Phytoplankton succession in shrimp ponds in Leganes, Iloilo, Philippines

Succession and diversity of phytoplankton community were studied in commercial shrimp (Penaeus monodon Fabricus) ponds in one grow-out season (May-Oct 2005) in Leganes, Iloilo, Philippines. 'Green water' technology was adopted for the three different postlarval stocking densities assigned within a randomized complete block design (RCBD) of two replicates, as treatment 1 (T1), 10 post larvae (PL).sq m, treatment 2 (T2), 15 PL/sq m, treatment 3 (T3), 20 PL.sq m. Weekly qualitative and quantitative analysis of phytoplankton composition was done along with the physicochemical analyses of the pond waters. Of the various classes of algae, the green algae (Chlorophyceae) were the most numerous group of algae (69% of the total algae) at the beginning up to 35 days of culture period in all treatments, while the blue-green algae (Cyanophyceae) had the most number of algae at the end of the culture period or at 84-126 days (44% of the total algae). The most dominant green alga was Nannochloropsis sp. while the most dominant blue green alga was a species of Oscillatoria. Throughout the culture period, T2 had significantly the highest mean number of phytoplankton (769,110 units/mL) over the other two treatments. While its Shannon-Wiener index of diversity was H'=1.66 and was not significantly different from the highest value obtained in T3 (H'=1.72). The lowest significant value was H'=1.42 that was observed in treatment 1. Of the different physico-chemical analysis done, the highest average value for dissolved oxygen (4.82 micro g/L) was observed in T2. Salinity was noted to be significantly low in T2 (25.44 ppt), while T3 had significantly low values for dissolved oxygen (3.78 micro g/L) and pH (7.5). Among the three treatments, T2 had the highest phytoplankton density, shrimp biomass and dissolved oxygen values, and had a high species diversity index. On the other hand, T3 had the lowest algal density, dissolved oxygen values and shrimp biomass. It seems that the importance of the algae on the growth of the shrimps depends on the diversity of algal species and on their effects on water quality.