Ocean Studies
Introduction to Oceanography
American Meteorological Society
Chapter 9
Marine Ecosystems
© AMS
Case in Point
– Blooms of phytoplankton, microscopic algae, occur
widely in surface ocean waters.
• Sometimes a bloom of a single species occurs and can
cause serious harm to the environment, the demise of marine
organisms, economic losses, and human health problems,
including illness and death.
• Amnesic shellfish poisoning is one of the many possible
consequences of harmful algal blooms (HABs) or red
tides.
– Red tides are becoming more frequent worldwide.
– Blooms of toxic algae have been blamed for fish kills
and the deaths of marine mammals around the world.
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Case in Point
– A second type of
harmful algal bloom,
known as a high
biomass bloom,
depletes the supply of
dissolved oxygen in
seawater.
– Harmful algal blooms
develop in the waters
of almost every
coastal state of the
U.S.
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Marine Ecosystems
• Driving Question:
– What are the basic components and structure
of marine ecosystems and what is their
source of energy?
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Marine Ecosystems
• In this chapter, we examine:
– How interactions among the biosphere, atmosphere, and ocean
govern the distribution and abundance of life in the ocean
– The components of marine ecosystems including producers,
consumers, and decomposers
– The processes operating within marine ecosystems including
energy supply for growth and reproduction, factors influencing
biological production in the ocean, the role of nutrients and trace
elements as limiting factors, and the importance of microbes in
marine ecosystems
– The physical and biological processes governing the flux of
carbon into and out of the ocean over a range of time scales
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Requirements for Marine Life
– One major difference between life in the
ocean and life on land is the much greater
space and variety of marine habitats.
– In addition to water and energy, marine life
forms require nutrients and trace elements in
sufficient quantities.
– According to the law of the minimum, the
growth and well being of an organism is
limited by the essential resource that is in
lowest supply relative to what is required.
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Structure of Marine Ecosystems
– An ecosystem is a fundamental subdivision
of the Earth system in which communities of
organisms interact with one another and with
the physical conditions and chemical
substances of their habitats.
• All ecosystems have both living (biotic) and
nonliving (abiotic) components plus energy
sources.
• Biotic components include producers, consumers,
and decomposers whereas abiotic components
constitute their physical and chemical environment.
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Structure of Marine Ecosystems
• PRODUCERS
– Photosynthesizing organisms
– Plants, algae, and some bacteria
– Floating, microscopic unicellular algae, photosynthetic
bacteria, and other groups of organisms capable of
photosynthesis are responsible for more than 99% of
biological production in the photic zone.
• referred to as phytoplankton
• diatoms, coccolithophorids, dinoflagellates, and bacteria
– The smallest and probably the most numerous of
phytoplanktonic organisms are certain groups of
bacteria.
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Structure of Marine Ecosystems
• PRODUCERS
– Diatoms are a diverse group of protista (or
protoctista) having properties of both plants
and animals.
– Coccolithophorids are a single-cell
photosynthesizing organism covered with tiny
calcium carbonate plates.
• Appear to thrive in nutrient-poor waters, forming
large blooms that turn surface waters greenish
blue as viewed from space
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Structure of Marine Ecosystems
A large red tide formed by the
dinoflagellate Noctiluca.
Coccolithophorid bloom in
the Bering Sea off Alaska
© AMS
Structure of Marine Ecosystems
• PRODUCERS
– Dinoflagellates have two flagella (thread-like
structures) and no solid covering so that they
are not preserved in marine sediment
deposits.
• some species form red tides
– The smallest and probably the most
numerous of phytoplanktonic organisms are
certain groups of bacteria.
• lack internal cellular structure such as nuclei,
chloroplasts and mitochondria
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Structure of Marine Ecosystems
• PRODUCERS
– Scientists have discovered
extraordinary ecosystems
in the deep ocean in which
chemical energy, rather
than sunlight, drives the
basic biological processes
• referred to as
chemosynthesis
– The total amount of carbon
converted from carbon
dioxide into organic matter
is known as primary
production
• referred to as carbon
fixing
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Cyanobacteria are the most
abundant photosynthetic
organisms in the ocean
Structure of Marine Ecosystems
• CONSUMERS
– Single-cell and multi-cellular consumers that
drift passively with ocean currents or are
weak swimmers are referred to collectively as
zooplankton.
