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

How nutrient retention and TN:TP ratios depend on ecosystem state in thousands of Chinese lakes

Worldwide, anthropogenic activities threaten surface water quality by aggravating eutrophication and increasing total nitrogen to total phosphorus (TN:TP) ratios. In hydrologically connected systems, water quality management may benefit from in-ecosystem nutrient retention by preventing nutrient transport to downstream systems. However, nutrient retention may also alter TN:TP ratios with unforeseen consequences for downstream water quality. Here, we aim to increase understanding of how nutrient retention may influence nutrient transport to downstream systems to improve long-term water quality management. We analyzed lake ecosystem state, in-lake nutrient retention, and nutrient transport (ratios) for 3482 Chinese lakes using the lake process-based ecosystem model PCLake+. We compared a low climate change and sustainability-, and a high climate change and economy-focused scenario for 2050 against 2012. In both scenarios, the effect of nutrient input reduction outweighs that of temperature rise, resulting in more lakes with good ecological water quality (i.e., macrophyte-dominated) than in 2012. Generally, the sustainability-focused scenario shows a more promising future for water quality than the economy-focused scenario. Nevertheless, most lakes remain phytoplankton-dominated. The shift to more macrophyte-dominated lakes in 2050 is accompanied by higher nutrient retention fractions and less nutrient transport to downstream waterbodies. In-lake nutrient retention also alters the water's TN:TP ratio, depending on the inflow TN:TP ratio and the ecosystem state. In 2050 higher TN:TP ratios are expected in the outflows of lakes than in 2012, especially for the sustainability-focused scenario with strong TP loading reduction. However, the downstream impact of increased TN:TP ratios depends on actual nutrient loadings and the limiting nutrient in the receiving system. We conclude that nutrient input reductions, improved water quality, higher in-lake nutrient retention fractions, and lower nutrient transport to downstream waterbodies go hand in hand. Therefore, water quality management could benefit even more from nutrient pollution reduction than one would expect at first sight.

Regime shifts in shallow lakes explained by critical turbidity

Worldwide, water quality managers target a clear, macrophyte-dominated state over a turbid, phytoplankton-dominated state in shallow lakes. The competition mechanisms underlying these ecological states were explored in the 1990s, but the concept of critical turbidity seems neglected in contemporary water quality models. In particular, a simple mechanistic model of alternative stable states in shallow lakes accounting for resource competition mechanisms and critical turbidity is lacking. To this end, we combined Scheffer's theory on critical turbidity with insights from nutrient and light competition theory founded by Tilman, Huisman and Weissing. This resulted in a novel graphical and mathematical model, GPLake-M, that is relatively simple and mechanistically understandable and yet captures the essential mechanisms leading to alternative stable states in shallow lakes. The process-based PCLake model was used to parameterize the model parameters and to test GPLake-M using a pattern-oriented strategy. GPLake-M's application range and position in the model spectrum are discussed. We believe that our results support the fundamental understanding of regime shifts in shallow lakes and provide a starting point for further mechanistic and management-focused explorations and model development. Furthermore, the concept of critical turbidity and the relation between light-limited submerged macrophytes and nutrient-limited phytoplankton might provide a new focus for empirical aquatic ecological research and water quality monitoring programs.

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.

How will hydro-energy generation of the Nepalese Himalaya vary in the future? A climate change perspective

Despite being one of the proven clean-energy technologies, hydroelectricity is losing attention in global research. Hydroelectricity is extremely important for countries possessing the required water resources, already heavily reliant on it and those lacking the financial capacity to invest in other expensive energy technologies. This study assessed the possible impact of climate change (CC) on hydro-energy generation in the Nepalese Himalaya (possessing eight peaks out of 14 over 8000 m) with a tremendous hydropower potential (∼50,000 MW). A planned 1200 MW storage type Budhigandaki Hydroelectricity Project is taken as a case. We estimated the energy generation for the baseline as well as 10 CC scenarios considering RCPs 4.5 and 8.5 at monthly, seasonal, and annual temporal scales for the mid-century. Results show that energy generation is highly dependent on the reservoir operating rule. The average annual energy generation is expected to vary within -5 to +12% of the base case in the mid-century, with significant variations across the months. We also infer that designing hydro-projects based on ensembled climate values could lead to a "rosy" but less probable and risky picture of energy generation in the future. Therefore, assessment of a wide spectrum of plausible CC scenarios are recommended. Storage type projects with provision of flexible operating rules considering finer temporal resolution and allocation to competing users (in case of multipurpose projects) supported by appropriate policies are desirable for climate resiliency. Complementing the existing energy generation mix with other technologies in areas where hydroelectricity is expected to undergo adverse impacts of CC is warranted for attaining future energy security and environmental safeguarding. Possibility of additional energy due to CC is a strong motivation for this region to focus on hydroelectricity development in the future.

Environmental impacts and risks of bridges and tunnels across lakes: An overview

Bridges and tunnels are built across lakes, especially in China, to improve road connectivity for transportation. However, their environmental impacts and risks have not received adequate attention. In this study, the magnitude of bridges and tunnels across 142 lakes in China were investigated. The investigation revealed that 37 bridges and 10 tunnels (a total length of 56.82 km) were built across 26 lakes during 2000-2020. From 2011 to 2020, the construction rate of bridges and tunnels across lakes in China was ∼6 times higher than the average value worldwide. Their environmental impacts and risks on lakes were summarised by analyzing previous publications screened from the Web of Science platform. The environmental impacts and risks during the bridge/tunnel construction period generally include decrease in water exchange, habitat destruction, biological reduction, increase in suspended solids, and water quality pollution. Tunnel construction may also affect the ecological conditions of groundwater due to its disturbances in the underground area. During the bridge/tunnel operation period, environmental impacts and risks were mainly induced by rainfall events and water pollution accidents. The impacts and risks were highly related to the construction location, bridge/tunnel properties, moving vehicles, fixing activities, and atmospheric deposition. Owing to the existing weaknesses in management practices, a framework, aiming to reduce the environmental impacts and risks caused by bridges and tunnels across lakes, was proposed. The framework identified the critical issues and their corresponding management strategies during the bridge/tunnel construction and operation periods and enabled the generation of the best management strategy for each specific period.

Anthropogenic impacts on the biodiversity and anti-interference ability of microbial communities in lakes

Lakes are critical for biogeochemical and ecological processes and are sensitive and vulnerable to anthropogenic disturbances, but how and to what extent human activities disturb the biodiversity in lakes remain unknown. Here, we showed the microbial diversity in 46 lakes and assessed the influence of 27 anthropogenic factors. We found that the economic level (e.g., per capita gross domestic product) was strongly negatively correlated (r = -0.97) with bacterial diversity but positively correlated (r = 0.17) with fungal diversity in lakes. The composition of the microbial community significantly changed with increasing economic level. Bacteria are more sensitive than fungi to anthropogenic impacts. Expanding the population size and increasing the economic level may promote the development of fungal diversity but inhibit bacterial diversity. Air quality, urbanization and ozone were negatively correlated with bacterial diversity, and fisheries had a negative correlation with fungal diversity. The anti-interference ability of lake microorganisms in the middle economic level zones (45,000-90,000 yuan/person) was stronger than that in high-level (> 90,000 yuan/person) and low-level (> 45,000 yuan/person) economic zones. Overall, our investigation provides national-scale evidence that changes in the microbial diversity in lakes were related to economic levels.

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.