Hydrological regulation drives regime shifts: evidence from paleolimnology and ecosystem modeling of a large shallow Chinese lake

Both nutrients and macrophytes regulate organic carbon burial: Insights from high-resolution spatiotemporal records of a large shallow lake (Baiyangdian) in eastern China

Both ecological regime shifts and carbon cycling in lakes have been the subject of global debates in recent years. However, the direct linkage between them is poorly understood. Lake Baiyangdian, a representative large shallow lake with the coexistence of a macrophyte-dominated area (MDA) and an algae-dominated area (ADA) in eastern China, allowing better understanding of the relationship between regime shifts and organic carbon (OC) burial in lakes. On the basis of Bayesian isotopic mixing modelling of C/N ratios and δ13C values, the sediment OC is primarily of autochthonous origin. The mean OC burial rate (OCBR) was 39 g C m-2 yr-1 before eutrophication occurred in 1990 and increased approximately 2.7-fold to 106 g C m-2 yr-1 after eutrophication. Partial least squares path modelling revealed that this change can be largely attributed to enhanced primary productivity and rapid burial as a result of intensified human perturbation. In terms of spatial patterns, the OCBR was greater in the MDA than in the ADA, which may be related to the different burial and mineralization processes of debris from macrophytes and algae. It then deduced that a decrease in the OCBR and an increase in the mineralization rate might have occurred after a shift from a macrophyte-dominated state to an algae-dominated state. Our findings highlight that eutrophication generally increases OC burial by enhancing lake primary productivity. However, once nutrient levels reach a critical range, lake ecosystems may shift from a macrophyte-dominated state to an algae-dominated state, which can lead to a significant reduction in the carbon burial capacity of lakes. Therefore, more attention should be given to avoiding shifts in eutrophic lakes, as such shifts can alter carbon cycling.

Leveraging Bayesian network to reveal the importance of water level in a shallow lake ecosystem: A study based on Paleo-diatom and fish community

Lake ecological processes and nutrient patterns are increasingly affected by water level variation around the world. Still, the long-term effects of water level change on lake ecosystems and their implications for suitable lake level management have rarely been studied. Here, we studied the ecosystem dynamics of a mesotrophic lake located in the cold and arid region of northern China based on long-term paleo-diatom and fishery records. Utilizing a novel Copula-Bayesian Network model, possible hydrological-driven ecosystem evolution was discussed. Results show that increased nutrient concentration caused by the first water level drop in the early 1980s incurred a transition of sedimental diatoms towards pollution-resistant species, and the following water level rise in the mid-1980s brought about considerable external loading, which attributed to eutrophication and caused the miniaturization of fishery structure. In the 21st century, a continuous water level plummet further reduced the sediment diatom biomass and the fish biomass by altering nutrient concentration. However, with the implementation of the water diversion project in 2011, oligotrophic species increased, and the ecosystem developed for the better. From the perspective of water quality protection requirements and the ecological well-being of Lake Hulun, the appropriate water level should be around 542.42-544.15 m. In summary, our study highlights the coupling effect of water level and water quality on Lake Hulun ecosystem and gives shed to lake water level operation and management under future climate change and human activities.

Driver-response relationships in a large shallow lake since the Anthropocene: Short-term abrupt perturbations versus long-term sustainable

Lakes, as integral social-ecological systems, are hotspots for exploring climatic and anthropogenic impacts, with crucial pathways revealed by continuous sediment records. However, the response of multi-proxies in large shallow lakes to typical abrupt events and sustained drivers since the Anthropocene remains unclear. Here, we explored the driver-identification relationships between multi-proxy peaks and natural and anthropogenic events as well as the attribution of short-term perturbations and long-term pressures. To this end, sediment core records, socio-ecological data, and documented events from official records were integrated into a large shallow lake (Dongting Lake, China). Significant causal cascades and path effects (goodness-of-fit: 0.488; total effect: -1.10; p < .001) were observed among catchment environmental proxies, lake biogenic proxies, and mixed-source proxies. The peak-event identification rate (PEIR) and event-peak driving rate were proposed, and values of 28.57%-46.43% and 50%-81.25% were obtained, respectively. The incomplete accuracy of depicting event perturbations using sediment proxies was caused by various information filters both inside and outside the lake. PEIRs for compound events were 1.41 (±0.72) and 1.09 (±0.46) times greater than those for anthropogenic-dominated and natural-dominated events, respectively. Furthermore, socio-economic activity, hydrologic dynamics, land-use changes, and agriculture exerted significant and persistent pressures, cumulatively contributing 55.3%-80.9% to alterations in sediment proxies. Relatively synergistic or antagonistic trends in temporal contributions of these forces were observed after 2000, which were primarily attributed to the "Grain for Green" project and the Three Gorges Dam. This study represents one of the few investigations to distinguish the driver-response relationship of multiple proxies in large shallow lakes under typical event perturbations and long-term sustained pressures since the Anthropocene. The findings will help policymakers and managers address ecological perturbations triggered by climate change and human activities over long-term periods.

Hydro-meteorological factors and inflowing nutrients drive water quality in an impounded lake of China's South-to-North Water Diversion Project

Freshwater lakes play a vital role in global hydrological and biogeochemical cycles, serving various functions and maintaining ecological balance. However, freshwater resources are more vulnerable to deterioration due to multiple stressors. Gaoyou Lake is one of the impounded lakes of the Eastern route of South-to-North Water Diversion Project in China, and as an important source of drinking water, the lake has been routinely monitored. Long-term monitoring of water quality in Gaoyou Lake showed that concentrations of nutrients and chlorophyll a as well as trophic state in the water column increased while water transparency decreased, indicating that the water quality has declined during the last 12 years. Specifically, there was a notable and statistically significant increase in chlorophyll a concentrations, averaging an annual rate of 9.9%. Despite a slight decline in trophic level index until 2014, subsequent years saw an upward trend, ranging from 50.7 to 56.4 and indicating a light eutrophic state. Spatially, the western area displayed higher nutrient and chlorophyll a concentrations. Changes in hydro-meteorological variables and nutrients from inflowing rivers were the main factors correlated with water quality in Gaoyou Lake. Thus, pollution source apportionment and management within Huaihe River basin should be considered to reduce the external loadings of nutrients in order to improve and sustain long-term water quality.

Ecological threshold of phosphorus load in Baiyangdian Lake based on a PCLake model and ecological network analysis

Ecological thresholds are a useful indicator for implementing ecological management. Many studies determine the thresholds for nutrient loads in lakes based on the maximum allowable concentration of chlorophyll a (Chla), although this neglects the overall performance of the ecosystem. A PCLake model of Baiyangdian (BYD) Lake in northern China was constructed with six ecological network analysis (ENA) indicators that characterized the ecosystem function, system maturity, and food web structure to quantify the overall status of the BYD ecosystem. To my knowledge, this is the first study on the system level responses of the BYD Lake to phosphorus load interference. Different phosphorus load scenarios were designed to simulate the ecological responses of BYD Lake. The simulated results were employed to calculate the ENA indicators. Ecological thresholds were determined through the driving response relationship between the phosphorus load gradient and the ENA indicators. The results show a non-linear transition response of ENA indicator under phosphorus load gradient. As phosphorus load increases, D/H, SOI, and FCI decreases while A/DC, TPP/TR, and TPP/TB increases. This indicates that the overall structure and function of the ecosystem will deteriorate if phosphorus load increases. The phosphorus load thresholds for the overall performance of BYD Lake were 0.50-1.32 mg m-2 d-1, slightly wider than that of Chla (0.53-1.26 mg m-2 d-1). The model results clearly indicate that there is a time-lag phenomenon at the switch points in the response of ENA indicators compared to that of single functional group. In addition, the A/DC, TPP/TR, SOI, and FCI present more time-lag than that of other ENA indicators. These time-lag effects provide a particular opportunity for biodiversity conservation. Therefore, a possible management strategy is proposed to combine system-level and function group-level thresholds, with the ENA-based threshold as the bottom line and the phytoplankton's threshold as the early-warning indicator. This design is expected to be more precise and efficient, by exploiting the advantages of two thresholds, and may benefit for ecological management practices.

Identifying the true drivers of abrupt changes in ecosystem state with a focus on time lags: Extreme precipitation can determine water quality in shallow lakes

A better understanding of abrupt ecosystem changes is needed to improve prediction of future ecosystem states under climate change. Chronological analysis based on long-term monitoring data is an effective way to estimate the frequency and magnitude of abrupt ecosystem changes. In this study, we used abrupt-change detection to differentiate changes of algal community composition in two Japanese lakes and to identify the causes of long-term ecological transitions. Additionally, we focused on finding statistically significant relationships between abrupt changes to aid with factor analysis. To estimate the strengths of driver-response relationships underlying abrupt algal transitions, the timing of the algal transitions was compared to that of abrupt changes in climate and basin characteristics to identify any synchronicities between them. The timing of abrupt algal changes in the two study lakes corresponded most closely to that of heavy runoff events during the past 30-40 years. This strongly suggests that changes in the frequency of extreme events (e.g., heavy rain, prolonged drought) have a greater effect on lake chemistry and community composition than do shifts in the means of climate and basin factors. Our analysis of synchronicity (with a focus on time lags) could provide an easy method to identify better adaptative strategies for future climate change.

