EFFECT OF DIFFERENT LEVELS OF PHOSPHORUS INPUT ON PHYTOPLANKTON PRODUCTIVITY IN LIMED ACID SULFATE SOILS FISHPOND

One of problems of building lishponds in a<:id sulfate soil areas is their poor response to fertilization. An experiment to assess the effect of different doses of triple super phosphate (TSP) on phytoplankton productivity in acid sulfate soil fishponds has been conducted for one month using lli earthenponcls ol'2Om2 each. Prior to fertilizer application the ponds were limed at 1.351<g/rlz and urea fertilizer was applied at l2glm2. The results showed that there were positive relationships between rloses of TSP applied and chlorophvll-o t:oncentralions. Fishponds receiving TSP at a rate oI'28glmz had mean chlorophyll-cr con<;etrtrations ol 90.76mg/n.r'1.,',,nr1rr,,ed trr .l.90mg/m3 in <:ontrol ponds.


IN'TRODUCTION
Phosphorus is a key nutrient in pond fertilization (tsoyd and Musig, 1981), because its natural concenbration in pond wi:rter is usually too low to support abundant phyto- plankton populations (Bot'd,19ti2).Several types of phosphorus fertilizers are r:onrnronly used in fishponds (Boyd, 1979).
Inorganic phosphate added to fish ponds in fertilizers is removed from the water primarily by phytoplankton, bacteria and sedirlent (Boyd, 1979).Phosphat,e aclsorption by pond soil increases with increrasing r:lay content, exchange acidity, exchangeable ir<-rn, alunrinum and calcium, but decreases with increasing organic matter and soil pH (I'ahila, 1990;Shresta,1994).
F-ishponds built on acid sulfate soils re- quire extra phosphorus input as the preva- lence of acid catiorrs elfectivelv inrmobilizes phosphorus.
F'ertilizers are applied to fishponds to increase inorganic nutrient concentrations thereby supportirrg greater phytoplankton growth.The growth rate of an alga will de- cline if the concentration of a given nutrient drops below that supporting an uptake rate sufficient to maintairr the existing growth rate (l)arley, 1982).
The ohjective of' this experiment is to evaluate the eI'fects of' diff'erent doses of phosphate fertilizer, plus a constant amount of urea, in the enhancement of phytoplankton productivity in lirned acid sulfate soil fishponds.

Pond Prepuratiort
Fifteen ponds of 20m2 each were used.
Ponds were deacidified before fertilizer appli- cation by repeated water inundation and draining, followed by liming at a rate of 1.35kg/m2.Lime (CaCO3) was incorporated and mixed with soil mass to 10cm depth.*) Researcher of t.lre Palenrbang Research St,ation fbr.Fr.eshwat.crI,'isheries Fertilization I'he ponds were itrundatcd wit,h water from a reservoir oanal 2 days after linring, and then fertilized ll days after inrttrdation.The fertilizers wero dissolve d in rn trtc'r' in rt plastic bucket and splashed over the pond surface.

CON(II,US I0N
Fishpond built in acid sulfate soils after deacidified b-v application of'lime was suffer- ing in low plant nutrients, especially phos- phorus.Phytoplankton have poor growth in ponds fertilized by urea only, whereas addi- tion TSP fertilizer give a strong response to increase phytoplankton bionrass.Under the present condition there was a linear relationship between 'l'SI) input ancl phyboplankton biornass.In practice, fertilization with 0.29 P/rnr/day or nrore with N to p ratio 1:1 woulcl be suibable lbr limed acicl sulfate soils fish- pond.
[,oss of soluble orthoplrosphat.ef ronr pond water as described bv lloyd and Musig (1981)  were t:rken up bv ph.ytoplankton ernd bacte- ria.and absorbed by pond sedinrent.phos- phorus absorbed b-t' the sediment of'acid soil could be the nrajor parts ofphosphurus loss I'rorn pond waber.'l'he pond soil whir:h was high in clay content, and high in iron and alurninunr was <:onducive for phosphate fixation.
'l'he findings in this study shows that soluble orthophosphate existed in pond watcr for longer period in ponds receive higher level ol''fSP input, therefore, by a single applit:a- tion higher level o{' 'lSI, gave a greater p availability to phytoplankton.Increasing'l'SI) inptrt resulted tn prop<lrtional incretrser in chlorophyll-o conr:elrtrations.[)onds which was trcated with 2tlg TSIr/rrr2 1t).19g lrli4 I day) with N:P rabio 1 : 1 produced nroan

Figure 1 .
Figure 1.Souluble irthophosphate concentrabion in pond water during one month of experimental period related to differerrt TSI) input.