Microbial populations in tropical reservoirs using flow cytometry

Flow cytometry was applied in the study of bacteria and phytoplankton populations in five tropical reservoirs. Water quality between different reservoirs was compared and correlation analyses were carried out to investigate how the biomass of bacteria and phytoplankton related to other water quality parameters measured (i.e., temperature, dissolved oxygen, pH, conductivity, water transparency, turbidity, chlorophyll a, and total nitrogen and phosphorus). Average chlorophyll a concentrations were typically greater than 20 mu g/L. Bacteria populations detected with flow cytometry were generally small in size (typically <0.08 mu m(3) or 0.3 mu m equivalent spherical diameter) and contributed less than 13% of the total microbial biomass. Subpopulations of pico-, ultra-, and net phytoplankton were discriminated flow cytometrically by their red and orange autofluorescence. Cyanobacteria dominated four out of the five reservoirs in terms of numbers but only contributed more than 50% of the microbial biomass in two of the reservoirs. In general, local reservoirs were found to be phosphorus limited and alkaline conditions favored the growth of phytoplankton and bacteria.