Restoring gradual land-water transitions in a shallow lake improved phytoplankton quantity and quality with cascading effects on zooplankton production

Land-water transition areas play a significant role in the functioning of aquatic ecosystems. However, anthropogenic pressures are posing severe threats on land-water transition areas, which leads to degradation of the ecological integrity of many lakes worldwide. Enhancing habitat complexity and heterogeneity by restoring land-water transition areas in lake systems is deemed a suitable method to restore lakes bottom-up by stimulating lower trophic levels. Stimulating productivity of lower trophic levels (phytoplankton, zooplankton) generates important food sources for declining higher trophic levels (fish, birds). Here, we study ecosystem restoration project Marker Wadden in Lake Markermeer, The Netherlands. This project involved the construction of a 700-ha archipelago of five islands in a degrading shallow lake, aiming to create additional sheltered land-water transition areas to stimulate food web development from its base by improving phytoplankton quantity and quality. We found that phytoplankton quantity (chlorophyll-a concentration) and quality (inversed carbon:nutrient ratio) in the shallow waters inside the Marker Wadden archipelago were significantly improved, likely due to higher nutrient availabilities, while light availability remained sufficient, compared to the surrounding lake. Higher phytoplankton quantity and quality was positively correlated with zooplankton biomass, which was higher inside the archipelago than in the surrounding lake due to improved trophic transfer efficiency between phytoplankton and zooplankton. We conclude that creating new land-water transition areas can be used to increase light and nutrient availabilities and thereby enhancing primary productivity, which in turn can stimulate higher trophic levels in degrading aquatic ecosystems.

Synergistic Effects of Warming and Internal Nutrient Loading Interfere with the Long-Term Stability of Lake Restoration and Induce Sudden Re-eutrophication

Phosphorus (P) precipitation is among the most effective treatments to mitigate lake eutrophication. However, after a period of high effectiveness, studies have shown possible re-eutrophication and the return of harmful algal blooms. While such abrupt ecological changes were attributed to the internal P loading, the role of lake warming and its potential synergistic effects with internal loading, thus far, has been understudied. Here, in a eutrophic lake in central Germany, we quantified the driving mechanisms of the abrupt re-eutrophication and cyanobacterial blooms in 2016 (30 years after the first P precipitation). A process-based lake ecosystem model (GOTM-WET) was established using a high-frequency monitoring data set covering contrasting trophic states. Model analyses suggested that the internal P release accounted for 68% of the cyanobacterial biomass proliferation, while lake warming contributed to 32%, including direct effects via promoting growth (18%) and synergistic effects via intensifying internal P loading (14%). The model further showed that the synergy was attributed to prolonged lake hypolimnion warming and oxygen depletion. Our study unravels the substantial role of lake warming in promoting cyanobacterial blooms in re-eutrophicated lakes. The warming effects on cyanobacteria via promoting internal loading need more attention in lake management, particularly for urban lakes.

Disentangling the effects of phosphorus loading on food web stability in a large shallow lake

Excessive nutrient loads reduce ecosystem resilience, resulting in fundamental changes in ecosystem structure and function when exceeding a certain threshold. However, quantitative analysis of the processes by which nutrient loading affects ecosystem resilience requires further exploration. Food web stability is at the heart of ecosystem resilience. In this study, we simulated the dynamics of the food web under different phosphorus loads for Lake Baiyangdian using the PCLake model and calculated the food web stability. Our results showed that there was a good correspondence between the food web stability and ecosystem state response to phosphorus loads. This relationship confirmed that food web stability could be regarded as a signal for the state transition in a real lake ecosystem. Moreover, our estimates suggested that food web stability was influenced only by several functional groups and their interaction strength. Diatoms and zooplankton were the key functional groups that affected food web stability. Phosphorus loads alter the distribution of functional group biomass, which in turn affects energy delivery and, ultimately, the stability of the food web. Corresponding to functional groups, the interactions among zooplankton, diatoms and detritus had the greatest impact, and the interaction strength of the three was positively correlated with food web stability. Overall, our study explained that food-web stability was critical to characterize ecosystem resilience response to external disturbances and can be turned into a scientific tool for lake ecosystem management.

Environmental laws and ecological restoration projects enhancing ecosystem services in China: A meta-analysis

In recent decades, China has implemented ecological restoration projects (ERPs) to improve biodiversity and ecosystem services (ESs), accordingly, a series of environmental laws were issued to guide ecological restoration. However, quantitative evaluation of the effectiveness of ERPs remains ambiguous. To respond to the UN Decade on Ecosystem Restoration (UNDER), we conducted a meta-analysis of 85 peer-reviewed publications and an interdisciplinary evaluation framework based on China's environmental protection and land administration laws (EPLALs) were established to assess the effectiveness of ERPs. We found that ERPs enhanced ESs by 15-58%. Specifically, ERPs implemented in industrial/mining, and wetland regions significantly increased regulating and cultural services, and in arid and semi-arid regions mainly enhance provisioning services (72.98%). Climate factors were found to be crucial for ecological restoration effectiveness (temperature: r = -0.582, significance <0.05; precipitation: r = 0.635, significance <0.05). China's environmental laws emphasized management and investment in ecological restoration. However, the disclosure, public participation and real-time monitoring of ecological conditions need to be improved urgently. We therefore developed ERP-related policy recommendations and global lessons to help improve the effectiveness of ecological restoration.

Synergistic effects of climate warming and atmospheric nutrient deposition on the alpine lake ecosystem in the south-eastern Tibetan Plateau during the Anthropocene

Alpine lakes on the Tibetan Plateau are highly sensitive to global change and have been recognized as the sentinel of climate warming. However, anthropogenic impacts in populated area are migrating to these remote areas via transporting particulate nutrients by atmospheric deposition. Whether warming and nutrient deposition would impose additive or synergistic effects on the lake ecosystem remains largely unknown. Here, we present multi-proxy (sediment pigment and geochemistry) records during the past two centuries at the Cuoqia Lake in the southeast Tibetan Plateau. We found that the lake exhibited rapid ecological changes since 1980 AD characterized by an increase in primary productivity due to algal proliferation, with more rapid growth of green algae and diatoms. These findings are in concert with many other lakes (e.g., Moon Lake and Shade Co) in the same area, suggesting a consistent pattern of ecosystem evolution at the region scale. Statistical analyses suggested that nutrient deposition and climate warming were strongly associated with the variations in primary productivity and algae composition, exerting both individual and interactive effects. In addition, scenario analyses with a well-established process-based ecosystem model further revealed that the two factors not only individually, but also synergistically promoted the algal proliferation and community succession. Such synergy is evident in that the effect of lake warming would be more pronounced under higher nutrient deposition scenario, which is potentially due to higher temperature-driven mineralization in warmer conditions, and higher efficiency of nutrient utilization under enhanced light availability attributing to declining ice thickness and duration in cold seasons. Overall, our study proposes the existence and quantifies the synergistic impacts of climate warming and anthropogenic activities in driving the ecological changes in remote alpine lakes on the Tibetan Plateau. The lake ecological consequences driven by individual factor would be worsen by such synergy, so that we cannot predict the lake ecosystem trajectory in the future based on each factor separately, and more efforts than previously expected would be needed for the lake restoration and management.

Effect of different factors dominated by water level environment on wetland carbon emissions

The exacerbation of global warming has led to changes in wetland carbon emissions worldwide. Therefore, we conducted a meta-analysis of methane (CH₄) and carbon dioxide (CO₂) emissions in wetland ecosystem and explored the underlying mechanisms. Our finding indicated that (1) water level of -50 to 30 cm (the negative value represents the depth of the groundwater table, whereas the positive value represents the height of the above-ground water table) and -10 cm will result in a large CH₄ and CO₂ emissions, respectively; (2) CO₂ and CH₄ massive emissions occurred at the temperature range of 15-20 °C and > 20 °C, respectively; (3) CH₄ and CO₂ emissions were higher when the mean annual precipitation (MAP) was between 400 and 800 mm, but lower at an range of 800-1200 mm; (4) there was no significant difference in CH₄ and CO₂ emissions in marsh over time; however, CO₂ emissions in fen were relatively high; (5) there was no significant difference in CO₂ emissions between the forest, grass, and shrub groups; there was also no significant difference in CH₄ emission within the forest group; and (6) MAP has a low impact (0.577) on the CO₂ emissions of wetlands. Collectively, our findings highlight the characteristics of wetland CH₄ and CO₂ emissions under different conditions dominated by water level, enhance our understanding of the potential mechanisms that govern these effects, and provide basis for future wetland management and restoration in the future.

