Effects of Cyanobacteria on Phosphorus Cycling and Other Aquatic Organisms in Simulated Eutrophic Ecosystems

Effects of nutrient enrichment on freshwater macrophyte and invertebrate abundance: A meta-analysis

External nutrient loading can cause large changes in freshwater ecosystems. Many local field and laboratory experiments have investigated ecological responses to nutrient addition. However, these findings are difficult to generalize, as the responses observed may depend on the local context and the resulting nutrient concentrations in the receiving water bodies. In this research, we combined and analysed data from 131 experimental studies containing 3054 treatment-control abundance ratios to assess the responses of freshwater taxa along a gradient of elevated nutrient concentrations. We carried out a systematic literature search in order to identify studies that report the abundance of invertebrate, macrophyte, and fish taxa in relation to the addition of nitrogen, phosphorus, or both. Next, we established mixed-effect meta-regression models to relate the biotic responses to the concentration gradients of both nutrients. We quantified the responses based on various abundance-based metrics. We found no responses to the mere addition of nutrients, apart from an overall increase of total invertebrate abundance. However, when we considered the gradients of N and P enrichment, we found responses to both nutrients for all abundance metrics. Abundance tended to increase at low levels of N enrichment, yet decreased at the high end of the concentration gradient (1-10 mg/L, depending on the P concentration). Responses to increasing P concentrations were mostly positive. For fish, we found too few data to perform a meaningful analysis. The results of our research highlight the need to consider the level of nutrient enrichment rather than the mere addition of nutrients in order to better understand broad-scale responses of freshwater biota to eutrophication, as a key step to identify effective conservation strategies for freshwater ecosystems.

Feedback mechanisms of periphytic biofilms to ZnO nanoparticles toxicity at different phosphorus levels

The increasing use of nanoparticles (NPs) has raised concerns about their potential environmental risks. Many researches on NPs focused on the toxicity mechanism to microorganisms, but neglect the toxicity effects in relation to nutritional conditions. Here, we evaluated the interactive effects of zinc oxide (ZnO) NPs and phosphorus (P) levels on the bacterial community and functioning of periphytic biofilms. Results showed that long-term exposure to ZnO NPs significantly reduced alkaline phosphatase activity (APA) of periphytic biofilms just in P-limited conditions. Co-occurrence network analysis indicated that ZnO NPs exposure reduced network complexity between bacterial taxa in P-limited conditions, while the opposite trend was observed in P-replete conditions. Correlation analysis and random forest modeling suggested that excessive Zn²⁺ released and high reactive oxygen species (ROS) production might be mainly responsible for the inhibition of APA induced by ZnO NPs under P-limited conditions, while adjustment of bacterial diversity and improvement of keystone taxa cooperation were the main mechanisms in maintaining APA when subjected to weak toxicity of ZnO NPs in P-replete conditions. Taken together, our results provide insights into the biological feedback mechanism involved in ZnO NPs exposure on the ecological function of periphytic biofilms in different P nutritional conditions.