.
Seawater Acidity
The relative acidity of seawater is determined by the concentration of hydrogen ions in the
water. Acidity of a solution is quantified by a factor called pH which represents an exponent. For
example, a solution with a pH of 8 contains 108 grams of hydrogen ions per liter of solution. A pH
of 7 is considered neutral. A higher pH indicates an acidic solution, and a lower pH indicates an
alkaline solution. The pH of ocean water is remarkably stable due to the complexities of carbonate chemistry. When carbon dioxide dissolves in water, it forms a weak acid solution. If seawater becomes more acidic through the acquisition of hydrogen ions, bicarbonate compounds will
form. These compounds absorb acid, maintaining a buffer that tends to stabilize ocean pH levels.
Sunlight and Photosynthesis
The vast majority of life in the ocean is based on using sunlight for photosynthesis. Solar
radiation is rapidly absorbed by water and thus only penetrates to a certain depth. Absorption is
most rapid for very long
and very short wavelength radiations, visible
light penetrates further.
The layer of near-surface
waters where there is
light, and photosynthesis
is possible, is called the
photic zone [Fig. 5]. The
bottom of the photic zone
is defined as the depth at
which the light intensity is
about one percent of its
intensity at the ocean surface. The depth of this
layer at a given ocean location is not constant, because the angle of incoming sunlight varies from
day to day and seasonally,
however typical values
are a few tens of meters to Figure 5. The photic zone. This is the layer where most marine life exists.
a hundred meters.
Sunlight penetrates this thin layer and promotes the growth of plankton,
which are the base of the food chain.
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Pressure
Water pressure increases with ocean depth, and becomes extremely great at the bottom of
deep ocean trenches. In fact, the hulls of many modern submarines cannot withstand the pressure at depths of more than a few hundred meters. Marine organisms that have adapted to life
at high pressure can literally fall apart when brought to the surface because of the large pressure
change. Approximately every 10 meters (33 ft) of seawater adds one atmosphere of pressure,
which is the pressure felt at sea level. So the pressure at 30 meters (100 ft) depth is 4 times what
one feels at the surface (3 atmospheres worth of water, and 1 atmosphere from air).
BIOLOGICAL PATTERNS
Major surface currents and circulation patterns define large scale biological provinces for marine life. Distinct groups of marine organisms are supported by current gyres, which are very
large scale circular current patterns. The principal gyres are found at subpolar (about 60º latitude), subtropical (about 30º latitude), and equatorial latitudes. The subpolar gyre is characterized by relatively few species but a large total biomass, which is the total weight or number of
organisms. The subtropical gyre features a large number of diverse species, but a small total
biomass. The equatorial gyre includes both a large number of species and a large biomass. The
equatorial gyre shows a great deal of variation as a function of longitude. It should be noted that
most phytoplankton are very widespread and not limited to a particular gyre.
CLASSIFYING AND UNDERSTANDING THE DIVERSE LIFE IN THE OCEANS
Linnaeus System
Biologists today use a system to name and classify living organisms that was developed by
the Swedish botanist Carolus Linnaeus in 1758. In this system organism names fall into seven
nested categories or taxa. The seven taxa are kingdom, phyla, class, order, family, genus, and species.
Members of the same species can interbreed to produce fertile offspring and share the most
characteristics of any of the taxa. Members of the same genus evolved fairly recently (in terms of
the history of life on Earth) from a common ancestor, but cannot interbreed. Members of the
same family also evolved from a common ancestor, but one that lived much further back in time,
and so on. The two major kingdoms of life on Earth are the plants and the animals. The principal
distinction is that plants produce their own food through photosynthesis, or less commonly,
chemosynthesis, whereas animals must consume plants or other animals for food.
Genetic Mutations
Recent advances in the understanding of the molecular building blocks of life has led to a
new vision of the classical tree of life as defined by Linnaeus. The distance between individual
species is estimated in terms of the number of mutations in the genetic material. This internal
record of history has provided much information on one-celled organisms, particularly bacteria,
whose simplicity made it hard to distinguish them using more traditional methods. Our vision
of the evolution of life has been revolutionized by these new techniques but they have not changed
the classification schemes for most organisms.
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Dichotomous Keys
Once species are classified and their characteristics determined, there are several different
ways that a biologist can actually identify organisms in a particular ecosystem. Correctly identifying species is important because it allows scientists to describe and monitor changes within
species and between species, and to better understand the whole ecosystem. In the field, scientists identify plants and animals by carefully describing all distinguishing characteristics such as
shape, color, size, etc. They then find the species in field guides or by using a key, most commonly a dichotomous key. A dichotomous key uses a series of steps, each of which has two choices
to eventually classify the particular organism. For example, it is green or it is not green. Each
step leads to the next step in classification. In the case of fish, scientists would record the overall
shape, coloration, fin shape, and even behavior for correct identification.
