PRIMARY
PRODUCTION
Primary production is the production of organic compounds from
atmospheric or aquatic carbon
dioxide, principally through the process of photosynthesis, with chemosynthesis being
much less important. All life on earth is directly or indirectly reliant on
primary production. The organisms responsible for primary production are known
as primary producers or autotrophs,
and form the base of the food chain.
In terrestrial
ecoregions, these are mainly plants, while in aquatic ecoregions algae are primarily responsible. Primary
production is distinguished as either net or gross, the former
accounting for losses to processes such as cellular
respiration, the latter not.
Overview
The Calvin cycle
of photosynthesis
Primary production is the production of chemical energy in
organic compounds by living organisms.
The main source of this energy is sunlight but a minute
fraction of primary production is driven by lithotrophic organisms
using the chemical energy of inorganic
molecules.
Regardless of its source, this energy is used to
synthesize complex organic
molecules from simpler inorganic compounds such as carbon dioxide (CO2)
and water (H2O).
The following two equations are simplified representations of photosynthesis
(top) and (one form of) chemosynthesis (bottom) :
light
CO2 + H2O CH2O + O2CO2 + O2 + 4 H2S CH2O + 4 S + 3 H2O
In both cases, the end point is reduced carbohydrate (CH2O),
typically molecules such as glucose
or other sugars. These relatively
simple molecules may be then used to synthesise further more complicated
molecules, including proteins,
complex
carbohydrates, lipids,
and nucleic acids, or be respired
to perform work.
Consumption of primary producers by heterotrophic organisms,
such as animals, then transfers
these organic molecules (and the energy stored within them) up the food web, fueling all of
the Earth's living systems.
GPP and NPP
Gross Primary Production (GPP) is the rate at which an ecosystem's producers capture and store a
given amount of chemical energy as biomass in a given length of time. Some
fraction of this fixed energy is used by primary producers for cellular
respiration and maintenance of existing tissues. The remaining fixed
energy is referred to as Net Primary Production
(NPP).
NPP
= GPP - respiration
Net primary production is the rate at which all the
plants in an ecosystem produce net useful chemical energy; equal to the
difference between the rate at which the plants in an ecosystem produce useful
chemical energy (GPP) and the rate at which they use some of that energy through
cellular respiration. Some net primary production will go towards growth and
reproduction of primary producers, while some will be consumed by herbivores.
Both gross and net primary production are in units of
mass / area / time. In terrestrial ecosystems, mass of carbon per unit area per
year is most often used as the unit of measurement.
Terrestrial production
An oak tree; a typical
modern, terrestrial autotroph
On the land, almost all primary production is now
performed by vascular plants, although a small fraction comes from algae
and non-vascular plants such as mosses and liverworts.
However, before the evolution
of vascular plants, non-vascular plants played a more significant role. Primary
production on land is a function of many factors, but principally local hydrology and temperature
(the latter covaries to an extent with light, the source of energy for
photosynthesis). While plants cover much of the Earth's surface, they are
strongly curtailed wherever temperatures are too extreme or where necessary
plant resources (principally water and light) are limiting, such as deserts or polar
regions.
Water is "consumed" in plants by the processes
of photosynthesis (see above) and transpiration.
The latter process (which is responsible for about 90% of water use) is driven
by the evaporation
of water from the leaves of
plants. It allows plants to transport water and mineral nutrients from the soil to growth regions,
and also cools a plant down. It can be regulated by structures known as stomata, but these also
regulate the supply of carbon dioxide from the atmosphere, so that decreasing
water loss also decreases carbon dioxide gain. Crassulacean acid metabolism (CAM) and C4 plants use physiological
and anatomical
workarounds to increase their water-use efficiency and allow increased primary
production to take place under conditions that would limit "normal" C3 plants (the majority of plant species).
Oceanic production
Marine diatoms; an example of planktonic microalgae
In a reversal of the pattern on land, in the oceans,
almost all primary production is performed by algae, with a small fraction
contributed by vascular plants
and other groups. Algae encompass a diverse range of organisms, ranging from
single floating cells to attached seaweeds. They include photoautotrophs from a
variety of groups: prokaryotic bacteria
(both eubacteria and archaea); and three eukaryote categories the green, brown and red algae. Vascular plants are represented in
the ocean by groups such as the seagrasses.
In another departure from the situation on land, the
majority of primary production in the ocean is performed by microscopic organisms,
the phytoplankton.