Climate, hydrology, and human disturbance drive long-term (1988-2018) macrophyte patterns in water diversion lakes

Macrophytes are affected by many natural and human stressors globally but their long-term responses to these multiple stressors are not often quantified. We employed remote sensing and statistical tools to analyze datasets from both short-term (2017-2018) field investigations to explore seasonal patterns, and long-term (1988-2018) Landsat remote-sensing images to detect annual patterns of macrophyte distributions and study their responses to changes in climate, hydrology, and anthropogenic activities in a chain of water diversion lakes in eastern China. We found: 1) biomass and species richness of macrophytes peaked in summer with dominant species of submerged macrophytes Ceratophyllum demersum, Potamogeton pectinatus, and Potamogeton maackianus and floating macrophytes Trapa bispinosa, and non-native species Cabomba caroliniana spread in midstream Luoma Lake and Nansi Lake in summer, while Potamogeton crispus was dominant in all the lakes in spring; 2) water physicochemical parameters (chloride and water depth), lake characteristics (area and water storage), climate factors (air temperature and precipitation), and anthropogenic activities (commercial fishery and urban development) were significantly correlated to the seasonal distribution of macrophytes; 3) long-term data showed a significantly negative correlation between coverage of floating macrophytes and precipitation where the wettest year of 2003 had the lowest coverage of floating macrophytes; and 4) climate (air temperature) and hydrology (water level) were positively correlated with total macrophyte coverage, but human disturbance indexed by the gross domestic product was negatively driving long-term coverage of macrophytes. Our study has important implications for understanding the long-term succession of macrophytes under both natural and human stressors, and for future environmental management and ecological restoration of freshwater lakes.

Features of Plant Community and Driving Forces of Plant Community Succession in the Typical Desert Wetlands

Desert wetlands play a significant role in flood regulation, water purification, biodiversity maintenance, and regional ecological environment improvement. Vegetation is a key factor affecting wetland function and it is important to study the features of plant community and the driving forces of plant community succession. The Ningxia Habahu National Nature Reserve, a typical desert wetland ecosystem, was selected to study the features of wetland plant communities including plant density, biomass and frequency, and vegetation coverage, as well as the habitats, structural characteristics, species composition, dominant population structure, and other characteristics of different ecosystems. Data was collected using long-term fixed-point observation, sampling monitoring, and other methods. The results showed that the total plant density, total biomass, Magalef index and Shannon–Wiener index of the different desert wetlands in the reserve area were all relatively low, which was caused by the poor habitat and salinization of the arid environment. There was no significant difference between the root–shoot ratio of the wetland plants in the reserve area and that of the construct species of other ecosystems. The specific leaf area of the wetland plants was also not significantly different from that of the construct species of other ecosystems. Vegetation nitrogen-to-phosphorus (N/P) ratios were found to be the key driving force for the succession of plant community in the desert wetlands. These results not only provide underlying insights for the improvement of species diversity and ecological environment, but also provide a scientific basis for the sustainable protection and restoration of typical desert wetlands.

Plant Macrofossils Reveal Aquatic Macrophyte Successions of a Typical Shallow Lake (Huanggai Lake, China) in the Past Century

Aquatic macrophytes are one of the important biotic components of shallow lake ecosystems. Understanding the long-term evolution of the macrophyte community is crucial for lake management. Huanggai Lake, a typical shallow lake in the middle reach of the Yangtze River, was selected as the research site for this study. Based on ²¹⁰Pb/¹³⁷Cs dating, aquatic plant macrofossils were used to reconstruct the succession of aquatic macrophytes in the past century. Our results show that the lake maintained a consistent natural state before 1940, with a relatively low abundance of aquatic plants dominated by species such as Najas minor. From 1940 to 1974, human activities gradually intensified in the lake leading to the emergence of eutrophic species such as Potamogeton maackianus, along with the increasing abundance of other emergent and floating aquatic macrophytes. Since 1974, more pollution-resistant, emergent species such as Potamogeton maackianus and Potamogeton crispus have become dominant. The abundance of aquatic macrophytes reached its maximum in the early 1990s. Combined with macrofossil succession and other multiple sedimentary proxy analyses, driving mechanisms for aquatic macrophytes are discussed. Both the nearby Liangzi Lake and Huanggai Lake share many common features of aquatic plant evolution. This study is the first of its kind to use plant macrofossils (with identifiable images) as a proxy for aquatic macrophyte succession in a shallow Yangtze lake. In absence of long-term monitoring records, this study highlights the increased application of plant macrofossils for reconstructing the vegetation dynamics and restoration of degraded lakes exposed to severe anthropogenic impacts over the past century.

Can biogenic n-heptadecane be utilized to represent algae cell density dynamics in water environment? Evidences from field investigation and laboratory validation

The monitoring and prediction of algae cell density are the fundamental supports for eutrophication management. As the molecular marker of algae, n-heptadecane has been successfully utilized to reflect algae biomass in sedimentary studies. However, whether biogenic n-heptadecane (bio C17) can be utilized to represent algae cell density dynamics in water environment still remains an issue. Current study aims to provide novel evidences from both field investigation and laboratory validation for it. Firstly, we found a strinkingly positive correlation between algae cell density dynamics and bio C17 variations (p = 4.34 × 10-10) via meta-analysis using field dataset in Lake Chaohu. Then, we selected Microcystis aeruginosa, Chlorella vulgaris and Melosira sp. as model species of cyanobacteria, green algae and diatom, respectively, for laboratory validation. Our results illustrated that n-heptadecane was synchronized with the growth for cyanobacteria and green algae but not for diatom. The association between bio C17 and algae cell density was species-dependent, and the relationship between bio C17 and cell density was linear within 107 cells∙mL-1. Therefore, we established and optimized a generalized additive model to fit observed algae cell density in Lake Chaohu. In the optimal model, bio C17, Pielou evenness index J and Shannon-Wiener index H' were included, totally explaining 66% of the variance of algae cell density. Model comparisons revealed that considering algae community could indeed improve the interpretation of algae cell density in natural environment. In conclusion, our study provided novel evidences that bio C17 can be utilized to represent the cell density dynamics of cyanobacteria and green algae in the environment.

Hydrological Modeling in the Chaohu Lake Basin of China—Driven by Open-Access Gridded Meteorological and Remote Sensing Precipitation Products

This study assessed the performance of two well-known gridded meteorological datasets, CFSR (Climate Forecast System Reanalysis) and CMADS (China Meteorological Assimilation Driving Datasets), and three satellite-based precipitation datasets, TRMM (Tropical Rainfall Measuring Mission), CMORPH (Climate Prediction Center morphing technique), and CHIRPS (Climate Hazards Group InfraRed Precipitation with Station data), in driving the SWAT (Soil and Water Assessment Tool) model for streamflow simulation in the Fengle watershed in the middle–lower Yangtze Plain, China. Eighteen model scenarios were generated by forcing the SWAT model with different combinations of three meteorological datasets and six precipitation datasets. Our results showed that (1) the three satellite-based precipitation datasets (i.e., TRMM, CMORPH, and CHIRPS) generally provided more accurate precipitation estimates than CFSR and CMADS. CFSR and CMADS agreed fairly well with the gauged measurements in maximum temperature, minimum temperature, and relative humidity, but large discrepancies existed for the solar radiation and wind speed. (2) The impact of precipitation data on simulated streamflow was much larger than that of other meteorological variables. Satisfactory simulations were achieved using the CMORPH precipitation data for daily streamflow simulation and the TRMM and CHIRPS precipitation data for monthly streamflow simulation. This suggests that different precipitation datasets can be used for optimal simulations at different temporal scales.

Reconstruction of Ecological Transitions in a Temperate Shallow Lake of the Middle Yangtze River Basin in the Last Century

Exogenous drivers may cause a gradual and reversible change in a lake equilibrium, or they may force it over a threshold to a persistent alternative stable state, described as a regime shift in the ecosystem. In the mid-and-lower Yangtze River Basin (MLYB), major environmental problems in shallow lakes have been eutrophication and abrupt algal blooms under anthropogenic disturbances for the recent century. Much value is therefore placed on understanding the changes in shallow-lake ecosystems that characteristically precede changes in the state of the lake. Here, we describe a case study of the paleolimnological signature in diatom assemblages of various types of regime shifts caused by historically documented anthropogenic drivers in a temperate shallow lake: Taibai Lake. We evaluate the effectiveness of paleolimnological data as a surrogate for long-term monitoring. Algorithms using sequential t and F statistics detected breakpoints in the time series of diatom assemblages, in 1994–1996, 1974–1977, 1952–1956, and 1931–1934, respectively. The regression statistics suggest that the hydrodynamic–ecosystem and aquacultural–ecosystem relationships fit better in the breakpoint regression model, and the relationship between nutrient loading and ecosystem state suits the linear model. Feedback loops help reconstruct dynamic changes in Taibai influenced by major stressors. Our study exemplifies the value of system approaches to identifying regime shifts and their possible causes in shallow lakes from paleolimnological records. The case study of Taibai set an example of reconstructing the ecological regime shifts in shallow lakes in the MLYB and understanding the state changes in lake ecosystems, which will benefit effective lake management.

