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Nutrient Cycles in Marine Ecosystems


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Title: Nutrient Cycles in Marine Ecosystems

Nutrient Cycles in Marine Ecosystems
  • Inputs and outputs to the reservoir of dissolved
  • The biological uses of nutrients.
  • Nutrient availability and productivity.

  • Nutrient chemical that an organism needs to live
    and grow (or a substance used in an organisms
    metabolism) which must be taken from the

Demonstrate an understanding that there is a
reservoir of nutrients dissolved in the surface
layer of the ocean
  • Algae require light for photosynthesis.
  • Light intensity decreases as the depth of the
    ocean increases and therefore photosynthesis is
    restricted to a surface layer in which there is
    sufficient light.
  • This layer (referred to as the photic zone)
    varies in depth from about 30 m to 150 m,
    although it is considerably less is turbid water.

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Demonstrate an understanding that there is a
reservoir of nutrients dissolved in the surface
layer of the ocean
Demonstrate an understanding that there is a
reservoir of nutrients dissolved in the surface
layer of the ocean
  • The surface layer of the ocean contains many
    different ions, some of which are shown below

Demonstrate an understanding that there is a
reservoir of nutrients dissolved in the surface
layer of the ocean
  • These ions, together with nitrate and phosphate
    ions, form a reservoir of nutrients for the
    growth of algae and other primary producers.
  • Nitrate and phosphate ions occur at low
    concentrations in seawater
  • the mean concentration of nitrate is 0.5 parts
    per million (ppm)
  • the mean concentration of phosphate in seawater
    is 0.07 ppm.

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The Sea-Surface Microlayer
  • incredibly thin (few hundred µm)
  • important for the chemistry of the ocean
  • covers 71 of surface of the planet therefore
    it is the largest single ecosystem
  • not well understood (difficult to sample such a
    tiny vertical section of the water column)
  • critical link between ocean and atmosphere

  • Receives and transmits
  • Energy/gases/solids
  • Collects matter transported by winds from above
    and by water below

The Sea-Surface Microlayer
  • Especially dense concentration of minerals,
    organic chemicals, protozoans and
  • Upper 70 mm has dense concentrations of slightly
    larger organisms, including fish eggs, fish
    larvae, and crustaceans. Larger, floating
    jellyfish and seaweeds are found in the upper 30
    cm. There are many transient creatures that move
    up and down in tune to the sunlight.

  • The plants and animals that live in the water
    excrete many organic compounds, such as amino
    acids, proteins, and fatty acids that serve as
    nutrients for bacterial growth. These rise to the
    surface where they are concentrated the thin
    organic skin of the water. This happens in fresh
    water as well as salt water.
  • When aquatic organisms die, the oils in their
    bodies may float to the surface before they
    completely decompose.
  • The thin layer of oily material on the surface
    of the sea is an important part of the water
    cycle as it helps control the rate of
    evaporation. It is also a highly nutritious food
    source for many species of microscopic plants and
    animals (ie plankton).

The Sea-Surface Microlayer
  • Wind pushes the oil into long ribbons of calm
    water known as "wind slicks" or "wind rows."
  • You can see these on most days when looking at
    the sea from an overlook or from a boat.
  • Samples show the plankton and nutrients are
    thousands of times more concentrated in the
    windrows than in water only a few cm deeper or in
    adjacent areas.
  • Unfortunately, the oily surface of the sea is
    also the first to receive pollutants from the
    atmosphere. Scientists believe more than 30 per
    cent of all ocean pollution comes from tiny
    particles of dust and smoke in the air - often
    called fallout.
  • This settles on the most sensitive and vulnerable
    part of the ocean - its skin.
  • The pollutants contain pesticides, heavy metals,
    and industrial and motor vehicle toxins such as
    sulphuric acid, chlorine, and dioxin.

