Productivity in an ecosystem can be defined as the amount of organic matter produced by photosynthetic organism during which light energy from sun is converted to chemical energy by plants.
MEASUREMENT OF PRODUCTIVITY
Productivity is usually measured as the rate at
which energy or biomass is produced per unit area per unit time. This rate is
expressed in such terms as kilocalories per square per year (kcal/m2/yr) a
measured or energy or grams per square meter per year a measure of biomass.
Various techniques used to estimate primary productivity are given below:
The method adopted for the measurement of
production in plants varies depending on the type of plant, the specific
research objective, and the available resources. Here are some common methods
used for measuring production in plants:
- Light and Dark Bottle Method
- Radioactive Tracer Method
- Chlorophyll Concentration
- Carbon Dioxide Flux
- Harvest Analysis
- Dimension Analysis
1.Light and Dark
Bottle Method:
This method is employed
to measure primary productivity in an aquatic ecosystem such as a pond.
It is based upon the
assumption that the amount of oxygen produced is proportional to gross
production because one molecule of oxygen is produced for each atom of carbon
fixed.
Samples of water from
different depths are placed in paired bottles. One of the paired bottles is
covered with black tape or aluminium foil to exclude light and other is kept
clear to admit light and allow photosynthesis. The oxygen concentration of the bottles
is determined by Winkles method. The bottles are suspended to the same depth
from where samples were collected with the help of string. After 24 hours the
bottles are removed their oxygen concentration is determined and compared with
the concentration at the beginning.
The decline of oxygen
in the dark bottle indicates the amount of respiration by producers and
consumers where the oxygen change in the light bottle reflects the net result
of oxygen consumed by respiration and oxygen produced. Adding respiration and
production together or subtracting final oxygen concentration in the dark
bottle from that in the light bottles give an estimate of gross productivity
for 24 hours. This method has its problems. Some of the respiration attributed
to phytoplankton may be bacterial. The phytoplankton population may increase in
the bottle during the experimental time but not in dark bottle.
The procedure is based
on the assumption that respiration in the dark is the same as in the light.
2.Radioactive Tracer
Method:
It is one of the most
useful and sensitive method useful to estimate primary productivity in an
aquatic ecosystem especially in lakes and oceans. It is based upon measurement
of CO2 uptake.
The method involves
addition of radioactive carbon as carbonate to a sample of water in clear and
unstable carbon are assimilated into carbohydrates and become part of the
protoplasm of phytoplankton.
The plankton material
is removed from the water washed dried and place in a counting chamber to
determine the level of radioactivity. The radioactive counts are used to
calculate the amount of carbon dioxide fix in photosynthesis using the
photosynthetic equation.
The estimate is based
on the assumption that of radioactive carbon is proportional to that of stable
carbon. However in practice these may not be proportional therefore a
correction factor for 14C* adsorption in dark bottle is required.
The technique has its
deficiencies. It does not discriminate between respiration of phytoplankton and
bacteria the uptake and release of 14C* by bacteria and zooplankton and the
abilities of different species of phytoplankton to use available light.
3.Chlorophyll
Concentration:
This method is based
upon estimation of producer from chlorophyll concentration and light intensity
through the water column. The technique evolved from the discovery by plant
physiologist that a close relationship exists between chlorophyll concentration
and photosynthesis at any given light intensity.
If the assimilation
ratio and the available light are known then gross production can be estimated
by the extracting pigments and then measuring the chlorophyll concentration
with a spectrophotometer.
The method was first
used to estimate primary productivity in large water bodies such as sea but
later applied to terrestrial ecosystem as well as.
This method involves
the determination of chlorophyll contents of phytoplankton in a given volume of
water. Because all plant need chlorophyll to carry on photosynthesis the amount
of chlorophyll in a given amount of water is a direct measure of the total
biomass or standing crop of phytoplankton it contains. The chlorophyll is
extracted chemically and the amount of chlorophyll is measured. The deeper the
colour the greater is the concentration of the chlorophyll and therefore
phytoplankton biomass. The technique has a problem that chlorophyll
concentration varies with phytoplankton species and even within the cells of
species. Further extraction technique may alter the chlorophyll.
4.Carbon Dioxide Flux:
It is one of the most
useful methods for estimating primary productivity in terrestrial ecosystems.
It helps measuring both
gross and net primary production. It involves measurement of the uptake of
carbon dioxide in photosynthesis and its release in respiration.
In this method a simple
of community which may be twig and its leaves a segment of a tree stem the
ground cover and soil surface or even a portion of the total community such as
on-site sample of grassland is enclosed in a clear plastic tent. Air is drawn
through the enclosure and the carbon dioxide concentration in incoming air and
outgoing air is measured with an infrared gas analyzer or by absorption on a
KOH column.
The assumption is that
any carbon dioxide removed from the incoming air during the day has been
incorporated into organic matter. Therefore the quantity of carbon dioxide in
the enclosure is equivalent to photosynthesis minus respiration. A similar
sample may be enclosed in dark enclosure.
The amount of carbon dioxide is produced in the dark bag is a measure of respiration when photosynthesis stops. The quantity of carbon in the light and dark enclosure added together estimates gross production.
5. Harvest
Analysis (Standing Crop Method):
Harvest method is
widely used to estimate in terrestrial ecosystem. It is most useful for
estimating the production of cultivated land range and communities of annual
plants were production starts from zero at seedling or planting time becomes
maximum at harvest and is subject to minimal use by consumers.
The technique involves
removing vegetation at periodic intervals and drying the samples to a constant
weight. To obtain ‘n’ accurate estimate the production of plant biomass must be
sampled throughout the growing season and the contribution of each species must
be determined. Different species of plants reach their peak production at
different times during the growing season. The difference in standing crop
biomass between harvests periods expressed as germs per square meter per unit
time provides an estimate of net primary productivity. Caloric value of the
material can be determined through use of a calorimeter and biomass can be
converted to calories. Net primary productivity is then expressed as
kilocalories per square meter per year.
Harvest method provides
information about above ground productivity usually because low ground
productivity requires the samples of root biomass which is difficult at best.
Although the roots of some annual and crops plant may be removed from the soil
the task become more difficult with grass and herbaceous species and even more
so with forest trees.
6.Dimension Analysis:
Because plant of
different age, size and species make up the forest community modified harvest
technique called dimension analysis is used by ecologist.
This method help to
estimate standing crop and productivity from less intensive sampling. Dimension
analysis involves the measurement of light, diameter or height and diameter
growth and age is determined by growth ring analysis. Total weight both fresh
and dry of leaves and branches as well as the weight of also determined. Often
the roots are excavated and weighted.
Similarly biomass of
ground vegetation litter fall is also determined. Net annual production of
wood, bark, leaves, twigs, roots and flower is calculated. All these
information are used to calculate production of trees and other vegetation in a
sample unit. After dimension analysis the standing crop can be predicated by
using data such as (diameter) 2 x height x and annual productivity by measuring
current wood growth.
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