What is the pollution limit? Comparing nutrient loads with thresholds to improve water quality in Lake Baiyangdian

Ecological thresholds are useful indicators for water quality managers to define limits to nutrient pollution. A common approach to estimating ecological thresholds is using critical nutrient loads. Critical nutrient loads are typically defined as the loads at which the phytoplankton chlorophyll-a exceeds a certain concentration. However, national policies, such as in China, use chemical indicators (nitrogen and phosphorus concentrations) rather than ecological indicators (phytoplankton chlorophyll-a) to assess water quality. In this study, we uniquely define the critical nutrient loads based on maximum allowable nutrient concentrations for lake Baiyangdian. We assess whether current and future nutrient loads in this lake comply with the Chinese Water Quality standards. To this end, we link two models (MARINA-Lakes and PCLake+). The PCLake+ model was applied to estimate the critical nutrient loads related to ecological thresholds for total nitrogen, total phosphorus and chlorophyll-a. The current (i.e., 2012) and future (i.e., 2050) nutrient loads were derived from the water quality MARINA-Lakes model. Nitrogen loads exceeded the nitrogen threshold in 2012. Phosphorus loads were below all ecological thresholds in 2012. Ecological thresholds are exceeded in 2050 with limited environmental policies, and urbanization may increase nutrient loads above the ecological thresholds in 2050. Recycling and reallocating animal manure is needed to avoid future water pollution in Lake Baiyangdian. Our study highlights the need for effective policies for clean water based on policy-relevant indicators.

Metacommunity Concepts Provide New Insights in Explaining Zooplankton Spatial Patterns within Large Floodplain Systems

Flood pulse related physical variables (FLOOD) can affect zooplankton community structure through local factors directly and can also influence through regional dispersal factors of metacommunity concepts indirectly. Therefore, we infer that spatial patterns of zooplankton communities could be related to metacommunity concepts and their importance may depend on the size of the aquatic/terrestrial transition zone (ATTZ). Herein, we explored the relative importance of limnological (LIMNO) and FLOOD variables in zooplankton community by analyzing data from 272 sites across three floodplain lakes in the middle reaches of the Yangtze River. Our results showed that the variation in the zooplankton community can be well explained by the LIMNO and FLOOD variables in all of the lakes under the low water level season. However, during the high water level season, neither LIMNO nor FLOOD can explain the spatial variances of zooplankton. Therefore, our results indicated that testing biogeographical theories and macroecological laws using zooplankton should consider temporal aspects of flood pulse. Furthermore, we noted that the number of explained variance by local variables is negatively correlated with the size of the ATTZ. Metacommunity concepts provide complementary insights in explaining zooplankton spatial patterns within large floodplain systems, which also provide a theoretical basis for ATTZ protection in floodplain management.

The long-term changes in food web structure and ecosystem functioning of a shallow lake: Implications for the lake management

The food web structure (FWS) and ecosystem functioning (EF) of lakes worldwide are impacted by multiple disturbances. The historical evolution of the FWS and EF are not well understood due to the lack of sufficient long-term records of biotic variations. This study reconstructed the food web models in the 1950s, 1980s, 1990s, 2000s, and 2010s for Baiyangdian Lake (BYDL), the largest shallow lake in northern China, using the Linear Inverse Modeling (LIM) and investigated EF in different periods. Our results confirmed that the FWS and EF of BYDL have undergone significant changes. The biomass of phytoplankton continuously increased, and the primary productivity of phytoplankton began to replace the primary productivity of submerged macrophytes in the 2000s and became the largest energy flow in the food web. Changes in the energy flow of primary producers are transmitted to high-trophic functional groups, which affects the diet composition of fish. Based on the ecological network analysis indices and food web stability indicators, it was concluded that after a turning point in the 1990s, the ecosystem showed initial stability and then gradually became unstable. Water level fluctuations and nutrient enrichment may be the key driving factors for changes in ecosystem state. Therefore, to maintain a good state of the ecosystem, we recommend implementing comprehensive management measures of hydrological management, nutrient-loading reduction, and biomanipulation. Furthermore, this study extended LIM to lake ecosystems, which may provide a new method for lake ecological environment management.

Sensitivity of phytoplankton to climatic factors in a large shallow lake revealed by column-integrated algal biomass from long-term satellite observations

There are some uncertainties of using chlorophyll a (Chla) concentrations in water surface to address phytoplankton dynamics, especially in large shallow lakes, because of the dramatic vertical migration of phytoplankton. The column-integrated algal biomass (CAB) can reflect the whole water column information, so it is considered as a better indicator for phytoplankton total biomass. An algal biomass index (ABI) and an empirical algorithm were proposed previously to measure algal biomass inside and outside euphotic zone from the Moderate Resolution Imaging Spectrometer (MODIS) data. A long-term CAB time series was generated in this study to clarify the temporal and spatial changes in phytoplankton and address its sensitivity to climatic factors in Lake Chaohu, a shallow eutrophic lake in China, from 2000 to 2018. Overall, the CAB for Lake Chaohu showed significant temporal and spatial dynamics. Temporally, the annual average CAB (total CBA within the whole lake) was increased at rate of 0.569 t Chla/y, ranging from 62.06±8.89 t Chla to 76.03±10.01 t Chla during the 19-year period. Seasonal and periodic variations in total CAB presented a bimodal annual cycle every year, the total CAB was highest in summer, followed by that in autumn, and it was the lowest in winter. The pixel-based CAB (total CAB of a unit water column), ranging from 112.42 to 166.85 mg Chla, was the highest in the western segment, especially its northern part, and was the lowest in the central parts of eastern and central segments. The sensitivity of CAB dynamics to climatic conditions was found to vary by region and time scale. Specifically, the change of pixel-based algal biomass was more sensitive to the temperature change on the monthly and annual scales, while wind speed impacted directly on the short-term spatial-temporal redistribution of algal biomass. High temperature and low wind speed could prompt the growth of total CAB for the whole lake, and the hydrodynamic situations affected by wind and so on determined the spatial details. It also indicated that Lake Chaohu may face more severe challenges with the future climatic warming. This study may serve as a reference to support algal bloom forecasting and early warning management for other large eutrophic lakes with similar problems.

Environmental controls of harmful cyanobacterial blooms in Chinese inland waters

Harmful cyanobacterial blooms (CyanoHABs) are expanding world-wide, adversely affecting aquatic food production, recreational and tourism activities and safe drinking water supplies. China's inland waters have been increasingly threatened by CyanoHABs during the past several decades. The environmental factors controlling CyanoHABs are highly variable in space and time in China due to significant variations in climate, geography, geological and geochemical conditions among its many regions. Here, we synthesize diverse examples among Chinese water bodies regarding interactive effects of anthropogenic, climatic and geographic drivers influencing CyanoHAB potentials and dynamics in lakes and reservoirs; in order to provide a perspective and integrative approach to mitigating CyanoHABs. In China's many shallow water bodies, water quality is highly susceptible to human activity and to changing climatic and hydrological conditions, when compared to deeper lakes. Rapid increases in population, economic activity, and wastewater have accelerated CyanoHABs in China since 1980s, especially in the heavily urbanized, agricultural and industrial regions in the middle and lower Yangtze River basins. Climatic changes have provided an additional catalyst for expansion of CyanoHABs. In particular, rising spring temperatures have accelerated the onset and proliferation of Microcystis spp, blooms in the middle and lower reaches of Yangtze River basin. Large hydroelectric and water supply projects, like the Three Gorges Reservoir (TGR), have altered hydrological regimes, and have led to an increase of CyanoHABs in reservoirs and tributaries due to increases in water residence times. Manipulating water level fluctuations in the TGR may prove useful for controlling CyanoHAB in its tributary bays. Overall,CyanoHAB mitigation strategies will have to incorporate both N and P input reductions in these shallow systems. Furthermore, nutrient reduction strategies must consider climate change-induced increases in extreme weather events, including more intense rainfall and protracted heat waves and droughts, which can extend the magnitudes and duration of CyanoHABs. Ensuring the maintenance of natural hydrologic connectivity between lakes and rivers is of utmost importance in mitigating CyanoHABs throughout China.

