Assessing nutrient limitation in a subtropical reservoir

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.