Wind Slicks
  • Windslick research link

The Sea-Surface Microlayer
  • A polluted surface microlayer has the potential
    to poison much of the complex marine food web,
    including fish, crustaceans, whales, and
  • Destruction of the microlayer may alter the
    exchange of materials between the atmosphere and
    the ocean, thereby affecting global climate.
  • Oil pollution also floats on the surface of the
    sea and quickly contaminates this fragile
    environment with chemical toxins.
  • Oil, even a very thin layer, spreading over the
    surface of the water at the same time fish are
    releasing their floating eggs can devastate their
    reproductive success

The Sea-Surface Microlayer
  • Heavy metals, and other toxins, are hundreds of
    times more concentrated in the surface windrows
    of the sea than in deeper water. Pesticides are
    found concentrated millions of times greater than
    in the rest of the water.
  • As the ozone layer in the upper atmosphere breaks
    down from air pollution, ultraviolet radiation
    increases. This has been shown to have a severe
    impact on the phytoplankton and the eggs of sea
    creatures when they concentrate at the surface.

Explain the process by which the reservoir of
dissolved nutrients is replenished, including
upwelling from land and dissolving atmospheric
  • Upwelling is the movement of water from deep in
    the ocean to the surface layer, where the
    nutrients become available to primary producers.
  • brought about by several processes including the
    deflection of deep water currents upwards and the
    movement of water away from a coast by the action
    of wind.

Upwelling refers to deep water that is brought to
the surface.
Areas of upwelling are created by surface winds
that pull water away from an area. This deficit
of water on the surface invites water to come up
from deeper regions.
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Explain the process by which the reservoir of
dissolved nutrients is replenished, including
upwelling, runoff from the land, and dissolving
of atmospheric gases
  • Upwelling is the movement of water from deep in
    the ocean to the surface layer, where the
    nutrients become available to primary producers
  • Upwelling brought about by several processes
  • Deflection of deep water currents upwards
  • Movement of water away from the coast (due to
  • Upwelling Animation

Mechanisms that create ocean upwelling
  • Wind
  • Coriolis Effect
  • Ekman Transport

Due to friction between the layers of water in
the ocean and the Coriolis Effect, the net result
of wind blowing across the surface of the water
is transportation of a layer of water 90 degrees
to the direction of the wind. This is known as
Ekman Transport.
To understand upwelling, you must be familiar
with how the Coriolis Force affects ocean surface
currents. The Coriolis Effect acts on moving
water, because it is not attached to the rotating
Earth. As water flows over the rotating earth,
it appears to deflect to the right in the
Northern Hemisphere and the left in the Southern.
  • Coriolis Effect link
  • Coriolis Video

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Seasonal upwelling
Onshore winds pile water up on shore, thus
surface water will be forced downward. This is
Offshore winds take water away from shore, thus
water from depth will upwell to the surface.
Even though upwelling areas account for only 1
of the ocean surface, they support 50 of the
worlds fisheries.
Productivity (phytoplankton growth) of an area
is determined by the rate and the duration of
  • Rate of upwelling determines phytoplankton cell
  • Duration of upwelling determines the total amount
    of phytoplankton.

small vs. large
few vs. many
-Moderate rates of upwelling for long duration (8
months or longer) provide the ultimate
combination for a large fishery. -With too low
or too high a rate, phytoplankton are small, so
there is a trophic level between the algae and
the fish.therefore the fish receive less energy.
Classification of upwelling systems in terms of
rate and duration
After Thurman, H.V. (1994)
Explain the process by which the reservoir of
dissolved nutrients is replenished, including
upwelling from land and dissolving atmospheric
  • Run-off from the land is part of the hydrological
    cycle and the water may leach nutrients,
    including nitrates and phosphates, from the soil.
  • Carbon dioxide in the atmosphere dissolves in
    seawater forming hydrogen carbonate ions (HCO3),
    making carbon dioxide available for fixation in
    the process of photosynthesis, by primary

Show that each of the nutrient cycles listed
below can be summarized as shown in Fig 4.1 and
state the biological use of each nutrient
  • nitrogen which is used to make proteins
  • carbon which is used to make all organic
  • magnesium which is used to make chlorophyll
  • calcium which is used to make bones, corals, and
  • phosphorous which is used to make DNA and bone

  • Figure 4.1 summarizes the ways in nutrients are
    cycled in marine ecosystems. Nutrients may be
    derived from both land and the atmosphere,
    forming a reservoir in the surface layer of the
  • From here, nutrients are taken up by living
    organisms and incorporated into food chains.
    Nutrients may be removed by harvesting, sinking
    to the sea bed, or incorporation into coral
    reefs. Nutrients from the sea bed may be returned
    to the surface layer of the sea by the process of

