The presence of cyanobacterially-produced microcystins (cyclic peptide hepatotoxins) was determined
by analysis of Microcystis spp. in scum and water samples collected from a surface cyanobacterial bloom at
a giant freshwater prawn (Macrobrachium rosenbergii) farm in Teung District, Chiang Rai Province, in
northern Thailand during March to August 2004. M. aeruginasa and M. wesenbergii were the dominant
species of cyanobacteria at concentrations between 850,000±190,000 and 302,000±73,000 colonies l-1.
Microcystins were present at 0.44±0.020 g kg-1 dry weight with microcystin-LR and microcystin-RR as the
dominant microcystin types, accounting for 45% and 48% of the total microcystins detected, respectively.
Microcystins in pond water were present at between 2.2±3.0 µg l-1 and 9.4±2.0 µg l-1. Total microcystin
concentrations in water seemed to be positively correlated with the number of Microcystis colonies. A
decrease of microcystins in water was observed from April to August 2004, which may have resulted from
removal by mechanisms not examined in this study. The total microcystin in water was slightly negatively
correlated with total culturable bacteria numbers. Microcystis spp. colony number showed a significant
negative correlation with soluble reactive phosphorus (r = -0.98, p<0.05). Nitrate-N, ammonium-N and
soluble reactive phosphorus concentrations were between 1.2-1.9, 0.85-1.15 and 0.9-1.1 mg l-1, respectively.
Phosphorus concentrations were higher than the permitted limit for waste water from a fishery farm (less
than 0.4 mg l-1). This study suggested that surface blooms of Microcystis species in cultivation ponds may
present a risk for microcystin bioaccumulation in prawns, either directly or via other organisms in the food
web.