The effect of using paddle wheel aerator in the pond was not significant for growth rate of plankton (phyto- and zooplankton) and benthic organisms (Tables 2,3 and 7; Appendices 23 and 24). The pattern of plankton and benthic organisms development is the same as without paddlewheel aeretion. Other experiments of using paddlewheel aeratior to increase DO concentration during emergency conditions was not enough to maintain sufficient DO concentration for good fish growth (Boyd and Tucker, 1979). Paddlewheel aerator is good when used in small ponds (Rapparort et al., 1976). While Busch et al. (1978) used six paddlewheel aerator in 0.6 Ha pond area removed temperature stratification in the daytime and eliminated oxygen gradient in the nighttime. The result show that development of plankton and benthic organisms was not depend on agitation and increasing DO in the pond water by using paddlewheel aerator. Plankton development in rearing pond are influenced by interactions between temperature, photoperiod, water quality, nutrient availability, and fish predation (Geiger, 1983). Especially the major sources photoynthesis of phytoplankton of phytoplankton (algae) is CO2 from carbonate-bicarbonate equillibrium system in the pond water (Boyd, 1972). Positivelly correlationship between the growth rate of phytoplankton with light and nutrient (Laws and Bannister, 1980) and temperature (Goldman, 1977). Temperature influences zooplankton growth, filtering rates, and reproduction; and light and photoperiod influence the methabolic control mechanism of growth and reproduction (Starkweather, 1972), while benthic organisms growth depend on nutrient-sediment soils and fish predation (White, 1983).
Positivelly correlationship between increasing phyto- and zoo -plankton population in the pond at present study (Appendix 25). Although the zooplankton population increased over time, the phytoplankton still remained abundant in the ponds water (Table 2 and 3; Appendices 1 and 13). The results show that zooplankton is not as useful as phytoplankton as feed a source. According to Allan (1976) in Geiger (1983) Rotifer and Cladocera compete for a smaller range of food particles (1–20 um; 1–50 um) and Copepoda seem to prefer large particles (2–100 um); but also are more selective in what they eat. Observations in the experiment ponds show that Rotifer more abundant is than the other groups. This result, because Rotifer have the shortest lifespans of their peak reproduction period. Rotifer have live-spans and peak reproductive of 3.5 days, while Cladocera amd Copepoda have a lifespan of 50 days each, with 14–15 and 24 respectively to reach their peak reproduction period (Allan, 1976 in Geiger, 1983).
Chironomid and Nematoda are dominant benthic organisms found in the pond bottom during experiment period (appendix 19), with Chironomid more often dominanting. For addition, it was also observed that fluctuation in benthic organisms do occur. This result, because its was partly utilised by prawn after the third month of growing period (Table 7 and 0; Appendex 21), while in 6th sampling benthic organisms increased directly with nutrient amount of pond bottom increase. (Table 6). Dominance of Chironomid in the pond may be due to the fact that they have shortest lifespans and sufficient nutrients in the pond bottom to suport reproduction. According Bardach et al. (1972) Chironomids larva are ubiquitous in fine sediment or virtually in all aquatic systems and often became particularly abundant. There is a direct relationship between silika and phosphorous levels in overlying water and the presence and abundance of particular Chironomid species (Holdren and Amstrong, 1980 in White, 1983).
Chorophyll a concentration increased directly with phytoplankton population increase (Appendix 25). Increasing Chlorophyll a concentration in the ponds will be dangerous to organisms culture, especially in the night when the oxygen demand is high. In the present study, DO diurnal day to day ranges of oxygen concentration in the day and the night time become broaden (Appendix 26). Bannister (1979) suggested the concentration of chlorophyll a better less than 100 mg m-3. In the experiment ponds this concentration is reached since week 14 (in WPWA) and week 16 (in WOPWA) during the culture period till termination of experiment. Infact, with the use of a paddle wheel aerator, chlorophyll a concentration in pond increased faster than ponds without paddle wheel aeration. Laws and Malecha (1981) have the adverse belief that prawn farmers should maintain the pond water to keep chlorophyll a concentration in the range of 150–400 mg m-3, and O2 level should not fall below 4 ppm. (Environmental Protection Agency, 1973) in Laws and Malecha, 1981). In this case, chlorophyll a concentration is still in the toleratn range, but DO concentration ever been deplete to below 4 ppm.
