M.Sc. (Aquatic Health) / The relationships between water quality variables and phytoplankton diversity in the Hartbeespoort Dam were assessed spatially and temporally from February 2011 to March 2012 to evaluate the effects of the water quality variables on cyanobacterial bloom (Microcystis aeruginosa) hence aquatic macrophytes growth (Eichhornia crassipes) in the dam. Variables measured using standard methods included; temperature, pH, electrical conductivity, total suspended solids, dissolved oxygen, nitrate, nitrite, total phosphorous , ammonium, trace metals, chlorophyll-a and the phytoplankton community. The physical parameters ranged between: temperature (11.8-28 oC), electrical conductivity (282-796 ƒÊS/cm), dissolved oxygen (0.33-32.2 mg/L), pH (6.95-9.91) and total suspended solids (2-372 mg/L). Chemical variables ranged between; total phosphorous (0.02-3.5 mg/L), nitrate (0.03-21.2 mg/L), nitrite (0.02-0.48 mg/L) and ammonium (0.01-1.58 mg/L), chlorophyll-a (0.13-8693 ƒÊg/L), and exceed the TWQR values of the South African Water Quality Guidelines for aquatic ecosystem health health. Metal concentrations in water had the following decreasing order; macro elements: potassium > calcium >sodium > magnesium. Microelements: iron >zinc > aluminium > copper > nickel > manganese > chromium> selenium > lead > silver > arsenic > cadmium. Iron had the highest concentration among microelement of 631.62 ƒÊg/L and potassium the highest concentration amongst macro element of 34.49 mg/L. Six Different algal divisions were found in the dam with cyanophyta (cyanobacteria) been the most dominant group (95 %) and M.aeruginosa the most dominant species (69 %). The current study revealed an increase in physical parameters, chlorophyll-a and phytoplankton community and a decrease in chemical parameters in the summer months. An inverse relationship was observed in the winter months at all sites. One-way ANOVA showed a significant differences for physical variables (p <0.05) between months, with no significant differences noted (p > 0.05) between sites and between depths. Chemical variables however, showed a significant differences between months, sites and between depths (p <0.05). A 2-tailed Pearson correlation revealed negative correlations between temperatures and phosphorus, ammonia, nitrate, nitrite, electrical conductivity and iron (r=-0.298;-0.232;-0.099;-0.461;-0.441;-0.260) respectively and positive correlations between temperatures and chlorophyll-a and pH (r= 0.240; 0.609 ;) respectively (p <0.05; p <0.01). Canonical discriminant functions analysis revealed similarities and dissimilarities in water quality variables temporally and spatially with eigenvalues of 84.6 % and 59.1 % respectively. There was an adverse impact of the physico-chemical variables on the phytoplankton community, therefore aquatic macrophytes growth in the dam. The current study revealed that temperature, pH, phosphorous, nitrate and probably iron, copper, zinc and selenium may have contributed to the hypertrophic state of the dam, hence cyanobacterial bloom and growth of aquatic macrophytes.