• include bacteria and dinoflagellates as small as 2
micrometers
• most zooplanktonic organisms are suspension or
filter feeders
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Structure of Marine Ecosystems
• CONSUMERS
– Most ubiquitous members of the zooplankton are
copepods, a diverse group of tiny crustaceans
covered with an exoskeleton
• Include both herbivorous and carnivorous species
• Play an important intermediate role in marine food chains,
feeding on phytoplankton and, in turn, being eaten by larval
and juvenile fish
– Larger members of the zooplankton community
include euphausiids and other shrimp-like
crustaceans called krill.
– Zooplankton is the main food source for larger
consumers in the sea.
© AMS
Structure of Marine Ecosystems
Copepods are tiny crustaceans
that are an important food
source for juvenile fish and
shellfish.
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Euphausiids (krill) occur in large
swarms in the Southern Ocean
and feed on algae on the under
surface of sea ice
Structure of Marine Ecosystems
• DECOMPOSERS
– Consumers that feed on dead organic matter,
either on the ocean bottom or in the water
column
– Break down organic matter and recycle
nutrients back into the ecosystem
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Structure of Marine Ecosystems
• TROPHIC
STRUCTURE OF
ECOSYSTEMS
– Complex of linked food
chains is described as
a food web
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• Each feeding position
occupied by an
organism within a food
web is called a trophic
level.
• Energy is transferred
from lower trophic
levels to higher trophic
levels.
Structure of Marine Ecosystems
• TROPHIC STRUCTURE OF ECOSYSTEMS
– Ecological efficiency is defined as the fraction of the
total energy available at one trophic level that is
transformed into work or some other usable form of
energy at the next higher trophic level.
• Varies depending on types of organisms and ecosystems
– A complex food web with diverse food sources tends
to be more stable and less vulnerable to
environmental change than is a simple ecosystem.
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Structure of Marine Ecosystems
• BIOACCUMULATION
– Some persistent toxins move
from one trophic level to the
next, building in concentration
along the way as one
organism consumes another.
• Process of continually
increasing concentration
within a food web is called
bioaccumulation
• Consequence of the
ecological inefficiencies that
prevail between trophic
levels—the biomass declines
at each successively higher
level but the toxins remain
© AMS
Field studies in the freshwater
ecosystem of Saginaw Bay, Lake Huron,
MI found bioaccumulation of PCBs in
successively higher trophic levels
Ecosystem Processes
• ENERGY FOR GROWTH AND
REPRODUCTION
– Photosynthesis converts solar energy into chemical
energy.
– Because solar radiation is essential for
photosynthesis, this process occurs only during
daylight and primarily in relatively shallow waters.
• Most sunlight is absorbed very near the ocean’s surface;
typically 80% is absorbed in the upper 10 m (33 ft) of water.
• The depth to which sunlight penetrates ocean water depends
on the clarity of the water.
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Ecosystem Processes
• ENERGY FOR GROWTH AND
REPRODUCTION
– Energy-releasing chemical reactions can
occur in the ocean without sunlight.
– Many microbes synthesize organic matter,
obtaining energy from chemical reactions
fueled by energy-rich substances, such as
hydrocarbons (petroleum or methane gas),
hydrogen sulfide, and various metals.
• discharged in the waters of hydrothermal vents
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Ecosystem Processes
• PRODUCTION IN THE PHOTIC ZONE
– Net production: the amount of organic matter
produced during photosynthesis that exceeds the
amount consumed via cellular respiration
• confined to surface sun-lit waters
– The total amount of carbon fixed into organic matter
through photosynthesis in a given unit of time is the
gross primary production.
– No net primary production occurs below the
compensation depth—usually the ocean depth
where the light level diminishes to about 1% of what it
is at the surface.
– Primary production using recycled nutrients is called
© AMS regenerated production.
