The density (number - per liter) of phytoplankton in the different ponds was recorded (Appendix 1) according to the time of sampling. Figure 1 and Table 2 show the average changes in composition of phytoplankton groups in ponds with paddle wheel aeration (WPWA) and that with the control (WOPWA) during the experiment period. Result of phytoplankton composition in each pond are presented in Appendices 11 and 12.
Maximum population of phytoplankton groups and total population were not reached at the same time for each pond (Table 2). Total maximum phytoplankton were 14,893 and 44,384 (x 1,000 plankter per liter) in pond WPWA in weeks 22 and 20 respectively. On the other hand, in pond WOPWA, maximum population reached was 27,227 and 21,407 (x 1,000 plankter per liter) respectively in weeks 20 and 24. Before the first prawns partial harvest, the mean maximum population was reached in same time (20 weeks). The mean maximum population were 27,600 and 17,109 (x 1,000 plankter per liter) (Table 1 and Figure 1). Analyses of variance of exponential growth rate to reach maximum population does not show significant difference (P < 0.05) for all groups or total phytoplankton (Appendix 24).
During the experiment, the first 10 weeks of prawns stocking till termination showed that Cyanophyta was the dominant phytoplankton in all ponds, with Chrysophyta and Chlorophyta replacing it at some later time (Appendix 2).
Types of phytoplankton genera found in the pond are presented in Appendices 7, 8, 9 and 10. In the first and second pond WPWA were Chlorophyta (19 and 15 genera), Chrysophyta (16 and 11 genera), Cyanophyta (7 and 9 genera), Euglenophyta (3 and 2 genera), and Phyrophyta (one genera each), while in the first and second pond WOPWA were Chlorophyta (16 and 18 genera), Chrysophyta (12 and 16 genera), Cyanophyta (10 and 7 genera), Euglenophyta (2 genera each) and Phyrophyta (one genera each).
Oscillatoria was identified as the most abundant genera of phytoplankton present (Appendices 3, 4, 5 and 6). Oscillatoria dominated in the first 8 weeks after prawn stocking with the exception in pond no. 2 WOPWA where its dominance was visible only the 10th week. Other genera of considerable number were Cyanophyta (Anabacna, Merismopedia, Lyngbya, Chroococcus), Chrysophyta (Navicula, Synedra, Cyclotella, Gyrosigma, Dynobryon), Chlorophyta (Dispora, Protococcus, Cosmarium, Crucigenia), Euglenophyta (Euglena), and Phyrophyta (Ceratium).
The experiment shows a clear relationship between phytoplankton and zooplankton in both the ponds WPWA and WOPWA. Increase in Phytoplankton population led to an increase zooplankton population (Appendix 25). With the exception in pond no. 1 WPWA were it nows observed that phytoplankton population decrease if there was an increase in zooplankton.
Appendix 13 shows the status of zooplankton population densities in the various experimental ponds. Composition of zooplankton in each pond are presented in Appendices 15 and 16. Figure 2 and Table 3 show the changes of average zooplankton composition with time.
During the experiment it was observed that at different times there was dominance by different zooplankton groups occured. During most times rotifer dominated the composition of zooplankton compared to others such as Cladocera, Copepoda Nauplius and Adult (Appendix 14). However, at times, Copepoda Nauplius appear to be more dominant than the rotifer. At the termination of the experiment, Copepoda Nauplius or adult was dominant in all ponds except pond no. 1 WOPWA. While Cladocera sometimes disappeared from zooplankton composition.
Maximum zooplankton population for each treatment was not reach at the same time (Table 3). In ponds WPWA the total maximum zooplankton population reached was 3,944 and 1,562 per liter; while in ponds WOPWA it was 3,863 and 1,319 per liter. Analyses of variance of exponential growth rate to reach maximum population within each group and total was not statistically significant (P < 0.05) (Appendix 24).
Mean population of zooplankton and population in each treatment was reached at the same time (Figure 2). Highest zooplankton population in pond WPWA and WOPWA was 2,778 and 2,340 plankter per liter after 18 weeks interval of prawns stocking (Table 3). Rotifer was 2,425 plankter per liter in the 18th week; Cladocera was 83 and 27 plankter per liter in weeks 2 and 6. Copepoda Nauplius and Adult were 353 and 220; 278 and 257 plankter per liter in weeks 16 and 24 and in week 20.
Measurements conducted on Chlorophyll a concentration are presented in Table 5. Figure 3 shows the changes in average concentration of Chlorophyll a in pond WPWA and WOPWA over time. Observations on Chlorophyll a concentration in each pond were carried out (Appendix 17) every tow weeks (Biweekly).
For both ponds, Chlorophyll a concentration increased 8 weeks after prawns stocking (Appendix 17). Maximum concentration was reached in weeks 20 and 24 for ponds no. 1 and 2 WPWA and in pond WOPWA it was in weeks 16 and 24 respectively. Concentration of Chlorophyll a reached more than 150 ug/liter (150 ug m-3) in each pond after 14 weeks of prawns stocking (Table 3). Maximum concentration in pond WPWA was 267 mg m-3 and 304 mg m-3 while in WOPWA it was 264 mg m-3.
There was a visible increase in mean concentration of Chlorophyll a in pond WPWA and WOPWA on the eighth week of prawns stocking. Maximum concentration reached was 265 mg m-3 and 285 mg m-3 in weeks 24 and 16 (Table 30). Analyses of variance of Chlorophyll a concentration did not result in a significant difference (P< 0.05) (Appendix 23).
