Partitioning of wastewater treatment high rate algal pond biomass and particulate carbon

The partitioning of algae, bacteria, grazers and detritus in two wastewater treatment high rate algal ponds (HRAPs) was investigated in relation to particulate carbon (PC) over a year. Algae dominated the pond biomass accounting for ~ 61% of the PC. Changes in algal biomass correlated with changes in PC with both varying seasonally and having spring or summer maxima. The algal biomass itself was dominated by larger cells or colonies in the 20-200 μm size fraction with Pediastrum sp. prominent. Bacterial biomass, in contrast, only accounted for ~ 13.5% of the PC and varied less seasonally. Grazer biomass was lowest at ~ 4% of the PC on average and was dominated by either zooplankton or microzooplankton. Grazer biomass however, varied the most and reached ~ 14% of the PC during a spring zooplankton bloom that markedly reduced algal biomass. The remaining ~ 21.5% of the PC was made up of dead algal, detrital matter, and mucilage that tended to aggregated into bio-floccs. The PC of an efficiently operating HRAP is shown to be driven by high algal biomass with low bacterial and grazer biomass. If this balance is lost grazers may grow to levels that enable them to reduce algal biomass and productivity compounding pond instability.