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Plant growth in the
oceans:
Resources and controls
Plant growth
Resource limitation
Grazing control
Summary
Plant growth
Resource limitation
Grazing control
Summary
The most important plants in the
ocean are phytoplankton
These tiny, single-celled plants grow
simply by dividing into two new cells
If the conditions are good for
growth, the number of cells will
double each day
DAY 1
1 cell
DAY 2
2 cells
DAY 3
4 cells
DAY 4
8 cells
Population (number of cells)
Starts with one cell, one
cell division per day
This growth is exponential, and is
described by a model of the type:
Nt 2  Nt1 exp(k.(t2  t1))
where N is number of cells and t is time, and the
constant k is related to the doubling time:
ln(2)
k 
doublingtime
The exponential model is obviously
correct in describing the process of
continuous cell division, but it predicts
an unrealistic ever-increasing biomass
DAY 30 535 million cells
In reality, there is a limit to the
amount of phytoplankton biomass
that a habitat will support, so the rate
of population increase starts to
decline
Population (number of cells)
Same initial rate as the
exponential model, but
reaches an equilibrium value
This logistic model is more complex,
but makes a more realistic prediction
P
Pequil .Pst art
Pst art  ((Pequil  Pst art )exp( K.t)
Starting
population
Equilibrium
population
Rate
constant
Time
Plant growth
Resource limitation
Grazing control
Summary
What can limit phytoplankton growth?
Too little light
Lack of materials to build new cells
Like all plants, phytoplankton algae grow
by photosynthesis
LIGHT
CARBON
DIOXIDE
WATER
CARBOHYDRATE
OXYGEN
They build complex organic materials
from carbon dioxide and water, using light
as an energy source
STEP
Light decreases
as you go deeper
into the ocean
The decrease in light with depth is
also described well using an
exponential model, this time a decay:
Lz 2  Lz1 exp(  K(z2  z1))
Light at
depth ‘z2’
Light at
depth ‘z1’
Absorption
coefficient
Difference in
depths
Photosynthesis is dependent on light
Depth (metres)
Depth (metres)
There is more photosynthesis where
water is clear than where it is turbid
Photosynthetic rate
Photosynthetic rate
surface
Enough light here for growth
100 m
500 m
1000 m
Too dark for growth
3500 m
So phytoplankton will grow well when
they are close to the surface ...
… but phytoplankton that are deeper down
will not receive enough light for growth
The light available for phytoplankton
is controlled by a number of different
environmental factors
LIGHT
ENERGY
Phytoplankton
production
Phytoplankton
biomass
Ocean
circulation
Climate
Weather
Incident
solar radiation
Ice cover
Wind
mixing
Average light
in surface
mixed layer
Turbidity
Mixed layer
temperature
Phytoplankton
production
Phytoplankton
biomass
In addition to light, phytoplankton need
a wide range of chemical elements to
build organic material
Carbon is the most abundant element
in organic material
However, carbon is in plentiful supply
in the surface oceans
Carbon dioxide is readily soluble in
water, and can diffuse in from the
atmosphere
Other elements are needed to build
phytoplankton biomass
Some elements are required in large
amounts, whilst others are needed
only in minute quantities
These elements are usually called
nutrients
Nutrients needed in large amounts
include nitrogen, silicon and
phosphorus
Ocean
circulation
Climate
Weather
CHEMICAL
NUTRIENTS
Incident
solar radiation
Ice cover
Wind
mixing
Average light
in surface
mixed layer
Turbidity
Mixed layer
temperature
Phytoplankton
production
Phytoplankton
biomass
Again, there are complex
environmental controls on the
amount of nutrients available to
phytoplankton
Ocean
circulation
Climate
Chemical
nutrient supply
Weather
Incident
solar radiation
Ice cover
Average light
in surface
mixed layer
Turbidity
Mixed layer
temperature
Nutrient
availability
Wind
mixing
Phytoplankton
production
Phytoplankton
biomass
Biogenic
regeneration
Trace elements are elements that are
needed in very small amounts
Many are metals, such as iron, zinc and
cobalt. They may form parts of
enzymes, so that trace elements can be
important controls on growth and
uptake of nutrients
Ocean
circulation
Climate
Chemical
nutrient supply
Weather
Trace elements
Incident
solar radiation
Ice cover
Average light
in surface
mixed layer
Turbidity
Mixed layer
temperature
Nutrient
availability
Wind
mixing
Phytoplankton
production
Phytoplankton
biomass
Biogenic
regeneration
Plant growth
Resource limitation
Grazing control
Summary
So far, we have looked at controls on
phytoplankton growth rates
Removal of phytoplankton is an
important control on both biomass
and production
Ocean
circulation
Climate
Chemical
nutrient supply
Weather
Trace elements
Incident
solar radiation
Ice cover
Nutrient
availability
Wind
mixing
Average light
in surface
mixed layer
Phytoplankton
production
Phytoplankton
biomass
Turbidity
Mixed layer
temperature
BIOMASS
LOSSES
Biogenic
regeneration
Herbivorous animals that graze
phytoplankton are small, ranging in size
from fractions of a millimetre to a few
centimetres
These animals grow and reproduce
rapidly, so that their population
increases fast when phytoplankton
biomass increases
This means that grazing is an effective
control on phytoplankton
The overall picture of control on
phytoplankton involves many factors, often
inter-related
Ocean
circulation
Climate
Chemical
nutrient supply
Weather
Trace elements
Incident
solar radiation
Ice cover
Nutrient
availability
Wind
mixing
Average light
in mixed layer
Phytoplankton
production
Phytoplankton
biomass
Turbidity
Biogenic
regeneration
Grazing
Mixed layer
temperature
Sedimentation
Plant growth
Resource limitation
Grazing control
Summary
You have seen that Cell division gives rapid
population increase
Phytoplankton does not
increase exponentially
in nature
You have seen that Light decreases with
depth
Photosynthesis is
determined by light, so
decreases with depth
and turbidity
You have seen that Chemical nutrients are
also important for
phytoplankton growth
Grazing by herbivores
can control the
abundance of
phytoplankton
Ocean
circulation
Climate
Chemical
nutrient supply
Weather
Trace elements
Incident
solar radiation
Ice cover
Nutrient
availability
Wind
mixing
Average light
in mixed layer
Phytoplankton
production
Phytoplankton
biomass
Turbidity
Biogenic
regeneration
Grazing
Mixed layer
temperature
Sedimentation
NOTES for USERS
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