MFSc

Fish production in farms largely depends on the primary productivity
In the water bodies. Among many factors, plant nutrient levels and their
availability in absorbable forms for the plankton is the most important one
contributing to primary production. The plant nutrient element, phosphorus
often found to limit the production, mainly due to the behavior of phosphorus
compounds to remain in unavailable form in aquatic environment. Hence it
becomes imperative to supplement phosphorus in suitable chemical form to
achieve better production. While the dosage is also important, the salinity of
water bodies has an interactive effect. Thus species of compounds in
phosphatic fertilizer is vital.
In the present study, two types of phosphatic fertilizers and
combination of these fertilizers with additives were tested for their effect on
aquatic primary production. Synthetic fertilizer namely single superphosphate
and natural phosphatic fertilizer namely rock phosphate were selected for
study. Commercial grade trisodium phosphate was used as additive in
combination with single superphosphate and rockphosphate in 1: 1 ratio with
respect to phosphate- P.
All the above four fertilizer materials were tested in brackishwater
having two salinity levels, a low salinity level of approximately 10 ppt and a
high salinity level of approximately 20 ppt. Each fertilizer was added in three
different doses, ie, O.lppm, 0.2 ppm and 0.4 ppm of phosphate-P. A set of
control with two salinity level of water and without adding fertilizer were also
run. All the experiments were done in duplicate, in accordance with
statistically designed 2x4x4 Factorial Completely Randomized Design. The
experiments were conducted in wide mouth polythene drums having 25 litre
capacity using 20 liters of water. The experiment was set up in a protected,
undisturbed, elevated place and kept exposed to sun light for 30 days. Volume
loss due to evaporation was compensated by adding appropriate amount of
water having corresponding salinity.

140
Water samples were drawn on l " day (before the addition of fertilizer),
i\ 14th and so" days and analysed for available phosphate, total phosphate,
silicate, nitrite, nitrate, iron, water pH, dissolved oxygen, carbon dioxide, total
alkalinity, total hardness and calcium. Productivity was assessed on 1 st day and
so" day by estimating chlorophyll. The results were statistically analysed with
ANOV A technique.
In the experiment, maxrmum productivity in terms of chlorophyll
(14.6mg/m3) was found under the treatment with single super phosphate along
with additive trisodium phosphate when applied at a dose of 0.4 ppm of
phosphate-P in the high saline media. For the same type of treatment in low
salinity level, a little reduced value of productivity was observed (chloropyll
=13.9 mg/rrr'), The next effective treatment was rock phosphate with trisodium
phosphate at a concentration of 0.4 ppm of phosphate-P, here a chlorophyll
concentration of 13.6mg/m3 and 12.6 mg/rrr' were obtained in high and low
salinities respectively. The minimum productivity levels were observed in
treatments with rock phosphate alone in all concentrations, compared to other
treatments.
It could be concluded from the experiments the following i) single super
phosphate with a combination of trisodium phosphate was most effective
compared to rock phosphate with trisodium phosphate, which could be due to
the greater solubility of single super ph~sphate ii) single super phosphate was
found to be more effective than rock phosphate, which could be due to the
grater solubility as already explained. iii) the concentration level of 0.4 ppm of
phosphate-P was found to be the most effective in enhancing the primary
production compared to the lower doses tried ie, 0.1 and 0.2 ppm. iv) along
with an increase in the available phosphate from the added fertilizers there was
an increase in the primary production also v) salinity was found to have some
effect on production, higher the salinity grater the production. This can be due
to the availability of other minerals and intrinsic factors required for the
synthesis of chlorophyll and biomass.