Larger autotrophs, such as the seagrasses and macroalgal seaweeds are generally
confined to the littoral zone
and adjacent shallow waters, where they can attach to the underlying
substrate but still be within the photic zone.
There are exceptions, such as Sargassum,
but the vast majority of free-floating production takes place within
microscopic organisms.
Factors for Primary Production
The factors limiting primary production in the ocean are
also very different from those on land. The availability of water, obviously,
is not an issue (though its salinity
can be). Similarly, temperature, while affecting metabolic
rates, ranges less widely in the ocean than on land because the heat capacity
of seawater buffers temperature changes, and the formation of sea ice insulates it at lower temperatures. However, the availability
of light, the source of energy for photosynthesis, and mineral nutrients, the building
blocks for new growth, play crucial roles in regulating primary production in the
ocean.
Light
The sunlit zone of the ocean is called the photic zone
(or euphotic zone). This is a relatively thin layer (10-100 m) near the ocean's
surface where there is sufficient light for photosynthesis to occur. For
practical purposes, the thickness of the photic zone is typically defined by
the depth at which light reaches 1% of its surface value. Light is attenuated
down the water column by its absorption or scattering by
the water itself, and by dissolved or particulate material within it (including
phytoplankton).
Net photosynthesis in the water column is determined by
the interaction between the photic zone and the mixed layer. Turbulent mixing
by wind energy at the
ocean's surface homogenises the water column vertically until the turbulence dissipates
(creating the aforementioned mixed layer). The deeper the mixed layer, the
lower the average amount of light intercepted by phytoplankton within it. The
mixed layer can vary from being shallower than the photic zone, to being much
deeper than the photic zone. When it is much deeper than the photic zone, this
results in phytoplankton spending too much time in the dark for net growth to
occur. The maximum depth of the mixed layer in which net growth can occur is
called the critical depth. As long as there are adequate nutrients available,
net primary production occurs whenever the mixed layer is shallower than the
critical depth.
Both the magnitude of wind mixing and the availability of
light at the ocean's surface are affected across a range of space- and
time-scales. The most characteristic of these is the seasonal cycle (caused by the consequences
of the Earth's axial tilt), although
wind magnitudes additionally have strong spatial components. Consequently, primary
production in temperate regions such as the North Atlantic is highly seasonal, varying with
both incident light at the water's surface (reduced in winter) and the degree
of mixing (increased in winter). In tropical regions, such as the gyres
in the middle of the major basins, light may
only vary slightly across the year, and mixing may only occur episodically,
such as during large storms or hurricanes.
Nutrients
Mixing also plays an important role in the limitation of
primary production by nutrients. Inorganic nutrients, such as nitrate, phosphate and silicic acid are necessary for phytoplankton to synthesise their cells and cellular machinery.
Because of gravitational sinking of particulate material
(such as plankton, dead or faecal material), nutrients
are constantly lost from the photic zone, and are only replenished by mixing or
upwelling of deeper water. This is exacerbated
where summertime solar heating and reduced winds increases vertical
stratification and leads to a strong thermocline, since this makes it more difficult
for wind mixing to entrain deeper water. Consequently, between mixing events,
primary production (and the resulting processes that leads to sinking
particulate material) constantly acts to consume nutrients in the mixed layer,
and in many regions this leads to nutrient exhaustion and decreased mixed layer
production in the summer (even in the presence of abundant light). However, 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.
Iron
Another factor relatively recently discovered to play a
significant role in oceanic primary production is the micronutrient iron.[1] This is used as a cofactor
in enzymes involved in processes such as nitrate reduction and nitrogen fixation. A major source of iron to the
oceans is dust from the Earth's deserts, picked up and
delivered by the wind as eolian dust.
In regions of the ocean that are distant from deserts or
that are not reached by dust-carrying winds (for example, the Southern and North Pacific oceans), the lack of iron can
severely limit the amount of primary production that can occur. These areas are
sometimes known as HNLC (High-Nutrient, Low-Chlorophyll) regions,
because the scarcity of iron both limits phytoplankton growth and leaves a
surplus of other nutrients. Some scientists have suggested introducing iron
to these areas as a means of increasing primary productivity and sequestering
carbon dioxide from the atmosphere.[2]
Measurement of Primary Production
The methods for measurement of primary production vary
depending on whether gross vs net production is the desired measure, and
whether terrestrial or aquatic systems are the focus. Gross production is
almost always harder to measure than net, because of respiration, which is a
continuous and ongoing process that consumes some of the products of primary
production (i.e. sugars) before they can be accurately measured. Also,
terrestrial ecosystems are generally more difficult because a substantial
proportion of total productivity is shunted to below-ground organs and tissues,
where it is logistically difficult to measure. Shallow water aquatic systems
can also face this problem.
