KONINKLIJKE ACADEMIE VOOR OVERZEESE WETENSCHAPPEN ...

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Chlorophyta (14 taxa) and Euglenophyta (8 taxa) and finally Bacillariophyta
(4 taxa). These results show higher diversity in both treatments in Vinh Long
province in comparison with the number of phytoplankton taxa in culture systems
of the Mekong Delta.
Typically, the main species of phytoplankton were: Melosira varians,
Nitzachia acicularia, Synechonecystis aquatilis, Coelosphaerium kutzingia,
Oscillatoria limosa, Synechonecystis aquatilis, Polycystis incerta, Lyngbya
birgei, Pediastrum biradistum, Scenedesmus dimorphus, Pediastrum biradistum,
Euglena spirogyra, Phacus alata, Phacus pleuronectes. The mean of phytoplankton
density in both treatments ranged from 59.856 ± 6.154 cells. 1' to
212.694 ± 142.439 cell. 1 ' (fig. 14). The density of phytoplankton was the highest
in samples 2 and 3 and the lowest in sample 1 for both treatments. The reason
of low density of phytoplankton was low water level by selection for bigger
fish after nursing and new water was supplied. Hence, the flood did not affect the
water quality and fish decreased eating in cold weather and superfluous food was
increased. Flooding did not affect phytoplankton density in samples 2 and 3 but
cold weather decreased activity of fish eating. This led to superfluous food and
to the increase of nutrient in ponds and the improvement of phytoplankton density
in intensive culture systems.
3.2.2. Zooplankton
Time (months)
—• — Treatment 1 —■— Treatment II
Fig. 14. — Density of phytoplankton (cell/1) in intensive culture system.
The results presented in figure 15 show that the number of zooplankton taxa
was 36, the main species was Rotatoria (14 taxa), followed by Copepoda
(12 taxa) and Protozoa (10 taxa), and finally Cladocera (6 taxa). The main
species of zooplankton in both treatments were: Centropyxis ecomis, Difflugia