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Biological Processes Geological Processes
The Carbon Cycle
  • Key role in Earths thermostat
  • Absorbed by ocean, utilized by plants in
    photosynthesis, humans in digestion
  • Sinks (storage) in lithosphere (largest reservoir
    limestone and other sedimentary rock),
    hydrosphere (ocean), atmosphere (CO2) and in the
    biosphere (dead animals, wood, plants)
  • Released by fires, decomposition, volcanoes, and
    human respiration

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The Oxygen Cycle
  • Essential for animals
  • Taken in during respiration
  • Released by plants in photosynthesis
  • Disrupted by the same factors that disrupt the
    carbon cycle
  • Clear cutting of trees
  • Increased burning of fossil fuels
  • Pollution to phytoplankton containing water

The Nitrogen Cycle
  • 78 of troposphere is N2, 0 utilized in
  • Present in proteins, moves through food chain
  • Most complex cycle
  • Disrupted by
  • Burning fuel (releases nitric oxides)
  • Increased use of fertilizer
  • Removal from topsoil
  • Addition to aquatic ecosystems

Nitrogen Cycle
  • N2 gas is modified by nitrogen fixing bacteria
    into ammonia (NH3) (nitrogen fixation)
  • Bacteria turn nitrogenous waste and detritus into
    ammonia (ammonification)
  • NH3 is converted into nitrite (N02) which is used
    to produce nitrate (N03) (nitrification)
  • Other bacteria convert nitrite into gas which
    enters the air (denitrification)

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The Phosphorous Cycle
  • Very slow process
  • Found only in sedimentary rocks and water (not in
  • Released as rocks erode
  • Travels through food chain
  • Released by decomposition

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The Sulfur Cycle
  • Most stored underground
  • Released by volcanoes and swamps
  • Plants assimilate the sulfur
  • Bacteria break it down
  • 99 of all that reaches atmosphere is by humans
    (industries, burning fuel, refining petroleum)

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Explain the process by which the reservoir of
dissolved nutrients is replenished, including
upwelling from land and dissolving atmospheric
  • Atmospheric nitrogen gas is fixed by blue-green
    algae in Intertidal zones, resulting in the
    formation of nitrogen-containing organic
    compounds. In this way, nitrogen can enter marine

Explain the process by which the reservoir of
dissolved nutrients is replenished, including
upwelling from land and dissolving atmospheric
  • Some nutrients, including nitrates and phosphates
    are also recycled in the surface layer of the
    ocean as a result of excretion from zooplankton.

Demonstrate an understanding that the reservoir
of dissolved nutrients is depleted by uptake
into organisms in food chains.
  • One of the ways in which nutrients are removed
    from the surface waters of an ocean is by their
    uptake by primary producers, (phytoplankton) and
    their use for the synthesis of organic
  • EX, nitrate ions are used in the synthesis of
    amino acids and proteins. If the phytoplankton is
    eaten by zooplankton, the proteins will pass to
    the next trophic level.
  • Zooplankton may subsequently be eaten by small
    fish and, in this way, nutrients are passed along
    a food chain.

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Explain how productivity may be limited by the
availability of dissolved nutrients.
  • Inorganic nutrients, such as nitrate ions and
    phosphate ions are essential for the growth of
    primary producers. Since consumers depend on
    these primary producers for food, either directly
    on indirectly in food chains, the productivity of
    the primary producers will influence the
    productivity of higher trophic levels. In water
    where the nutrient levels are high, for example
    as a result of upwelling, the productivity is
    correspondingly high. One of the most productive
    ecosystems is the Benguela upwelling system, off
    the west coast of southern Africa.

Phytoplankton photosynthesize using specialized
color pigments called chlorophyll. Thus, Ocean
Color maps are another way to identify areas of
upwelling. Where on this ocean color map are
high phytoplankton concentrations?
Demonstrate an understanding that the nutrients
taken up by organisms in food chains may sink to
the sea floor in faeces or after death, may be
incorporated into coral reefs, or may be removed
by harvesting
  • Detritus (decaying organic materials), feces and
    dead organisms may gradually sink to the sea
    floor. This represents a loss of nutrients from
    the surface water. In deep water, these nutrients
    will tend to remain on the ocean floor, unless
    returned to surface waters by upwelling.
  • The growth of corals involves the deposition of
    calcium carbonate this represents another way in
    which nutrients may be removed from water.
  • Harvesting fish and other marine organisms, also
    results in the loss of nutrients from marine

