A review on the methods of determining ecological thresholds.

The drastic changes or threshold phenomena of ecosystems in response to environmental changes are main research focuses in ecology. However, the difficulty in quantitative detection of ecological thresholds hinders the further research progress and application of this topic. Based on typical cases, we analzyed the driving-response mechanism of S-shaped curve type, subsidy-stress type, step type of the potential ecological thresholds. Seven methods to determine ecological thre-shold were summarized, including locally weighted scatterplot smoothing, piecewise regression, Gaussian model, change-point analyzer, regime shift detection, threshold indicator taxa analysis and system dynamics simulation model. The advantages, disadvantages and applicability of those methods were also addressed. This study could provide references for quantitative detection of ecological threshold.

[Evaluation of eutrophication level changes in Baiyangdian Lake based on multiple biological groups]. / 应用多种生物类群评价白洋淀水环境质量变化.

Baiyangdian Lake is the largest shallow lake in the North China Plain. Due to the increases in upstream water consumption in recent decades, the amount of natural water entering the lake has decreased, resulting in a significant drop in the water level of Baiyangdian Lake. Severe eutrophication has occurred in Baiyangdian Lake owing to the discharge of domestic sewage around the sub-lakes. With the establishment of the Xiong'an New Area in 2017, the importance of environmental governance and protection of Baiyangdian Lake has been unprecedentedly increased. The implementation of ecological water supplement from upstream reservoirs and other basins has significantly increased water level of Baiyangdian Lake. Moreover, domestic sewage from surrounding rural areas was collected. In order to understand the current state of water quality in Baiyangdian Lake, and to evaluate the effects of the previous water replenishment and pollution control projects, we investigated water physicochemical variables and aquatic organisms of Baiyangdian Lake in August 2019. We evaluated water quality status of Baiyangdian Lake based on water quality, phytoplankton, zooplankton and macrozoobenthos, by comparing with the evaluation based on the survey data in 2010. In addition, submerged macrophytes were used to evaluate the water ecological status. The results showed that the water quality state of Baiyangdian Lake in 2019 was greatly improved compared with 2010. Among them, total phosphorus concentration decreased by 88.6%, total nitrogen concentration decreased by 83.9%, chlorophyll a concentration decreased by 47.8%, and the Seechi depth increased by 43.4%. The diversity of zooplankton and macrozoobenthos were significantly increased, the density of phytoplankton was significantly reduced, the dominant species of phytoplankton changed from a general highly eutrophic type to a eutrophic type, and the distribution of the clean-type submerged macrophytes was narrowed. The evaluation methods based on submerged macrophytes species and based on zooplankton diversity were not suitable for water quality evaluation in Baiyangdian Lake. In the early stage, the pollution source of Baiyangdian Lake was concentrated in the west, which led to lower water quality. Nowadays, due to the water replenishment from the west, north and south, the hydrodynamic conditions changed significantly. In the west, pollution control projects were carried out, and the spatial distribution of water quality in Baiyangdian Lake are showing a characteristics of homogenization.

[Research basis of ecological thresholds and regime shifts in China].

Non-linear changes widely occur in ecosystems, which manifest as sudden changes in system state with increasing pressure. To explain these changes, ecologists proposed the concepts of ecological threshold and regime shift, and continuously improved the theory and method system. Mechanistic and empirical studies further deepen our understanding for evolution mechanism of the complex system, which begun to be implemented into the environmental management. In recent decades, a large number of quantitative studies on stress-response in various types of ecosystems in China. Results from those studies are closely related to the theory paradigms of ecological threshold and regime shift in essence. Based on the "China Ecological Thresholds and Alternative Stable States Database", we screened and summarized the relevant ecological thresholds and explained the mechanisms underlying regime shift for 10 ecosystem types, including river, lake, wetland, forest, grassland, estuary and ocean, farmland, desert, city, and tundra. We linked case studies with theory paradigms of ecological threshold and regime shift with the aim to integrate multi-field scientific achievements and resources as the basis for ecosystem complexity research, and promote their application in ecological monitoring, ecological security early warning, and ecological standards innovation.