Communities
Marine organisms are named and classified according to the conventions of the Linnaeus
system. However, marine biologists have also found it useful to recognize and classify ocean
communities. A community is a group of organisms living in a common environment. The common classification criterion for marine communities is depth: for example, near-surface, on or
just above the ocean bottom, etc. Each environment is characterized by the communities that
have adapted to its conditions.
For example, for many organisms on the ocean shore life is regulated by the rising and falling of
the tides. Tide pools are small pools
maintained by water brought in
and flushed out by tides [Fig. 6].
A wide range of life forms can be
found in tidal communities. The location of an organism relative to
the tide lines significantly affects
the physical conditions it faces.
For example, tide pool residents
must be able to cope with a highly
variable environment.
Organisms that live near the
top of the high tide line are submersed in water only occasionally,
Figure 6. Tide pools in Nova Scotia.
although spray from breaking
waves can wet regions above the high tide line. Organisms that live near the low tide line are
almost constantly submerged. Plants and animals that attach themselves to rock outcrops along
shorelines, including certain algae and barnacles, must be capable of remaining anchored against
the intense action of waves. As the tides recede, these organisms are often exposed to extreme
temperatures, harsh winds, and direct sunlight. They must therefore be fairly robust creatures in
order to survive. More delicate organisms can sometimes prosper in crevices or better protected
areas such as tide pools.
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Rocky shore communities are coastal communities, with the important distinction that the coastal
boundary is primarily rock. Rocky shore organisms include sea anemones, shore crabs, sea stars,
muscles, and barnacles [Fig. 7].
Figure 7. Rocky shore community. This community encompasses four zones whose placement is
based on local tides. The spray zone is covered by water only during storms. The high tide zone is
mostly dry, covered by the highest high tides but not reached by the lowest high tides. The middle tide
zone is covered by all high tides and exposed during all low tides. The low tide zone, covered during the
highest low tides, is exposed during the lowest low tides.
Sandy beach communities, as the name suggests, consist of those organisms that live on or near
sandy ocean beaches. Organisms that live in sandy beach communities include seaweed, beach
hoppers, and sand crabs [Fig. 8].
An estuary [Fig. 9] is a semi-enclosed body of coastal water connected to the open ocean. The
water in an estuary is significantly diluted by fresh water runoff from inland sources, and is
therefore much less saline that ocean water. Estuary communities are typically rich with a wide
variety of plant life.
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Figure 8. Sandy beach community. Zonation is distinctive on steep sandy beaches such as the one
illustrated. However, unlike on rocky shores, animals tend to bury themselves and so are more difficult to
see. The upper four zones are similar to those described in Figure 7. The sublittoral zone is found below
the lowest low tide. This benthic environment extends to 200m (656 ft) depth.
The pelagic ocean community refers to those organisms that occupy the bulk of ocean water
mass, from the surface to just above the ocean floor. The pelagic province is divided into two
major divisions. Water above a depth of 200 meters (650 feet) is referred to as the neritic division,
and water deeper than 200 meters (650 feet) is called the oceanic division.
Located below the pelagic province is the benthic environment. Benthic communities include those marine organisms that live just below, on, or just above the ocean floor. Benthic
organisms include clams and sea stars, as well as burrowing sea urchins, which feed on deposits
of organic matter within the ocean floor.
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CONCLUSION
An extraordinary variety of
diverse life forms live in the
ocean, from the smallest to the
largest organisms that have
ever lived on our planet. The
nature and distribution of marine life is strongly influenced
by abiotic factors, including the
physical properties of water
and ocean current circulation
patterns. The naming system
created by Linnaeus in the 18th
century is still used today to
classify living organisms. Various communities of marine organisms are also recognized,
defined by their common environment.
Figure 9. Rhone river estuary in south France. Estuary is the mouth
of a river, bay, or lagoon that contains both fresh and salt water as
influenced by tides. They are often host to many types of wildlife.
VOCABULARY
abiotic
alkaline
bicarbonate compounds
buffer
class
crustaceans
ecosystem
exponent
genus
heat capacity
invertebrate
man-of-war
mutation
nutrients
order
pH
acidity
barnacle
biological province
calcareous
community
dichotomous key
estuary
family
grazer
heterotrophic
ions
medusa
nekton
ocean communities
osmotic pressure
photic zone
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algae
benthos
biomass
chemosynthesis
covalent bonds
dihydrogen oxide (H2O)
evaporation
food chain
gyre
intertidal
kingdom
metabolic
neritic division
oceanic division
pelagic
photosynthesis
phylum
polar molecule
salinity
semi-permeable membrane
symbiosis
tidal community
vertebrate
phytoplankton
polyp
sandy beach community
species
taxon
tide pool
zooplankton
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plankton
rocky shore community
saturation
substrate
taxonomy
vapor pressure