Scale also greatly affects measurement techniques. While
biochemically-based techniques are appropriate for plant tissues, organs, whole
plants, or plankton samples, they are decidedly inappropriate for large scale
terrestrial field situations. There, net primary production is almost always
the desired variable, and estimation techniques involve various methods of
estimating dry-weight biomass changes over time. Biomass estimates are often
converted to an energy measure, such as kilocalories, by an empirically determined
conversion factor.
Terrestrial
In terrestrial ecosystems, researchers generally measure
net primary production. Although its definition is straightforward, field
measurements used to estimate productivity vary according to investigator and
biome. Field estimates rarely account for below ground productivity, herbivory,
decomposition,
turnover, litterfall, volatile organic compounds,
root exudates, and allocation to symbiotic
microorganisms. Biomass based NPP estimates result in underestimation of NPP
due to incomplete accounting of these components. However, many field
measurements correlate well to NPP. There are a number of comprehensive reviews
of the field methods used to estimate NPP.
The major unaccounted for pool is belowground
productivity, especially production and turnover of roots. Belowground
components of NPP are difficult to measure. BNPP is often estimated based on a
ratio of ANPP:BNPP rather than direct measurements.
Grasslands
The Konza tallgrass prairie in the Flint Hills
of northeastern Kansas
Most frequently, peak standing biomass is assumed to
measure NPP. In systems with persistent standing litter, live biomass is
commonly reported. Measures of peak biomass are more reliable in if the system
is predominantly annuals. However, perennial measurements can be reliable if
there was a synchronous phenology driven by a strong seasonal climate. These
methods may underestimate ANPP in grasslands by as much as 2 (temperate) to 4 (tropical) fold[4]. Repeated measures of standing live and dead
biomass provide more accurate estimates of all grasslands, particularly those
with large turnover, rapid decomposition, and interspecific variation in timing
of peak biomass. Wetland
productivity (marshes and fens) is similarly measured. In Europe, annual mowing
makes the annual biomass increment of wetlands evident.
Forests
Methods used to measure forest productivity are more
diverse than those of grasslands. Biomass increment based on stand specific allometry plus
litterfall is considered a suitable although incomplete accounting of
above-ground net primary production (ANPP)[3]. Field measurements used as a proxy for ANPP
include annual litterfall, diameter or basal area increment (DBH or BAI), and volume increment.
Aquatic
In aquatic systems, primary production is typically
measured using one of three main techniques:
- variations in oxygen concentration
within a sealed bottle (developed by Gaarder and Gran in 1927)
- incorporation of inorganic carbon-14
(14C in the form of sodium bicarbonate) into organic matter
- fluorescence kinetics (technique
still a research topic)
The technique developed by Gaarder and Gran uses
variations in the concentration of oxygen under different experimental
conditions to infer gross primary production. Typically, three identical
transparent vessels are filled with sample water and stoppered.
The first is analysed immediately and used to determine the initial oxygen
concentration; usually this is done by performing a Winkler titration. The other two vessels
are incubated, one each in under light and darkened. After a fixed period of
time, the experiment ends, and the oxygen concentration in both vessels is
measured. As photosynthesis has not taken place in the dark vessel, it provides
a measure of respiration. The light vessel permits both photosynthesis and respiration,
so provides a measure of net photosynthesis (i.e. oxygen production via
photosynthesis subtract oxygen consumption by respiration). Gross primary
production is then obtained by subtracting oxygen consumption in the dark
vessel from net oxygen production in the light vessel.
The technique of using 14C incorporation (added as
labelled Na2CO3) to infer primary production is most
commonly used today because it is sensitive, and can be used in all ocean
environments. As 14C is radioactive (via beta decay),
it is relatively straightforward to measure its incorporation in organic
material using devices such as scintillation counters.
Depending upon the incubation time chosen, net or gross
primary production can be estimated. Gross primary production is best estimated
using relatively short incubation times (1 hour or less), since the loss of
incorporated 14C (by respiration and organic material excretion / exudation)
will be more limited. Net primary production is the fraction of gross
production remaining after these loss processes have consumed some of the fixed
carbon.