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Stormwater Runoff
  • the most common pollutant of streams, rivers, and

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Demonstrate an understanding that the reservoir
of dissolved nutrients is depleted by uptake into
organisms in food chains
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Primary production is the total amount of carbon
(C) in grams converted into organic material per
square meter of sea surface per year (gm C/m2/yr).
General Marine Productivity
  • Factors that limit plant growth and reduce
    primary production include solar radiation and
    nutrients as major factors and upwelling,
    turbulence, grazing intensity and turbidity as
    secondary factors.
  • Only 0.1 to 0.2 of the solar radiation is
    employed for photosynthesis and its energy stored
    in organic compounds.

  • Macronutrients and micronutrients are chemicals
    needed for survival, growth and reproduction in
    large and small quantities, respectively.
  • Upwelling and turbulence return nutrients to the
  • Overgrazing of autotrophs depletes the population
    and leads to a decline in productivity.
  • Turbidity reduces the depth of light penetration
    and restricts productivity even if nutrients are

Productivity varies greatly in different parts of
the ocean in response to the availability of
nutrients and sunlight.
  • In the tropics and subtropics sunlight is
    abundant, but it generates a strong thermocline
    that restricts upwelling of nutrients and results
    in lower productivity.
  • High productivity locally occurs in areas of
    coastal upwelling, in the tropical waters between
    the gyres, and in coral reefs.

  • In temperate regions productivity is distinctly
  • Polar waters are nutrient-rich all year but
    productivity is only high in the summer when
    light is abundant.

Variations in Primary Productivity
Mixing plays an important role in the limitation
of primary production by nutrients.
  • Mixing plays an important role in the limitation
    of primary production by nutrients.
  • Inorganic nutrients, such as nitrate, phosphate,
    and silicate acid are necessary for phytoplankton
    to synthesize their cells and cellular machinery.
  • Because of gravitational sinking of particulate
    material (such as plankton, dead or fecal
    material), nutrients are constantly lost from the
    photic zone, and are only replenished by mixing
    or upwelling of deeper water.
  • Summer increased solar heating, reduced winds
    leads to vertical stratification (thermocline)
    which makes it more difficult for upwellings

Mixing plays an important role in the limitation
of primary production by nutrients.
  • Between mixing events, primary production (and
    the resulting processes that leads to sinking
    particulate material) constantly acts to consume
    nutrients in the mixed layer
  • In many regions, this leads to nutrient
    exhaustion and decreased mixed layer production
    in the summer
  • Even in the presence of abundant light not
    always a limiting factor!
  • As long as the photic zone is deep enough,
    primary production may continue below the mixed
    layer where light-limited growth rates mean that
    nutrients are often more abundant

Primary productivity varies from 25 to 1250 gm
C/m2/yr in the marine environment and is highest
in estuaries and lowest in the open ocean.
  • In the open ocean primary productivity
    distribution resembles a bulls eye pattern
    with lowest productivity in the center and
    highest at the edge of the basin.
  • Water in the center of the ocean is a clear blue
    because it is an area of downwelling, above a
    strong thermocline and is almost devoid of
    biological activity.

  • Continental shelves display moderate productivity
    between 50 and 200 gm C/m2/yr because nutrients
    wash in from the land, and tide- and wave-
    generated turbulence recycle nutrients from the
    bottom water.
  • Polar areas have high productivity because there
    is no pycnocline to inhibit mixing.
  • Equatorial waters have high productivity because
    of upwelling.
  • Centers of circulation gyres, which occupy most
    of the open ocean, are biological deserts.

The Sargasso Sea and Vertical Profiles
  • Although rate of productivity is very low for the
    open ocean compared to areas of upwelling, the
    open ocean has the greatest biomass productivity
    because of its enormous size.

  • Some heterotrophs (e.g., earthworms, flies,
    beetles, crabs, sea cucumbers, ants, vultures,
    hyenas, etc.) depend on detritus (dead organic
    material/biomass), rather than live organic
  • Note animals can be both a herbivore and a
    detrivore, or a carnivore or omnivore and a
    detrivore, i.e., they eat both living and dead

  • Decomposers play a very important role in
    mineralisation by breaking down organic
    substances into inorganic compounds that are
    again available for reuse by producers.