Ecosystem Processes
• PRODUCTION IN THE PHOTIC ZONE
– Primary production requires sunlight, nutrients, and
phytoplankton (above the compensation depth) and varies
with both location and season.
– Primary production is very low in much of the tropical
ocean due to a lack of nutrients.
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Ecosystem Processes
• PRODUCTION IN THE PHOTIC ZONE
– At temperate latitudes, production varies with the
season.
– In winter, sunlight is weak and surface waters become
more dense and sink.
– In spring, more intense solar heating reestablishes
the pycnocline and stratification, halting the deep
mixing of autotrophs.
• Conditions are favorable for a dramatic increase in
phytoplankton populations, an event known as the spring
bloom.
– Explosive growth of zooplankton populations that graze on the
phytoplankton
– In polar regions sunlight limits production although
ocean stratification is generally absent and nutrients
© AMS are abundant.
This satellite image
shows the spring bloom
around Arctic Canada
in May 2003. Areas of
high chlorophyll
concentration are shown
in red, orange, and yellow
whereas areas of lower
production are in shades
of green and blue.
© AMS
Ecosystem Processes
• NUTRIENTS AND TRACE ELEMENTS AS
LIMITING FACTORS
– Some areas of the ocean have sufficient nitrogen,
phosphorus, and sunlight but still have low levels of
production.
• These areas are often in the middle of ocean basins, far from
land, and are known as HNLC regions (for high nutrients,
low chlorophyll).
• This condition might arise from the lack of some element that
usually occurs in trace quantities.
– iron is often a limiting trace metal in marine ecosystems
• HNLC regions of the ocean may be so distant from terrestrial
sources of wind-borne iron particles that input of iron-rich
dust is insufficient to make these areas productive.
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Ecosystem Processes
• MICROBIAL MARINE ECOSYSTEMS
– Microbes, single-celled organisms including
bacteria, apparently dominate life in the ocean
along with viruses and a group of organisms
known as archaea.
– Microbes act as both producers and
consumers.
• Contribute enormously to global oceanic primary
production and are especially important in open
ocean waters
© AMS
Ecosystem Processes
© AMS
SeaWiFS image of marine biological production during
2007. Areas of relatively high production are shown in
red, orange, and yellow whereas areas of relatively
low production are in shades of green and blue.
Ecosystem Processes
• MICROBIAL MARINE ECOSYSTEMS
– The ocean is home to three major groups of microbes: bacteria,
archaea, and viruses.
– Archaea and bacteria together form the class of organisms
called prokaryotes.
• All prokaryotes are unicellular
• Essentially no internal cellular structure
– Eleven different types of microbes (nine bacteria and two
archaea) dominate life in the ocean in numbers and probably in
mass.
– Many different types of unicellular eukaryotes of very small sizes
also inhabit the ocean.
• collectively known as protozoa
• include ciliates and dinoflagellates
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Ecosystem Processes
• MICROBIAL MARINE ECOSYSTEMS
– Bacteria and archaea are significant consumers in
marine food webs.
• Can directly absorb and digest dissolved organic carbon
(DOC) that cannot be used as food by larger organisms
– Bacteria and archaea are food sources for the
smallest heterotrophs, including dinoflagellates and
ciliates.
– The organic carbon they consume seldom reaches
the rest of the marine food web, but is recycled within
a semi-separate food web, the so-called microbial
loop that operates parallel to the larger food web.
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Ecosystem Processes
A simple microbial loop linked to a
schematic marine food chain
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Ocean’s Role in the Global
Carbon Cycle
– The ocean—primarily the deep ocean—is the
most significant reservoir of carbon in the
Earth system.
• Holds about 50 times the amount of carbon dioxide
contained in the atmosphere and about 20 times
the amount of carbon stored in the biosphere
– Two different sets of processes, biological and
physical, govern the cycling of carbon into
and out of the ocean.
