There is debate about the relative importance of controlling anthropogenic nitrogen (N) versus phosphorus (P) inputs to limit algal growth in lakes and reservoirs. Our study examined nutrient responses in a subtropical reservoir using short-term algal bioassays on 3 occasions, once during the austral winter and twice during the austral summer. Measurement of photosynthetic yield (Fv/Fm) was used to determine the response to nutrient addition. For the 2 summer sampling occasions, the photosynthetic yields of the N+P treatments were significantly higher than the control. At some sites and on some occasions there was a response to P or N alone, but there was no consistent pattern. The one winter sampling occasion had no response to nutrient addition. Overall, the magnitude of the photosynthetic yield of algal samples correlated with nitrate/nitrite (NO2-;/NO3-;) and soluble reactive phosphorus (SRP) concentrations, but not with ammonium (NH4+), or dissolved organic N (DON) or P (DOP), despite relatively high concentrations of DON. Therefore we concluded that both N and P co-limited the growth of phytoplankton in the 2 austral summer sampling occasions. This contrasted with high N:P ratios and low P concentrations observed, which suggested that the reservoir was most likely to be P limited. This study highlights the importance of determining algal responses to nutrients and measures nutrient concentrations and ratios to determine whether N or P should be controlled to prevent algal blooms.