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

Primary production of lake phytoplankton, dominated by the cyanobacterium Cylindrospermopsis raciborskii, in response to irradiance and temperature

We present the first data on the interacting effect of temperature and light on primary production of the toxic cyanobacterium Cylindrospermopsis raciborskii in situ. C. raciborskii can be a dominant component of the phytoplankton community in tropical and subtropical lakes and reservoirs. We examined the interacting effects of a range of light (0, 2, 7, 17, 30, and 100% of ambient light) and temperature (20, 24, 28, and 32 °C) conditions, in terms of primary production rate and primary production irradiance model parameters, for a C. raciborskii-dominated phytoplankton community in a subtropical reservoir. Based on 13C-uptake experiments, phytoplankton preconditioned to temperatures between 24 and 26 °C had highest maximum primary production rates (2.25 ± 0.45 µg C µg Chl-a-1 h-1) at 28 °C and lowest at 32 °C (0.58 ± 0.13 µg C µg Chl-a-1 h-1). Temperature also had an effect on the response to light conditions. Phytoplankton preconditioned to a shallow euphotic depth (~2.3 m deep) had the lowest half saturation of primary production, Ik, at 28 °C and highest at 32 °C, while the highest temperature treatment also had the highest level of photoinhibition at 100% of ambient light. This suggests that the cyanobacterial community is adapted to a low light environment under optimal temperature conditions for primary productivity. These conditions are consistent with other studies showing that C. raciborskii is highly adapted to low light conditions. This work demonstrates the importance of considering temperature when comparing calibrated primary production parameters

Schindler’s legacy : from eutrophic lakes to the phosphorus utilization strategies of cyanobacteria

David Schindler and his colleagues pioneered studies in the 1970s on the role of phosphorus in stimulating cyanobacterial blooms in North American lakes. Our understanding of the nuances of phosphorus utilization by cyanobacteria has evolved since that time. We review the phosphorus utilization strategies used by cyanobacteria, such as use of organic forms, alternation between passive and active uptake, and luxury storage. While many aspects of physiological responses to phosphorus of cyanobacteria have been measured, our understanding of the critical processes that drive species diversity, adaptation and competition remains limited. We identify persistent critical knowledge gaps, particularly on the adaptation of cyanobacteria to low nutrient concentrations. We propose that traditional discipline-specific studies be adapted and expanded to encompass innovative new methodologies and take advantage of interdisciplinary opportunities among physiologists, molecular biologists, and modellers, to advance our understanding and prediction of toxic cyanobacteria, and ultimately to mitigate the occurrence of blooms