Phytoplankton bloom in commercial shrimp ponds using green-water technology

Phytoplankton community composition, density, and succession were studied in tropical commercial ponds with euryhaline tiger shrimp (Penaeus monodon Fabricius) using green-water technology at two different stocking densities [T-1 10 post-larvae (PL) m(-2) and T-2 15 PL m(-2)] in one grow-out season (May-October 2005) in Leganes, Iloilo, Philippines. Weekly qualitative and quantitative analyses of phytoplankton were done along with physicochemical analyses of the pond waters. A total of 103 taxa belonging to nine different algal classes were observed. Of these classes, the Chlorophyceae, Cyanophyceae and Bacillariophyceae constituted the great bulk of the phytoplankton population. The two treatments did not show any significant differences in the growth pattern of phytoplankton over time and in their diversity indices. Although T2 had higher values than T-1 for algal density and species diversity index, the differences were not significant. The mean Shannon-Wiener diversity index for T-2 (1.56) was higher than T-1 (1.39) but not significantly different. Both treatment ponds had Chlorophyceae as the dominant algae during the initial culture phase [0-35 days of culture (DOC)], which coincided with high salinity (average= 35.67 ppt) and relatively high N:P ratios (average= 1.95). The chlorophycean bloom was made up mostly of Nannochloropsis sp. The cyanophycean bloom occurred towards the final culture phase (84-112/126 DOC) when there was low salinity (average= 19.5 ppt) and relatively high N:P ratios (average 2.01). A short diatom bloom occurred in T-2 at the same time that the N:P ratios rose dramatically to 4.2 at 42 DOC. Among the eight physicochemical parameters examined, positive correlations were noted among alkalinity, ammonium-nitrogen, nitrite-nitrogen and phytoplankton community. High species diversity index and species richness could have enhanced the stability of favorable Nannochloropsis blooms, especially in T2. No differences were noted between the two treatments in terms of the shrimp's biomass at harvest time (T-1=28.9 and T-2=29.4 g fresh wt per shrimp), although a significantly higher survival rate (P<0.05) was observed in T-1 (97%) than in T-2 (56%). Both treatments were able to control the occurrence of the luminous bacterium Vibrio harveyi.