Characterizing 19 thousand Chinese lakes, ponds and reservoirs by morphometric, climate and sediment characteristics

Chinese lakes, including ponds and reservoirs, are increasingly threatened by algal blooms. Yet, each lake is unique, leading to large inter-lake variation in lake vulnerability to algal blooms. Here, we aim to assess the effects of unique lake characteristics on lake vulnerability to algal blooms. To this end, we built a novel and comprehensive database of lake morphometric, climate and sediment characteristics of 19,536 Chinese lakes, including ponds and reservoirs (>0.1 km²). We assessed lake characteristics for nine stratification classes and show that lakes, including ponds and reservoirs, in eastern China typically have a warm stratification class (T_avg>4 °C) and are slightly deeper than those in western China. Model results for representative lakes suggest that the most vulnerable lakes to algal blooms are in eastern China where pollution levels are also highest. Our characterization provides an important baseline to inform policymakers in what regions lakes are potentially most vulnerable to algal blooms.

Hydrologic and nutrient-driven regime shifts of cyanobacterial and eukaryotic algal communities in a large shallow lake: Evidence from empirical state indicator and ecological network analyses

The detection and prediction of lake ecosystem responses to environmental changes are pressing scientific challenge of major global relevance. Specifically, an understanding of lake ecosystem stability over long-term scales is urgently needed to identify impending ecosystem regime shifts induced by human activities and improve lake ecosystem protection. This study investigated regime shifts in cyanobacterial and eukaryotic algal communities in a large shallow lake over a century in response to nutrient enrichment and hydrologic regulation using evidence from empirical state indicators and ecological network analyses of sedimentary-inferred communities. The diversity and structure of cyanobacterial and eukaryotic algal communities were investigated from sedimentary DNA records and used, for the first time, as state variables of the lake ecosystem to detect lake stability. Two regime shifts were inferred in the 1970s and 2000s based on temporal analysis of empirical indicators. Co-occurrence network analysis based on taxonomic abundance distributions and presence/absence patterns also supported the two regime shifts based on architectural features of the ecological networks. Moreover, the associations of cyanobacterial and eukaryotic algal taxa were observed to be non-random across time. The abrupt driver-mediated regime shift in the 1970s is characterized by the disappearance of submerged vegetation, significantly increased relative abundances of Microcystis and Chlorophyta taxa, and was primarily caused by sluice construction. The critical transition observed in the 2000s was manifested by the occurrence of serious cyanobacterial blooms and was triggered by increased nutrient loading with the development of urbanization and agricultural intensification. This study reveals the important roles of hydrologic regulation and nutrient loading in the temporal successional dynamics of a shallow lake ecosystem, providing new insights into regime shifts of lake ecosystems that can help inform future efforts to predict important lake ecosystem state changes.

Water Level Regulation for Eco-social Services Under Climate Change in Erhai Lake Over the Past 68 years in China

Water level plays a crucial role in the function and social services of lakes. Studies on historical changes in water level and its eco-social function can give insights into future water conservation and management. In this study, interannual and seasonal changes in the water level of Erhai Lake were analyzed from 1952 to 2019 to explore water level responses to human activities and climate change. The time series was divided into three distinct periods, i.e., 1952–1971, 1972–2003, and 2004–2019. Results showed that the water level and fluctuation amplitude differed among the different time periods, i.e., 1965.8 and 1.3 m (1952–1971), 1964.4 and 1.9 m (1972–2003), and 1965.2 and 1.2 m (1972–2003), respectively. The construction and operation of a hydroelectric power plant along the outlet river significantly decreased the water level and increased fluctuation amplitude in the 1972–2003 period. Since 2004, due to the implementation of local government water level management laws for Erhai Lake, the water level has remained relatively high, with moderate fluctuation amplitude. In addition, compared to the increase in water level amplitude in response to increased wet season (May–October) precipitation in the 1952–1971 period, response sensitivity increased in the 1972–2003 period, but became non-significant in the 2004–2019 period. In regard to the multi-timescale relationship between water level and precipitation, precipitation decreased by 89 mm in the 2004–2019 period compared with that from 1952 to 1971, and artificial water-level regulation resulted in a time-lag of 2, 3–3.5, and 4 months between water level and precipitation during the 1952–1971, 1972–2003, and 2004–2019 periods, respectively. The eco-social aspects of changes in water level are discussed below, and water level regulation from an ecological perspective is recommended to gain economic returns in the future.

Eutrophication-induced regime shifts reduced sediment burial ability for polycyclic aromatic hydrocarbons: Evidence from Lake Taihu in China

Regime shifts from a vegetated state with clear water to a turbid state with high contents of phytoplankton and suspended particles have been found in numerous waters worldwide. The fate and risks of hydrophobic organic contaminants (HOCs) in such waters may be altered, and the effects on burial ability of HOCs remain unknown. Influences of regime shifts on sediment burial ability for 16 polycyclic aromatic hydrocarbons (PAHs) (defined as burial/emission ratio) were investigated based on the evidence from the third largest freshwater lake (Lake Taihu) in China. The results of δ¹³C, δ¹⁵N, atomic ratio of C_org/N, and the content of total organic carbon testing and historical data suggested that the regime shifted abruptly from macrophytes to phytoplankton dominance in Lake Taihu in the late 1980s. The annual burial ability for the PAHs decreased gradually over time by 63.2%-98.9% in the period from 1980 to 2016. Meanwhile, the decrease rates of PAH burial ability varied from -1.65% y^-1 to -2.98% y^-1, depending on the hydrophobicity of the compound. The PAH burial ability varied with the dominant primary producers associated with the trophic level index of the water column. Regime shifts had a stronger influence on the burial ability of PAHs with higher hydrophobicity. This study helps to understand the fate and potential risks of HOCs in waters due to eutrophication-induced regime shifts.

Climate and Nutrient-Driven Regime Shifts of Cyanobacterial Communities in Low-Latitude Plateau Lakes

Cyanobacterial blooms that form in response to climate warming and nutrient enrichment in freshwater lakes have become a global environmental challenge. Historical legacy effects and the mechanisms underlying cyanobacterial community succession are not well understood, especially for plateau lakes that are important global freshwater resources. This study investigated the temporal dynamics of cyanobacterial communities over centuries in response to nutrient enrichment and climate warming in low-latitude plateau lakes using high-throughput DNA sequencing of sedimentary DNA combined with traditional paleolimnological analyses. Our results confirmed that nutrients and climate warming drive shifts in cyanobacterial communities over time. Cyanobacterial community turnover was pronounced with regime shifts toward new ecological states, occurring after exceeding a tipping point of aquatic total phosphorus (TP). The inferred species interactions, niche differentiation, and identity of keystone taxa significantly changed after crossing the aquatic TP ecological threshold, as demonstrated by network analysis of cyanobacterial taxa. Further, the contribution of aquatic TP to cyanobacterial community dynamics was greater than that of air temperature when lakes were in an oligotrophic state. In contrast, as the aquatic TP threshold was exceeded, the contribution to community dynamics by air temperature increased and potentially surpassed that of aquatic TP. Overall, these results provide new evidence for how past nutrient levels in lacustrine ecosystems influence contemporary cyanobacterial community responses to global warming in low-latitude plateau lakes.

Geographical location and water depth are important driving factors for the differences of suspended particulate organic matter (SPOM) in lake environment across nationwide scale: Evidences from n-alkane fingerprints

Suspended particulate organic matter (SPOM) plays a connective role in global biogeochemical carbon cycles and energy flows in aquatic ecosystems. However, little is known about the occurrence and source of SPOM in lake environment and their driving factors across nationwide scale. Here, we utilize the molecular markers of n-alkanes and their fingerprints in 46 typical lakes and reservoirs with different water depths across China from both sides of the Hu Line to study this issue. Σ₂₉n-alkanes, Σ biogenic n-alkanes and Σ anthropogenic n-alkanes ranged from 104.8 to 10332 ng·L^-1, from 88.5 to 4843 ng·L^-1, and from 16.2 to 5488 ng·L^-1, respectively. Their occurrences were only associated with water depth. Then, we compared the differences of carbon-chain distribution of both biogenic and anthropogenic n-alkanes and related proxies in different lake groups. The profiles of different biogenic and anthropogenic n-alkanes posed large differences in different lake groups. Finally, linear discriminant analysis (LDA) was applied to test the possible effects of geographical location and water depth on the holistic differences of SPOM in different lakes and reservoirs across China. The results illustrated that both geographical location and water depth were important driving factors for the holistic differences of SPOM in different lakes and reservoirs across China. Intensive anthropogenic activities narrowed the differences between shallow and deep lakes in eastern China. In conclusion, this study provided new insights into the driving factor analysis of SPOM in lakes and reservoirs on large scale.