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Ocean’s Role in the Global
Carbon Cycle
• PHYSICAL PUMP
– An important process controlling the ocean
subcycle of the global carbon cycle
– Primarily physical in nature and is known as
the physical pump or solubility pump
– Acts in concert with ocean heat transport and
heat loss to the atmosphere to convey carbon
to the deep ocean where it may be
sequestered for millennia
© AMS
Ocean’s Role in the Global
Carbon Cycle
• PHYSICAL PUMP
– Situated above Antarctic Bottom Water (AABW) is a
large water mass that encircles the entire Southern
Ocean: Circumpolar Deep Water
– Produced by mixing of AABW with the overlying deep
waters of the Atlantic and Pacific Ocean basins
– Circumpolar Deep Water circulates slowly throughout
the ocean on a time scale of 1000 years
• Gradually warms and diffuses upward into the surface layer
where CO2 is released (outgassed) to the atmosphere
because carbon dioxide is less soluble in warmer water
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Ocean’s Role in the Global
Carbon Cycle
• BIOLOGICAL PUMP
– The downward transport of particulate organic carbon
© AMS
Biological processes that control the distribution of
carbon in the ocean constitute the biological pump.
Ocean’s Role in the Global
Carbon Cycle
• BIOLOGICAL PUMP
– Eighty percent of primary production in the ocean
takes place in the photic zone of the open ocean and
roughly equals the amount of organic matter
produced by all land plants annually.
– In the photic zone, phytoplankton and other
photosynthetic organisms convert inorganic carbon
into organic carbon and organisms grow and
reproduce rapidly.
• Particulate organic carbon (POC): carbon contained in
marine snow plus the carbon in the sinking bodies of
zooplankton and other marine animals
• POC may be eaten by zooplankton or decomposed by
bacteria or other microbes and converted back to dissolved
inorganic carbon.
© AMS
Ocean’s Role in the Global
Carbon Cycle
• BIOLOGICAL PUMP
– Particles of organic matter that settle to the sea floor
and are not recycled by bottom-dwelling organisms
are buried in sediment deposits, removing the carbon
they contain from the water and the biosphere for
perhaps millions of years.
– Another way in which organic carbon is recycled in
the ocean is by re-mineralization, by being converted
back to soluble inorganic forms.
– Cycling of carbon into and out of the ocean is a key
player in Earth’s climate system.
– Carbon dioxide fluxes at the interface between the
ocean and atmosphere vary greatly from one region
of the ocean to another.
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Ecosystem Observations and
Models
– Studies of the oceanic carbon sub-cycle on various
temporal and spatial scales rely on data obtained
remotely by satellite-borne instruments.
• NASA’s Sea-viewing Wide Field-of-View Sensor (SeaWiFS):
maps ocean color (the distribution of photosynthetic pigments
and particularly chlorophyll-a, the most significant
photosynthetic pigment in marine algae).
• Advanced Very High-Resolution Radiometer (AVHRR)
sensor onboard NOAA’s polar-orbiting satellites measures
sea surface temperatures on a global scale.
– For information on ocean properties at depths below
the range of satellite sensors, ocean scientists rely on
in situ observations taken at specific locations over a
lengthy period, spanning years, decades, or longer.
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Ecosystem Observations and
Models
– These time series data permit scientists to construct
numerical models ranging in complexity from those
applicable to a single location, to three-dimensional
models of the global ocean.
• The principal goals of these models are to identify processes
and explain observations, combine many types of data to
improve estimates of biogeochemical fluxes, and improve
predictability of future ocean states.
• Another major goal of ocean modelers is to produce coupled
models that integrate biogeochemical and physical
processes.
– Biogeochemical aspects of ocean modeling are
essential and data are being collected, processes are
being studied, and the gap between physical
modeling and biogeochemical modeling is closing.
© AMS
Conclusions
– Marine ecosystems are composed of producers,
consumers, and decomposers and their energy is
supplied via photosynthesis or chemosynthesis.
– Persistent toxic chemicals may enter marine food
webs and bioaccumulate, threatening the well being
of organisms (including humans) feeding at higher
trophic levels.
– Primary production in the ocean’s photic zone varies
spatially and temporally because of seasonal
changes in sunlight, weather, fluctuations in nutrient
supply, the availability of trace elements, stratification
of ocean waters, and upwelling.
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