Loss processes can range between 10-60% of incorporated
14C according to the incubation period, ambient environmental conditions
(especially temperature) and the experimental species used. Aside from
those caused by the physiology of the experimental subject itself, potential
losses due to the activity of consumers also need to be considered. This is
particularly true in experiments making use of natural assemblages of
microscopic autotrophs, where it is not possible to isolate them from their
consumers.
Global
As primary production in the biosphere is an important
part of the carbon cycle, estimating it at the global scale is important
in Earth system science.
However, quantifying primary production at this scale is difficult because of
the range of habitats on Earth, and because of the impact of weather events
(availability of sunlight, water) on its variability.
Using satellite-derived
estimates of the Normalized Difference Vegetation
Index (NDVI) for terrestrial habitats and sea-surface chlorophyll
for the oceans, it is estimated that total (photoautotrophic) primary
production for the Earth was 104.9 Gt
C yr-1.[9] Of this, 56.4 Gt C yr-1 (53.8%), was the
product of terrestrial organisms, while the remaining 48.5 Gt C yr-1, was
accounted for by oceanic production.
In areal
terms, it was estimated that land production was approximately 426 g C m-2 yr-1
(excluding areas with permanent ice cover), while that for the oceans was 140 g
C m-2 yr-1.[9] Another significant difference between the
land and the oceans lies in their standing stocks - while accounting for almost
half of total production, oceanic autotrophs only account for about 0.2% of the
total biomass.
In
the ecosystem mainly two components are proposed - Biotic factors and Abiotic
factors. Biotic factors have the efficiency to accumulate the
food. So, the amount of food that is
either produced or taken from
other factors is known as productivity.
The important component that can synthesize food itself, ae known as
producers. All the green plants are
producers. Basically, the food
synthesized through the green plants is known as primary production and this
primary productivity are of following types :
Gross Primary Productivity
The total amount of food
synthesized by plants per unit time per unit area is known as gross primary
productivity. It is also known as total
photosynthesis or total assimilation.
Net Primary Productivity
After synthesis of food, its some
part is used in the metabolic processes, so the amount of food retained after
metabolism, is known as net primary productivity. It can be calculated by following formula :
NPP
= GPP - R
Where NPP - Net primary productivity
GPP
- Gross primary productivity
R -
Respiration rate
Net Community Productivity
In the plants, large amount of food
is accumulated. But, it is used by
herbivores. SO, after taking of the food
by herbivores from plants, the remaining food is known as Net community
productivity.
NCP = NPP
- GSP
Where NCP - Net community productivity
GSP -
Gross secondary productivity
Methods for Determination of Primary Productivity
The best method to determine the
primary productivity is to determine the rate of energy flow, but it is too
difficult. So, in the different
conditions, different methods can be used to determine primary productivity. Some important methods are as follows :
1. Harvest
method : In this process, small plot
is selected, the standing crop is cut down and particular time is given for the
regeneration of crop which may be 7 to 15 days.
After this time period, standing crop is again cut down. It is dried away at 600C in
oven. Then, its dry weight is determined
or with the help of calorimeter, its value is determined in calorie. With the help of following formula, NPP is
determined :
NPP = Dry
weight of plants x Area
Time
in days
The unit of productivity will be gms/
day/ unit area.
But, with the help of this method,
we can find out only NPP, not GPP.
2. Oxygen
measurement (D.O. method or Light and dark bottle method) : This method is suitable for determination of
NPP and GPP in the aquatic ecosystem. It
is based on the oxygen measurement. Its
basic principle is to evolve oxygen during photosynthesis. It is clear that its rate of photosynthesis
is higher, and then the rate of oxygen evolution will be high. In this method, three bottles are taken and
the pond water is filled in these bottles.
Then, the D.O. of first bottle is determined. THen, one bottle is covered with black paper
or black cloth, which is known as dark bottle.
In this bottle, photosynthesis cannot occur but respiration will take
place. The third bottle is known as
light bottle where photosynthesis as well as respiration will occur. These bottles are hanged in the pond . After 6-8 hours, bottles are taken out and
D.O. is determined in both the bottles, and following observatiosn are taken :
a. D.O.
of bottle A (initial D.O.) = A ppm
b. D.O.
of dark bottle = B ppm
c. D.O.
of light bottle = C ppm
Now, the NPP and GPP are calculated
by the following formula :
a. Respiration
rate = (B-A) ppm
b. NPP =
(C-A) ppm
c. GPP = NPP
+ R
It is useful only for aquatic
ecosystem but not for terrestrial ecosystem.