Effects of hydrological and climatic variables on cyanobacterial blooms in four large shallow lakes fed by the Yangtze River

Shallow lakes, one of the most widespread water bodies in the world, are easily shifted to a new trophic state due to external interferences. Shifting hydrologic conditions and climate change can cause cyanobacterial harmful algal blooms (CyanoHABs) in shallow lakes, which pose serious threats to ecological integrity and human health. This study analyzed the effects of hydrologic and meteorological variables on cyanobacterial blooms in Yangtze-connected lakes (Lake Dongting and Poyang) and isolated lakes (Lake Chao and Tai). The results show that (i) chlorophyll-a (Chl-a) concentration tends to decrease exponentially with increasing relative lake level fluctuations (RLLF) and precipitation, but to increase linearly with increasing wind speed and air temperature; (ii) Chl-a concentrations in lakes were significantly higher when RLLF < 100, precipitation < 2.6 mm, wind speed > 2.6 m s^-1, or air temperature > 17.8 °C; (iii) the Chl-a concentration of Yangtze-isolated lakes was more significantly affected by water level amplitude, precipitation, wind speed and air temperature than the Yangtze-connected lakes; (iv) the RLLF and the ratio of wind speed to mean water depth could be innovative coupling factors to examine variation characteristics of Chl-a in shallow lakes with greater correlation than single factors.

New insights into spatiotemporal source apportionment of n-alkanes under mixed scenario: A pilot study on Lake Chaohu, China

N-alkanes are ideal molecular markers for the source apportionment of organic matter. However, the estimation of both biogenic and anthropogenic sources under mixed scenario using n-alkanes and their related proxies still remains an issue. In this study, we investigated spatiotemporal variations of n-alkanes in suspended particulate matter of Lake Chaohu for their source apportionment. Overall, Σ₂₉ n-alkanes ranged from 324.1 to 113685 ng·L^-1. C₁₇H₃₆ was the most abundant homologue and was followed by C₁₈, C₂₇ and C₂₉ homologues. Carbon preference indexes (CPIs) in most samples (at least 85%) were < 3, indicating mixed sources of n-alkanes in Lake Chaohu. Therefore, biogenic and anthropogenic n-alkanes were separated by subtraction to avoid potential bias. Our results showed Σ biogenic and Σ anthropogenic n-alkanes ranged from 14.8 to 3531.6 ng·L^-1 and 257.6 to 4938.5 ng·L^-1, respectively. For biogenic n-alkanes, their carbon-chain distributions posed a preponderant peak at C₁₇, indicating algae were the main contributors to biogenic n-alkanes in Lake Chaohu. Biogenic average chain length (ACL_bio) was developed to quantify the contributions of different biogenic sources. We recommended ACL_bio < 21 for algae dominance and ACL_bio > 26 for terrestrial plant dominance. For anthropogenic n-alkanes, their carbon-chain distribution presented obviously spatiotemporal variations. The sources of anthropogenic n-alkanes in summer and winter were typical dominances of light petroleum and incomplete fossil fuel burning/heavy oil emission, respectively. New developed proxies, anthropogenic average chain length (ACL_anthro) and the ratio of unit short- to long-carbon anthropogenic n-alkanes (L/H), are effective for quantifying the relative contributions of different anthropogenic sources. We recommend log₁₀L/H > 0.5 and ACL_anthro < 20.5 for light petroleum input dominance, log₁₀L/H < -0.5 and ACL_anthro > 26.5 for incomplete fossil fuel burning at high temperature/heavy oil emission dominance.

Retention of nitrogen and phosphorus in Lake Chaohu, China: implications for eutrophication management

Nutrient retention is an important process in lake nutrient cycling of lakes and can mitigate lake eutrophication. However, little is known about temporal lake nutrient retention efficiency and it varies due to changes in hydrological, ecological, and nutrient inputs to lake waters. Quantitative information about seasonal lake N and P retention is critical for developing strategies to reduce eutrophication in lake systems. This study investigated TN and TP retention efficiencies and retention masses using water and mass balance calculations, and statistically analyzed the seasonal variability of nutrient retention in Lake Chaohu, China, from 2014 to 2018. Lake Chaohu experienced large amounts of external loads inputs (23.2 g N m^-2 year^-1 and 1.3 g P m^-2 year^-1), and approximately 58% TN and 48% TP were retained annually. The lake acted more as a sink for N than for P. The mean annual TP retention efficiency decreased (P < 0.05) over the study period, indicating that TP retention capacity was gradually exceeded. Seasonal variability of TN and TP retention efficiency was high and ranged from - 18.7 to 144.1% and from - 58.8 to 170.7%, respectively, over the five study years. The internal P loads over the study period were equivalent to roughly 9% of the total external loads. The annual nutrient retention efficiency of TN and TP increased with hydraulic residence time, while water temperature was an essential factor for the contrasting seasonal variation patterns of TN and TP retention efficiencies.

Scaling responses of leaf nutrient stoichiometry to the lakeshore flooding duration gradient across different organizational levels

Most wetlands have been subject to changes in flooding regimes by climate change and human activities, resulting in widespread alteration of wetland plants at different organizational levels. However, scaling the responses of wetland plants to changes in flooding regimes is still challenging, because flooding could indirectly affect wetland plants through affecting environment factors (e.g. soil properties). During the non-flooding period, we investigated leaf N and P stoichiometry at three organizational levels (intra-species, inter-species, inter-community) along a flooding duration gradient in a lakeshore meadow of Poyang Lake floodplain, China. At the intra-species level, leaf N and P stoichiometry showed species-specific responses to flooding duration. At the inter-species level, leaf N or P contents or N:P ratio showed no significant response to flooding duration. At the inter-community level, leaf N and P contents significantly increased with flooding duration, while leaf N:P ratio decreased. At each organizational level, leaf N and P stoichiometry showed poor correlation with soil N and P stoichiometry. Moreover, intra-specific responses of leaf N and P contents to flooding duration and soil nutrient content increased with mean flooding duration of species distribution, which was the index of species hydrological niche. Intraspecific variation had lower contribution than species turnover to variations in community leaf nutrient stoichiometry. In all, flooding duration affected leaf N and P stoichiometry mainly through direct pathway at the intra-species and inter-community level, rather than the indirect pathway via soil nutrient stoichiometry. Therefore, our results have implications for scaling up from environmental conditions to ecosystem processes via wetland plant communities.

Responses of lake macrophyte species and functional traits to climate and land use changes

Aquatic plants are essential components in the regulation of microhabitat complexity and physico-chemical parameters in lake ecosystems. Increased eutrophication, land use change, modification of hydrological regimes, and expansion of invasive species are expected to impact aquatic plant community composition; however, historical pathways and response patterns are not well understood at the national scale. We analyzed temporal changes in aquatic plant communities in Japan from the early 1900s to the 2000s using field survey records from 248 lakes. Relationships of species associations with climate, land use, and lake characteristics were described using a joint species distribution model. The mean variation attributable to lake characteristics was 25.4%, followed by climate (14.0%), and land use (10.5%). Among the 13 functional traits used in our analysis, sexual and pollination traits showed marked responses to precipitation and land use. Hypohydrophily increased with precipitation, whereas monoecious aquatic plants increased in lakes surrounded by urbanized area. The relative ratio of floating to submerged plants has increased over time. Our results provide insight into long-term changes in aquatic plant communities and identify functional traits sensitive to environmental change.

Regime shifts in shallow lake ecosystems along an urban-rural gradient in central China

Due to differential exploitation pressure, ecosystems along the urban to rural gradients often exhibit different status in ecological structure and function. This can be challenging for lake restoration, given the relative strengths, magnitudes and speed of the exploitation. In this paper, we reconstructed the ecological changes over the past century and identified the regime shifts based on subfossil aquatic biota (chironomid records) in three shallow lakes (Shahu, Yanxi and Futou Lake) along an urban-rural gradient in the Yangtze floodplain, China. Our results illustrated the differences among lakes in trajectories, timing of critical transition and current ecological status. Eutrophic chironomid taxa increased markedly and replaced macrophyte-related taxa in urban Shahu Lake and suburban Yanxi Lake, indicated by the shift from a stable, vegetation-dominated state to an alternative, algal-dominated state in 1963 CE and 1975 CE respectively. The ecological regime in rural Futou Lake transited around 1980 CE but it is still in a relatively clear water state with abundant macrophytes due to anthropogenic hydrological controls. The greatest variance of chironomid compositional changes in both Shahu and Yanxi Lake was captured by anthropogenic pollutants, and analyses show that when these pressures are high they may be further amplified by climate warming. Responses along the urban-rural gradient are exemplified by urban Shahu Lake having shifted to a fragile regime with weak resistance and resilience, while rural Futou Lake has stabilized in a new regime with improved ecological resilience. Suburban Yanxi Lake is still moving toward a new state, and as such is unstable, because the types and magnitudes of external stressors are changing with urbanization in the city. It is suggested that active and precise management strategies for lakes should be established along the urban-rural gradient given their distinct development trajectories, drivers and current status.

