Primary productivity is the major energy fixation in the
freshwater ecosystem. According to the productivity, the lake is divided into
vertical and horizontal strata. Dominant photosynthetic organisms are also
differ one strata to other. Primary photosynthesis of littoral zone where is
the shallow water of edge of lakes is contributed by macrophysics. Primary photosynthesis of limentic zone where
is the top of the water column is dominated by the phytoplankton and algae.
Gross
primary productivity (GPP) is the total amount of the food produced by
photosynthetic organism in given time in a given area. A part of the produced food is consumed for
cellular respiration. After respiration, remaining of initial produced food is
defined as the Net Primary Productivity (NPP). NPP is available for the increase
of the biomass of organisms.
Dissolved
Oxygen (DO) is used as the index to predict the primary productivity of the
freshwater ecosystems. Dark and light bottle method and C14 method are used for
determination of DO.
Dark and light bottle method
Primary
productivity of the lake is measured at different depths from the water surface
in two hour time interval within the day. The procedure is done by as follows.
1)
Water
samples are collected from following depth 0cm, 25cm, 50cm, 75cm and 100cm from
the surface.
2)
It
is allowed the bottle to fill with water completely. As soon as the bottles are
filled, glass stoppers are inserted into the bottles without air bubbles under
the water.
3)
After
water filled, initial bottles are collected.
The light bottles and dark bottles are tied and fixed in relevant depth
for 2 hour period.
4)
The
DO in initial, light and dark bottles is determined by using the Winkler
method.
5) The procedure is repeated for several
time intervals.
Calculation
Initial bottles are used to determine initial DO in each time
intervals. Since the dark bottles are covered by black coat, it is not allowed
light to penetrate. Therefore photosynthesis does not happen in dark bottles.
Only produced DO is consumed. Only cellular respiration (CRR) is taken
place. But in the case of light bottles,
it is allowed light to penetrate. Therefore the photosynthesis is continued. So
both photosynthesis and cellular respiration is also taken place in the light
bottles.
CRR=
(DO) initial- (DO) dark
NPP=
(DO) light-(DO) initial
GPP=
NPP+CRR
So
GPP= (DO) light-(DO) dark
The variation of
primary productivity Vs depth
The maximum primary productivity is recorded slight below the
surface due to the higher light intensity. It causes photo-inhibition of
phytoplankton and reduces primary productivity.
Primary productivity is higher in the euphotic zone where the light
penetration is taken place.
With
the depth, transparency of the water column is reduced. So the light
penetration becomes limited. Therefore primary productivity is decreased
gradually downward. Since the primary
productivity is zero or limited in the bottom due to the absence of light, DO
in the bottom are carried from the top layer by the water circulation. DO is
consumed largely by decomposing organisms.
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