High Rate Algal Ponds (HRAPs) are effective and economical open pond to provide near tertiary-level wastewater (WW) treatment, with the nutrients recovered as algal biomass. However, WW treatment performance can be reduced by the establishment of zooplankton grazers which can consume much of the algal biomass within a few days. The high food availability and the near neutral pH in HRAPs offer optimal conditions for the establishment of zooplankton taxa including cladocerans and rotifers. The control of these zooplankton species is essential for the effective operation of HRAPs. This thesis research was aimed at designing cost-effective treatments to control zooplankton population densities applicable to full-scale HRAPs. In this thesis, I: 1) reviewed existing and potential methods for zooplankton control in HRAPs; 2) assessed environmental and biological parameters for two wastewater HRAPs with CO₂ addition using a 14 month large-scale field experiment; 3) assessed chemical CO₂ asphyxiation, biological control of rotifers using competitor species, mechanical disruption of zooplankton using hydrodynamic shear stress, and the mechanical removal of zooplankton using filtration, using controlled laboratory conditions; 4) validated chronic CO₂ asphyxiation of zooplankton, the biocontrol of rotifers using the cladoceran M. tenuicornis and the ostracod H. incongruens, the control of M. tenuicornis using filtration and the hydrodynamic disruption of zooplankton, using outdoor mesocosms with physicochemical and operational conditions similar to those of full scale HRAPs; 5) assessed night time CO₂ asphyxiation to control zooplankton densities in an 8 m³ HRAP over 14 months. During the 14 months of the field experiment, zooplankton dynamics in paired 8 m³ HRAPs eight species of zooplankton established, with higher water temperatures and longer detention times promoting larger populations. Grazing pressure was associated with changes in the dominance of microalgal species; large and rapid reductions of productivity; reduced nutrient removal; increased colony size, number of cells in colonies and formation of protective spines in microalgae; and higher biomass settleability. Maintaining a dominance of colonial microalgae, operating HRAPs with short retention times, and facilitating competition among zooplankton species showed to potentially reduce grazer populations. Promising options for zooplankton control included physical methods such as filtration, hydrodynamic cavitation, shear, bead mills; chemical methods such as increase of HRAP night-time CO₂ concentration, promotion of the lethal un-ionized ammonia toxicity, use of biocides, and the chitinase inhibitor chitosan; biocontrol using competitor and predatory organisms. In the laboratory tests CO₂ asphyxiation caused acute mortality of all zooplankton species (t<10 min). Increasing the cladoceran Moina tenuicornis to densities >2,500 individuals/L was associated with a decrease in rotifer populations that were ~23% of the population in the control. The ostracod Heterocypris incongruens at densities >1,000 individuals/L were also associated with a decrease in rotifer densities that were ~27% of the population in the control. Hydrodynamic shear stress killed 100% of M. tenuicornis and ~80% of the rotifer Brachionus calyciflorus after a single pass. In outdoor mesocosms a continuous CO₂ concentration of ~100 mg/L maintained low pond water zooplankton densities, while a continuous concentration of ~180 mg/L killed all microcrustaceans and rotifers present. As biocontrol agents, M. tenuicornis at ~2,000 individuals/L reduced average rotifer densities by 90%, and H. incongruens at ~1,000 individuals/L removed all rotifers. Mechanical filtration using 300 µm and 500 µm filters eradicated M. tenuicornis after one and four filtration periods, respectively. Mechanical hydrodynamic stress killed up to 100% of microcrustaceans, and ~50% of larger rotifers. Zooplankton control using night time CO₂ asphyxiation in an 8 m³ HRAP reduced the average population densities of some zooplankton species over the experimental period: M. tenuicornis (41.3%), the copepod Paracyclops fimbriatus (43.9%), the rotifer Filinia longiseta (59.8%), but was associated with higher average population densities of others: H. incongruens (174.4%), the rotifers Asplanchna sieboldi (177.8%), Cephalodella catellina (200.0%), and B. calyciflorus (234.9%). However, the population densities of B. calyciflorus and C. catellina were always reduced after CO₂ treatments with flow rates ≥2 L/min were applied. The cladoceran Daphnia thomsoni and the rotifer Brachionus urceolaris established only in the control HRAP. Zooplankton control by CO₂ asphyxiation improved the overall performance of the treated WW HRAP compared to the control in several ways, including increasing algal biomass (VSS) (150.8%), productivity (151.4%), chlorophyll-a concentration (161.8%), particle size (MCSA) (115.8%), and average settleability efficiency (189.2%). Furthermore, M. tenuicornis was concentrated in the upper 50 mm of a 300 mm deep water column using vertical migration induced by CO₂ concentrations of between 25-180 mg/L in laboratory experiments, and by phototaxis-induced migration in an 8 m³ HRAP. This suggested that mechanical treatments such as filtration and hydrodynamic stress could be performed to the upper layer of the pond water, reducing the amount of water processed and the overall treatment costs. Overall, CO₂ asphyxiation appeared to be the most reliable, versatile, and effective zooplankton control treatment. All treatments reduced or eradicated zooplankton populations and promoted higher productivity of microalgae cultures. However, the efficacy of treatments to control diverse zooplankton species differed, and the implementation of any control strategy in a given system requires a preliminary assessment of zooplankton succession to identify the species able to consume the dominant microalgae that could establish in the system. Treatments can be dosed and combined to selectively kill specific zooplankton species. Moreover, zooplankton at controlled densities could potentially be used as a bio tool to improve biomass settleability or to consume unwanted microalgal species.