Interannual and Seasonal Shift between Microcystis and Dolichospermum: A 7-Year Investigation in Lake Chaohu, China

The shifts among bloom-forming cyanobacteria have attracted increasing attention due to the reductions in nitrogen and phosphorus during the eutrophication mitigation process. However, knowledge is limited regarding the pattern and drivers of the shifts among these cyanobacterial genera. In this study, we performed a 7-year long, monthly investigation in Lake Chaohu, to analyze the interannual and seasonal shifts between Microcystis and Dolichospermum. Our results showed that Microcystis was the dominant cyanobacterium in the western lake region in summer, whereas Dolichospermum was dominant in the other regions and seasons. The Microcystis biomass and ratio were driven primarily by total phosphorus and temperature. The sensitivity of Dolichospermum to nutrients and temperature was relatively weak compared to that of Microcystis. The shifts between Microcystis and Dolichospermum might be led by Microcystis. If the temperature and phosphorus level were relatively high, then Microcystis grew rapidly, and competitively excluded Dolichospermum. If the nutrient level, especially the phosphorus level, was low, then the exclusive power of Microcystis was weak, and Dolichospermum maintained its dominance, even in summer. The key temperature (~17 °C) determined the dominance of the two cyanobacteria. Microcystis never dominated, while Dolichospermum was always dominant below the key temperature. Microcystis and Dolichospermum had different means of responding to the interaction of temperature, nitrogen and phosphorus. The Dolichospermum biomass was sensitive to the variation in nitrogen level, and the sensitivity depended on temperature. While the Microcystis biomass was sensitive to the variation in phosphorus level, and the sensitivity depended on temperature and total nitrogen. The different ways might contribute to the succession of the two cyanobacteria. Our findings will be helpful for improving the understanding of the shift process between Microcystis and Dolichospermum.

Water Level Fluctuation under the Impact of Lake Regulation and Ecological Implication in Huayang Lakes, China

Water level fluctuation (WLF) in shallow lakes in the middle and lower reaches of the Yangtze River has been a concern of many researchers. This work aims to investigate the effects of climate change and regulation of floodgates and the Three Gorges Dam (TGD) on WLF and lake volume in Huayang Lakes during the past 52 years. The results revealed that precipitation is the dominant factor that leads to seasonal variation of lake levels, whereas regulation of floodgates and TGD are the key drivers of hydrology regime change in the past 20 years. Natural lake regime has higher water level when there is more precipitation and less lake volume. Floodgates and TGD regulations have changed this pattern since 2003, causing less difference in water level in spite of more precipitation and lake recession. Under the combined impacts of floodgates and TGD regulations, Huayang Lakes have experienced a prolonged outflow time since 2003 and the contribution rate caused by the floodgates and TGD regulations has increased by 19.90%. Additionally, the water level of Huayang Lakes decreased by approximately 0.3~0.5 m from September to November, but it showed no alteration from January to March in the past two decades. This indicated that floodgate regulations used for agricultural irrigation and fishery culture dominate the hydrology regime in winter and early spring. This study is beneficial for aquatic ecosystem protection in floodgate-controlled lakes under the circumstance of climate change and vigorous anthropology activities.

Reconstructing Six Decades of Surface Temperatures at a Shallow Lake

Lake surface water temperature (LSWT) plays a fundamental role in the lake energy budget. However, direct observations of LSWT require considerable effort for acquisition and hence are rare relative to a large number of lakes. In lakes where LSWT has not been covered sufficiently by in situ measurements, remote sensing and lake modeling can be used to produce a fine spatio-temporal record of LSWTs. In our study, the Moderate-Resolution Imaging Spectroradiometer (MODIS) LSWT was used to compare with in situ data at the overpass times over the six sites in Lake Chaohu, a large shallow lake in China. MODIS-derived LSWT reflected the variation of lake surface temperature well, with a correlation coefficient of 0.96 and a cool bias of 1.25 °C. The bias was modified by an “Upper Envelop” smoothing method and then employed to evaluate the general lake model (GLM) performance, a one-dimensional hydrodynamic model. The GLM simulations showed good performance compared with MODIS LSWT data at an interannual time scale. A 57-year record of simulated LSWT was hindcast by the well-calibrated GLM for Lake Chaohu. The results showed that LSWT decreased by 0.08 °C/year from 1960 to 1981 and then increased by 0.05 °C/year. These trends were most likely caused by a cooling effect of decreased surface incident solar radiation and a warming effect of reduced wind speed. Our study promoted the use of MODIS-derived LSWT as an alternative data source, and then combined with a numerical model for inland water surface temperature, and also further provided an understanding of climate warming effect on such a shallow eutrophic lake. Key points: (1) Moderate-Resolution Imaging Spectroradiometer (MODIS) lake water surface temperature (LSWT) was validated with real-time in situ data collected at Lake Chaohu with high accuracy; (2) MODIS LSWT was modified by the bias correction and employed to evaluate a one-dimensional lake model at interannual and intraannual scale; The LSWT hindcast by a well-calibrated model at Lake Chaohu decreased by 0.08 °C/year from 1960 to 1981 and increased by 0.05 °C/year from 1982 to 2016.

Modeling Decreased Resilience of Shallow Lake Ecosystems toward Eutrophication due to Microplastic Ingestion across the Food Web

The discovery of microplastic (MP) being present in freshwaters has stimulated research on the impacts of MP on freshwater organisms. To date, research has focused on primary effects, leaving questions with respect to secondary effects at the level of freshwater food webs unanswered. Here, we use a theoretical modeling approach to investigate the hypothesis that MP imposes negative impacts on the level of freshwater shallow lake food webs. We find that increasing MP levels have the potential to affect the critical phosphorus loading (CPL), which is defined as the threshold for regime shifts between clear and turbid states of the water column. The possible occurrence of catastrophic cascades due to MP pollution is predominantly driven by the negative effects of MP on zooplankton. We explore the possible states of the food web by scenario analysis and show that the secondary effects of MP at current concentrations are likely to be negligible. However, at the current rate of MP production, a 20-40% reduction in the CPL would occur by the end of this century, suggesting a loss of resilience in shallow lakes that would be subject to abrupt changes in the food web under lower nutrient loading.

Combining species sensitivity distribution (SSD) model and thermodynamic index (exergy) for system-level ecological risk assessment of contaminates in aquatic ecosystems

After reviewing the species- and community-level ecological risk assessments (ERAs) of chemicals in the aquatic environment, the present study attempted to propose a third stage of ERA, i.e., the ecosystem-level ERA. Based on the species sensitivity distribution model (SSD) and thermodynamic theory, the exergy and biomass indicators of communities from various trophic levels (TLs) were introduced to improve ecological connotation of SSDs. The species were classified into three TLs based on algae (TL1), invertebrates (TL2), and vertebrates (TL3), and the weight of each TL was determined based on relative biomass and β value, which indicated a holistic contribution of each species or community to the ecosystem. Then, a system-level ERA protocol was successfully established, and the community- and system-level ecological risks of 10 typical toxic micro-organic pollutants in the western area of Lake Chaohu and its inflowing rivers were evaluated. System-level ERA curves (ExSSD) were mainly affected by the community-level SSD at TL2 for most chemicals in the present study. The uncertain boundary of ExSSD was mostly related to TLs with a wider uncertain boundary, but had little relation to the weight of each TL. The results of system-level ERAs revealed that dibutyl phthalate had the highest eco-risk, whereas γ-hexachlorocyclohexane presented the lowest eco-risk. Results of the system-level ERA were not fully consistent with the those of community-level ERA owing to the lack of a sufficient dataset, SSD model type, and ecosystem structure, as indicated by the weight of each TL. The successful application of ExSSD in Lake Chaohu signifies the start of the third stage of ERA at the system-level, and it also provides a scientific basis for ecosystem-level ERA, aquatic ecosystem protection, and future water safety management. However, there were some limitations, including sufficient data dependence, neglect of ecological interactions, and neglect of environmental parameters such as natural organic matter. We propose to employ toxicogenomics to enrich the toxicity database, to simulate the interaction using the ecological dynamic model, and to introduce the chemical fate model into the system-level ERA.

Climate change has weakened the ability of Chinese lakes to bury polycyclic aromatic hydrocarbons

Burial in sediments is a crucial way to reduce mobilization and risks of hydrophobic organic contaminants (HOCs), but ability of sediments to bury HOCs may be altered if the environment is changed. Whether the ability of sediments to bury HOCs has been affected by climate change remains largely unclear. We excluded the impacts of anthropogenic emissions and eutrophication from that of climate change, and for the first time found that not only the rising surface air temperature but also the declining wind speed and the reducing days with precipitation had weakened the ability of Chinese lakes to bury 16 polycyclic aromatic hydrocarbon (PAHs) by 69.2% ± 9.4%-85.7% ± 3.6% from 1951 to 2017. The relative contributions of the climatic variables to the reduced burial ability depended on the properties of the PAHs, and lakes. Burial ability of the PAHs responded differently to climate change, and was correlated to their volatilization and aqueous solubility, and lake area, catchment area/lake area ratio, and water depth. Our study suggests that not only the rising surface air temperature but also the declining wind speed and the reducing days with precipitation can undermine global efforts to reduce environmental and human exposure to PAHs.

