There is little known about the role of nutrients in affecting phytoplankton growth and composition in subtropical reservoirs, unlike their temperate or tropical counterparts. Subtropical reservoirs that are typically larger than their temperate counterparts, with long residence times and undergo intense stratification in summer. These characteristics favour growth of one phytoplankton group, cyanobacteria, which tend to be the dominant taxa in subtropical reservoirs. This study examined the role of nutrients in promoting phytoplankton growth and species composition in a subtropical reservoir during a period of low inflows from the catchment. Chlorophyll a concentrations in Wivenhoe reservoir were in the range of eutrophic to hypertrophic levels with the highest concentrations (>60 [mcg] L-1) recorded at the upstream sites compared to the dam wall downstream (~10 [mcg] L-1). Nutrient concentrations were also higher upstream and the phytoplankton biomass was positively correlated with depth of the water column and total dissolved phosphorus concentrations. Mixing of the water column ensured that nutrients from the bottom of the water column are circulated to the euphotic zone in the shallow upstream sites, whereas at the downstream sites the water column was deeper and more often stratified. Therefore availability of nutrients was likely to be a key factor in increasing phytoplankton biomass. The reservoir phytoplankton community composition also correlated with the depth of the site and was dominated by cyanobacteria, by cell concentrations, and by diatoms, in terms of cell biovolume. Some of the dominant cyanobacterial groups, i.e. Aphanocapsa spp., Aphanothece spp., Cyanodictyon spp., Cyanocatena spp., Cyanonephron spp., Cyanogranis spp., Planktolyngbya spp. and Pseudanabaena spp. in this study are poorly understood with little or no published literature on their ecology. The toxin producing species, Cylindrospermopsis raciborskii, was also present in relatively high concentrations near the dam wall.