Spirulina (Arthrospira) is an important microalga that can generate a variety of commercial products. A semi-continuous mode of cultivation is often employed in microalgal cultivation. The traditional view, however, has been that cultures with recycled medium are more likely to be contaminated by microzooplankton, which is regarded as one of the critical problems in mass algal cultivation. In this study, the relationship between the population dynamics of contaminants and the medium conditions was investigated in Spirulina cultures. Spirulina platensis was cultivated in three groups of raceway ponds: one with fresh culture medium, one with medium that had been recycled for a month, and the third with medium recycled for six months. The results showed that totally 13 species of microzooplankton were observed by light microscopy, and 42 operational taxonomic units (OTUs) of prokaryotic contaminants were detected using amplicon sequencing. Out of all the contaminants, Brachionus plicatilis and Euplaesiobystra hypersalinica were observed to be the most harmful species in Spirulina cultures, while Proteobacteria were the most commonly found non-Cyanobacteria OTUs. On the initial day, more species of microzooplankton were introduced to the cultures that had recycled medium (9 species) than to those that had fresh medium (5 species). By the end of the cultivation, the algal biomass in the fresh medium group was the highest (2.8 g L-1), being almost 5 times higher than in the other two groups (around 0.50 g L-1). Our results proved that Spirulina can grow the best with fresh medium and that the more the culture medium is recycled, the stronger the inhibition on the growth of microalgae and microzooplankton. In order to improve large-scale Spirulina production, it is necessary to both subject the recycled medium to appropriate treatment to reduce the presence of harmful predators and to find effective ways to control contaminants.