High frequency data provide new insights into evaluating and modeling nitrogen retention in reservoirs

Freshwater ecosystems including lakes and reservoirs are hot spots for retention of excess nitrogen (N) from anthropogenic sources, providing valuable ecological services for downstream and coastal ecosystems. Despite previous investigations, current quantitative understanding on the influential factors and underlying mechanisms of N retention in lentic freshwater systems is insufficient due to data paucity and limitation of modeling techniques. Our ability to reliably predict N retention for these systems therefore remains uncertain. Emerging high frequency monitoring techniques and well-developed ecosystem modeling shed light on this issue. In the present study, we explored the retention of NO₃-N during a five-year period (2013-2017) in both annual and weekly scales in a highly flushed reservoir in Germany. We found that annual-averaged NO₃-N retention efficiency could be up to 17% with an overall retention efficiency of ∼4% in such a system characterized by a water residence time (WRT) of ∼4 days. On the weekly scale, the reservoir displayed negative retention in winter (i.e. a source of NO₃-N) and high positive retention in summer (i.e. a sink for NO₃-N). We further identified the critical role of Chl-a concentration together with the well-recognized effects from WRT in dictating NO₃-N retention efficiency, implying the significance of biological processes including phytoplankton dynamics in driving NO₃-N retention. Furthermore, our modeling approach showed that an established process-based ecosystem model (PCLake) accounted for 58.0% of the variance in NO₃-N retention efficiency, whereas statistical models obtained a lower value (40.5%). This finding exemplified the superior predictive power of process-based models over statistical models whenever ecological processes were at play. Overall, our study highlights the importance of high frequency data in providing new insights into evaluating and modeling N retention in reservoirs.

Sedimentary DNA reveals over 150 years of ecosystem change by human activities in Lake Chao, China

Understanding the extent and directionality of the impact of human activities on ecosystems is directly related to their management and protection. However, the lack of historical data limits our understanding of ecosystem changes with long-term exposure to human activities. Recently, lake sedimentary DNA (sedDNA) has become a powerful tool for revealing changes in ecosystems at the century and millennium scales. Here, we used sedDNA to reveal the dynamic of the microbial community (including bacteria and micro-eukaryotes) in Lake Chao over the past 150 years, and further explored the effects of long-term nutrient and heavy metal loads on these communities. Our data show that nutrient and heavy metal loads in Lake Chao have increased by ca. 2 to 4-fold since the 1960s. In response, the community structure, diversity, and ecological network of bacteria and micro-eukaryotes changed significantly during the 1960s, the 1980s and the 2010s. Importantly, community structure was more sensitive to human activities than diversity. We also found that the relative abundance of some taxa associated with nitrification and algal blooms (e.g., taxa in Nitrospira sp., Peridinales) has increased ca. 100-fold since the 1960s. Nutrient could better explain the variation in the bacterial community (ca. twice as much as heavy metal), while heavy metal explained micro-eukaryotes better (ca. 3 or 5-fold as much as nutrient). In particular, based on parsimonious models from distance-based linear model (distLM), we further identified that Pb is the key factor affecting the bacterial and micro-eukaryotes community in Lake Chao in addition to nutrient. Our study reveals the impacts of long-term human activities on lake ecosystems from multiple perspectives of nutrient and heavy metal loads, community structure, diversity and ecological network, these findings will contribute to the management and conservation of lakes in the future.

A palaeolimnological perspective to understand regime-shift dynamics in two Yangtze-basin lakes

Natural and human disturbances have caused widespread regime shifts in shallow lakes of the lower Yangtze basin (LYB, China) resulting in a severe decline of ecosystem services. Improved understanding of the relationship between environmental forcing and ecosystem response, and the mechanisms behind regime shifts has significant implications for management. However, the patterns of these regime shifts and the underlying internal mechanisms are less known. In this study, two typical lakes (Chaohu and Zhangdu) from the LYB were selected to determine the trajectories of ecological regime shifts, both of which transitioned from vegetation- to plankton-dominated states several decades ago. Ecological trajectories since the 1900s in both lakes were reconstructed using palaeolimnological proxies, mainly diatom assemblages. Although results show that regime shifts occurred in both lakes in the 1970s and the 1950s, respectively, their inherent mechanisms were different. In Lake Zhangdu, altered hydrological conditions pushed the ecosystem across an ecological threshold, providing an example of a driver-mediated regime shift. In Lake Chaohu, ongoing nutrient loading influenced ecosystem processes and drove the lake to an alternative stable state, potentially presenting an example of a critical transition after a loss of resilience. This research indicates that palaeolimnological perspectives can provide insights into regime shift changes, as well as important information regarding which restoration methods should be tailored to individual lakes.

Morphological traits of submerged macrophytes reveal specific positive feedbacks to water clarity in freshwater ecosystems

Positive feedbacks are generally related to the interactions between biotic processes and abiotic drivers and may lead to the emergence of alternative stable states in ecosystems. Understanding the mechanisms of self-reinforcing feedbacks in a macrophyte-dominant clear state is critical for lake management. Based on a survey of 35 lakes in the Yangtze River floodplain and canonical correspondence analysis (CCA) with forward selection, the results showed that water clarity is the most limiting factor that influences the community structure and biomass of submerged macrophytes. The canopy length of tall macrophytes (i.e., Myriophyllum spicatum L. and Potamogeton malaianus Miq.) showed positive allometry with plant height, while the canopy length of small macrophytes (e.g., Potamogeton maackianus A. Benn.) showed isometry. Our results indicated the existence of positive feedbacks between macrophyte vegetation and water clarity in a "more vegetation, higher water clarity" pattern. We found that the relationships between monospecific community biomass and water clarity differed among community types, indicating that the strength of the positive feedback was interspecific. Furthermore, we found significant differences in the Secchi depth (SD), chlorophyll a (Chl a), light attenuation coefficient (K) and dissolved oxygen (DO) associated with monospecific macrophyte patches. Plant height had significant relationships with the mean values of SD, Chl a, total phosphorus (TP) and K, suggesting that plant height was one of the mechanisms underlying the positive feedbacks. In management practices, efforts to build and maintain the resilience of an ecosystem should be trait-based rather than merely focusing on vegetation abundance.

Success of lake restoration depends on spatial aspects of nutrient loading and hydrology

Many aquatic ecosystems have deteriorated due to human activities and their restoration is often troublesome. It is proposed here that the restoration success of deteriorated lakes critically depends on hitherto largely neglected spatial heterogeneity in nutrient loading and hydrology. A modelling approach is used to study this hypothesis by considering four lake types with contrasting nutrient loading (point versus diffuse) and hydrology (seepage versus drainage). By comparing the longterm effect of common restoration measures (nutrient load reduction, lake flushing or biomanipulation) in these four lake types, we found that restoration through reduction of nutrient loading is effective in all cases. In contrast, biomanipulation only works in seepage lakes with diffuse nutrient inputs, while lake flushing will even be counterproductive in lakes with nutrient point sources. The main conclusion of the presented analysis is that a priori assessment of spatial heterogeneity caused by nutrient loading and hydrology is essential for successful restoration of lake ecosystems.

Excess nutrient loads to Lake Taihu: Opportunities for nutrient reduction

Intensive agriculture and rapid urbanization have increased nutrient inputs to Lake Taihu in recent decades. This resulted in eutrophication. We aim to better understand the sources of river export of total dissolved nitrogen (TDN) and phosphorus (TDP) to Lake Taihu in relation to critical nutrient loads. We implemented the MARINA-Lake (Model to Assess River Inputs of Nutrients to seAs) model for Lake Taihu. The MARINA-Lake model quantifies river export of dissolved inorganic and organic N and P to the lake by source from sub-basins. Results from the PCLake model are used to identify to what extent river export of nutrients exceeds critical loads. We calculate that rivers exported 61 kton of TDN and 2 kton of TDP to Lake Taihu in 2012. More than half of these nutrients were from human activities (e.g., agriculture, urbanization) in Sub-basins I (north) and IV (south). Most of the nutrients were in dissolved inorganic forms. Diffuse sources contributed 90% to river export of TDN with a relatively large share of synthetic fertilizers. Point sources contributed 52% to river export of TDP with a relatively large share of sewage systems. The relative shares of diffuse and point sources varied greatly among nutrient forms and sub-basins. To meet critical loads, river export of TDN and TDP needs to be reduced by 46-92%, depending on the desired level of chlorophyll-a. There are different opportunities to meet the critical loads. Reducing N inputs from synthetic fertilizers and P from sewage systems may be sufficient to meet the least strict critical loads. A combination of reductions in diffuse and point sources is needed to meet the most strict critical loads. Combining improved nutrient use efficiencies and best available technologies in wastewater treatment may be an effective opportunity. Our study can support the formulation of effective solutions for lake restoration.