There existed a clear direct relationship between phytoplankton population and Chlorophyll a concentration. Chlorophyll a concentration increased directly with the phytoplankton population in the pond. In pond WPWA no. 1 and average regression and correlationship was clear (P> 0.05 and P> 0.01) (Appendix 25).
Data measurement of organic matter particles in the ponds during the experiment are presented in Table 6. Figure 5 shows the average changes in amount of organic matter in ponds WPWA and WOPWA over time. In the pond with covering equipment, amount of organic matter increased in the first six months in pond WPWA and WOPWA. Organic matter amount in the sixth month were 955 and 740 mg per m2 respectively. On the other hand, without the covering equipment showed on increase in the amount of organic matter till the termination of the experiment; while in pond WOPWA there was an initial increase in the organic matter amount for the first five months and a decline from there on. Maximum amount of organic matter was 455 and 221 mg m-2 respectively.
The description of organic matter for pond was carried out (Appendix 18). In ponds with covering equipment, organic matter particle increased for each pond every month based on samples taken till the finished of the experiment. Amount of organic matter reached in pond WPWA was 1002 and 907 mg m-2 while in pond WOPWA it was 825 and 655 mg m-2 (Table 6). Analyses of variance of organic matter particle with covering collected sample was not statistically significant (P <0.05) for every sample taken (Appendix 23).
For organic matter equipment without using a cover in pond no. 2 WPWA the organic matter increased till the finish of the experiment. Maximum amount of organic matter was 526 mg m-2. In pond no. 1 WPWA, organic matter initially in the first two months followed by a decrease in months 2 to 4. The organic matter started increasing again in month fifth. Maximum amount of organic matter sixth months was 384 mg m-2. For pond POPWA, pond no. 1 organic matter decreased in the first two months an increase in the second to the forth month and a decline from there on. For pond no.2 WOPWA, the organic matter increased in the irst two months followed by a decrease in months 2 and 3, and an increase in months 3 and 5. Maximum amount of organic matter recorded in pond WOPWA was 175 and 310 mg m-2 in the forth and fifth month respectivelly (Table 6). Analyses of variance of organic matter was not significantly diferent (P < 0.05) for each month of sampling, except in the sixth month (P > 0.05) (Appendix 23).
During the experiment, it was observed that there was a positive corelationship between increasing organic matter particles and zooplankton and phytoplankton yields (Appendix 25). However, it was just the reverse in the case of benthic organisms against zooplankton and phytoplankton population.
Data on the number of benthic organisms per sq. metre in the ponds during experiment are presented in the Table 7. Figure 4 shows the changes in the number of benthic organisms in the pond over time.
It was observed the experiment, that the dominant benthic organisms were the Chironomid and Nematoda (Appendix 19). The other group of benthos acounted for less than 50 percent of the organism population. In the first two months amount of total benthic organisms increased in each pond, except for pond no.2 WPWA where the increase was in month three (3rd sampling) (Table 7). Maximum number of benthic organisms in pond no.1 and 2 WPWA were 3,516 and 1,128; and in WOPWA 2,984 and 968 per sq. metre respectively. The mean maximum population in pond WPWA and WOPWA was reached at same time in the second month (2nd sampling), with population for each were 2,051 and 1,976 per sq. metre respectively.
The analyses of variance on the number of benthic organisms was not significantly different (P < 0.05) for the monthly sampling (every sampling) till finishing of experiment (Appendix 23).
Relationship between benthic organisms with organic matter particle (with and without cover) was negative (Appendix 25).
Stomach contents analyses of prawns conducted by Mean Bulk Index (MBI) is presented in Table 8. From samples five (first and second sampling) and sixteen prawns (third sampling) analysed, there was a variation is the amount of feeds in the stomach or intestine. In the first sampling, pellets (MBI means 1 and 1.35) were the most important prawn feeds in pond WPWA and WOPWA. This was followed by phytoplankton, zooplankton and animal. Detritus and aquatic plant were not much utilised as feeds by the prawns (Table 8). Phytoplankton genera was found in the stomach and intestine of many prawns such as the Cyanophyta (Oscillatoria, Merismopedia, Anabaena) and Chrysophyta (Diatoma, Navicula, Synedra). Only Copepoda Nauplius appears was consumed.
The second of prawn stomach contents, the pellets obtained MBI value of 1.0 and 1.5 (average) in pond WPWA and WOPWA, a low figure compared to other feeds. Pellets are the most important feeds of prawns followed by animals (including worm and insect), aquatic plants, phytoplankton and zooplankton. Varieties of phytoplankton genera found in stomach and intestine of prawns, include Cyanophyta (Oscillatoria, Merismopedia), Chrysophyta (Navicula, Diatoma) and Chlorophyta (Scenedesmus), while the main types of zooplankton consumed were Rotifer and Copepoda adult (Appendix 21).
In the third sampling, pellets were the most important feeds for adult prawns of marketable size. The pellets had MBI value 1.8 and 1.4 in pond WPWA and WOPWA. The important prawn feeds are phytoplankton, zooplankton, animals, detritus, and aquatic plants. The dominant phytoplankton found in prawn stomach are Cyanophyta (Oscillatoria, Merismopedia, Anabaena), Chlorophyta (Spyrogyra, Scenedesmus), Chrysophyta (Navicula, Gyrosigma, Synedra). On the other hand, the main types of zooplankton consumed by the adult prawns are Rotifer, Copepoda nauplius and adult.
In the intestine of the prawns the dominant phytoplankton include Cyanophyta (Oscillatoria, Merismopedia), Chrysophyta (Navicula, Synedra). Rotifer and Copepoda adult were found in higher numbers than Cladocera and Copepoda nauplius in the prawn intestine (Appendix 22).