3. Carbon
assimilation method : This method is
based on use of carbon dioxide during photosynthesis. In this process, 3 plants having similar
shape and size are selected. These
plants are covered with bell jars. Each
bell jar is having inlet for air and outlet is connected with calcium
hydroxide. Now, the first set is control
set, second set is covered with black paper or black cloth, which will indicate
only respiration and the third, will indicate photosynthesis as well as
respiration. This set up is kept in the
sunlight and after 6-8 hours, the quantity of calcium hydroxide is determined
in the limewater. Actually, CO2
reacts with limewater to form calcium carbonate. This calcium carbonate is taken out, dried
away and its weight is determined, and
with the help of molecular weights, the weight of carbon dioxide is calculated. Its basic principle is :
100 gm CaCo3 Eq.
= 44 gm of CO2
The main observations are following
:
a.
Amount of CO2 in Ist set =
A gms
b.
Amount of CO2 in IInd set =
B gms
c.
Amount of CO2 in IIIrd
set = C gms
With the help of these observations,
productivity can be calculated as follows:
a.
Respiration rate = (B-A) gms
b.
NPP = (A-C) gms
c.
GPP = NPP + R
4. pH
method : This is suitable for
aquatic ecosystem. In aquatic ecosystem,
CO2 is present as soluble gas
and presence of CO2 will
indicate lowering of pH. So, as the CO2 is used in the photosynthesis, pH parallels
increase. This increase in pH will be
directly proportional to rate of photosynthesis.
5. Disappearance
of raw material : Any synthetic
process needs the nutrients so, the calculated amount of nutrient is
transferred into the soil and the initial analysis of soil sample is carried
out. After 24 hours, again the analysis
of soil sample is carried out and the loss of nutrients during 24 hours is
calculated. The rate of photosynthesis
is directly proportional to loss of nutrients.
In the ecosystem, CO2 is
present as soluble gas and presence of CO2
will indicate lowering of pH.
So, as the CO2 is used
up in the photosynthesis, pH parallels increase. This increase in pH will be directly
proportional to rate of photosynthesis.
6. Radio
isotopic method : It is the standard
and exact method to determine the productivity.
In this process, 14CO2
is provided to the plant and after 2 hours, radioactivity is
measured in the plants. It is directly
proportional to the rate of photosynthesis.
7. Chlorophyll
method : It is indirect method. It is clear that photosynthesis is based on
the amount of chlorophyll. So, if the
plants have higher amount of chlorophyll, then the rate of photosynthesis will
be higher. In this method chlorophyll is
extracted from the leaves and its concentration is determined through
spectrophotometer. Generally, this
method is used for comparison of productivity of different communities.
8. Herbage
cover method : It is indirect method
because in it the plants or in community, amount of green canopy is higher,
then the rate of photosynthesis will be higher.
So, it is used for the comparison of productivity of different
communities.
9. Global
pattern of primary productivity :
"Annual average rate of net plant production. The number after the bar is K cal/M2/year;
the number within the parentheses is area in 106 Km2.
Factors controlling Primary Productivity
Primary productivity is controlled
through all these factors that control the rate of photosynthesis some important
factors are the following :
1. Size of community : If the community is large, then the
productivity of the community will be high.
2. Herbage cover : The green canopy of the plant is known as
herbage cover, and this is the plant part where photosynthesis occurs. So, larger the canopy more the productivity.
3. Availability of nutrients in soil: It is proved that the rate of absorption of
nutrients is directly proportional to productivity. If the soil is nutrient rich, then the productivity
will be high.
4. Concentration of CO2 : CO2
is the raw material for photosynthesis and as the CO2 concentration
increases, the rate of photosynthesis will increase up.
5. Types of plants : It is clear that tropical plants have the
higher rate of photosynthesis, because in the tropical plants, photorespiration
is absent. So, efficiency of synthesis
will rise up.
6. Density of vegetation : If community is large, but vegetation is
small, then productivity will be less.
But if community is smaller and vegetation is desnse, then productivity
will be high.
7. Rainfall: The water is the raw material for plant
photosynthesis so; higher rainfall will indicate higher productivity. It is the reason that tropical rain forest
will indicate highest productivity.
8. Solar radiations: Light is another important limiting factor
for photosynthesis. So, as the amount of
solar radiations increase, the rate of photosynthesis is also increased. Generally, solar radiations are not barrier
in terrestrial ecosystems. But it is
barrier for aquatic ecosystem.
9. Disturbances in community : If community is disturbed by anthropogenic
factors, and then its productivity will be low.
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