Dense phyto - and zooplankton population becomes a problem if large number or cells die, as organic matter particulate. In this case, organic matter particle increased during the experiment period with phyto - and zooplankton more abundant in the pond water (equipment with cover) (Table 6 and Appendix 25), while in the contract without covering equipment the increase of organic matter was not orderly, due to bexause benthic organisms or prwan is eaten. Organic matter decomposed by bacteria my reduce oxygen to dangerously low levels in the night time (Boyd, 1973; Boyd et al., 1975; boyd et al., 1978). This will increase the concentration of CO2 and ammonia to high levels and a decline in the pH due to plankton die-off (Boyd et al., 1975). Decomposition of organic matter in pond water also supplies enough CO2 to support good algae growth (Boyd, 1972). These results in increased organic matter in pond bottom which will supported algae bloom in the experiment ponds.
Dense algae population may also be a problem on calm nights even if the population is healthy (Laws and Malecha, 1981). Green and blue-green algae are usually most abundant in warm water fish pond (Lin, 1983), but green algae failed to grow at normal rates in ponds water which contained blooms of blue-green algae (Boyd, 1973). Excessive blooms occur if the water is nulrient rich and will therefore lead to most damaging situation in fish ponds. Blue-green algae produce geosmine a compound giving fish an off-flavor tastes (Lovell and Sackey, 1973) and Oscillatoria tenuis also produced this compound (Medsker et al., 1960 in Lovell and Sackey, 1973). A dense bloom of the blue-green algae, such as Anabaena and Microsystis causes fish kills (Swingle, 1968; Boyd et al., 1975). Unfortunately in the experiment ponds there occured the growth of blue-green algae and its growth (Appendix 2). This result, because was not useful for prawn feed and futher there was an increase in the nutrients in the pond water (especially ammonium concentration) from the pellets feed is not eaten and prawn feces. Although the orthophosporous concentration in pond water decreased Jayamanne (1986), while the photoperiod and temperature good were sufficient enough to support plankton growth. The most common algae found during the experiment period was the Oscillatoria (Appendices 3. 4, 5 and 6) which was found in abundance from the 8th week after prawns stocking until the finish of the experiment. Although there were not a lot of prawn die till the first prawn partial harvest Jayamanne (1986). Futhermore blue-green algae blooms may have an influence on prawn growth.
During the experiment period, blooms of blue-green algae is occur (Appendix 2). In fact, the prawn is not utilised all blue-green algae, as seen from the stomach content analyses (Appendices 20, 21 and 22). Although there were a lot of plankton in the prawn stomach, it was found that all of it was not digestable (as seen from intestine contents), such as blue-green algae (Appexdix 22). This shows that the prawns have difficulty in digesting blue-green algae. The fact that plankton are less prefered by prawns as feeds can be another contributing factor to the abundant blue-green algae in the pond (Table 9). According to Spatar and Zorn (1978), Tilapia nilotica digested 70–80% of blue-green algae assimilation by the secretion of stomach acid, which lyses the algae cells. Commonly the algae used are diatoms (Lin, 1983).
The Present study also concerns the economic importance of using paddle wheel aeration corresponding with electricity or fuel consumption. Use of two paddle wheel aerator in pond monoculture prawns was in sufficient to increase the oxygen concentration (Jayamanne, 1986) and also alga blooms is occured. The formation of plankton blooms in commercial fish or prawn ponds causes severe economic losses. Furthermore, to increase profitability in monoculture prawns system, sanitary planktovorous fish (e.g. silver carp, bighead, common carp, Tilapia) in the pond will be needed to elimanate algae blooms. Low level stocking rate of fish correlate with improved pond ecosystem stability, without affecting prawn yield, survival or average weight (Cohen et al., 1983). No algae blooms occur in ponds with sanitary fish and, in addition, provide increased income to the farmers.
