Submerged macrophytes play a crucial role in lake ecosystems, and their survival is dependent upon their ability to cope with variable environmental stress. Therefore, studying the plastic response of submerged macrophytes' resource allocation and functional traits to the environment may provide insights helpful for ecological restoration practices. In September 2021, a field survey was conducted in the Erhai Lake, where samples of Ottelia acuminata, and functional traits and biomass allocation in relation to water depth were measured. The study found that O. acuminata exhibited large intraspecific variations to adapt to environmental stress, and the average intraspecific variation was 55.86

Biological invasions in freshwater ecosystems are increasingly severe, posing significant threats to ecosystem health and economic development. Hydrilla verticillata (L.f.) Royle and Elodea densa (Planch.) Casp. are two of the most aggressive invasive submerged macrophytes worldwide, and often regarded as similar species due to their growth forms and habitat requirements, although there are few field coexistence records. Hydrilla verticillata is a native species in the large plateau Lake Erhai, where the non-native E. densa has been documented since 2017. This study aims at exploring the colonisation process of E. densa as well as its niche overlap and interspecific interactions with native H. verticillata in Lake Erhai. A continuous seven-year field investigation was conducted in Lake Erhai. Four indicators were used to assess the population distribution of H. verticillata and E. densa, including occurrence frequency, biomass, relative abundance and relative niche breadth. The logistic growth model was applied to analyse population dynamics. The Gaussian model was used to characterise their distribution with water depths. A stability index was employed to evaluate variations in measured indices across different water depths. Indicators of niche overlap and interspecific association were used to describe the coexistence and interactions between the two species. Elodea densa established several stable populations in Lake Erhai after years of colonisation and naturalisation, primarily colonising deeper areas of the lake than H. verticillata. Elodea densa tended to thrive at depths of around 4.0 m, whereas H. verticillata typically grew at depths from 2.0 to 3.0 m. Hydrilla verticillata demonstrated greater stability across varying water depths than E. densa. Niche overlap between the two species was minimal. Our findings indicated that in this large plateau lake, native H. verticillata exhibited higher competitiveness than alien E. densa. There was a distinct niche difference in water depths between H. verticillata and E. densa, which prevented competitive exclusion. The two species achieved stable coexistence at a lake-wide scale. This study provided the first field evidence for coexistence between two globally recognized invasive species, showing that in habitats with sufficient environmental gradients and filtering pressures, submerged macrophytes similar in taxonomy and appearance could avoid competitive exclusion by occupying different ecological niches, leading to stable coexistence.

1. An understanding of how biodiversity confers ecosystem stability is crucial in managing ecosystems under major environmental changes. Multiple biodiversity drivers can stabilize ecosystem functions over time. However, we know little about how local environmental conditions can influence these biodiversity drivers, and consequently how they indirectly shape the ecological stability of ecosystems. 2. We hypothesized that environmental factors can have opposite influences (i.e., not necessarily either positive or negative) on the temporal stability of communities in different environmental ranges depending on the biodiversity drivers involved. We tested this novel hypothesis by using data from a 4-year-long field study of submerged macrophyte across a water depth gradient in 8 heterogeneous bays of Erhai lake (with total sample size of 30,071 quadrats), a large lentic system in China. 3. Results indicate that a unimodal pattern of stability in temporal biomass measurements occurred along the water-depth gradient, and that multiple biodiversity drivers (the asynchrony in species dynamics, and the stability of dominant species) generally increased the temporal stability of aquatic primary producers. However, the effect of water depth either increased or decreased the stability of biomass according to the environmental conditions associated with sites along the water depth gradient. 4. Synthesis. These results reveal the influence of local environmental conditions on the biodiversity drivers of stability may help predict the functional consequences of biodiversity change across different scenarios of environmental change.

IntroductionWater depth (WD) and snail abundance (SA) are two key factors affecting the growth of submersed aquatic plants in freshwater lake ecosystems. Changes in WD and SA drive changes in nutrients and other primary producers that may have direct or indirect effects on submersed plant growth, but which factor dominates the impact of both on aquatic plants has not been fully studied.MethodsTo investigate the dominant factors that influence aquatic plant growth in plateau lakes, a one-year field study was conducted to study the growth of three dominant submersed macrophyte (i.e., Vallisneria natans, Potamogeton maackianus, and Potamogeton lucens) in Erhai Lake.ResultsThe results show that, the biomass of the three dominant plants, P.maackianus, is the highest, followed by P.lucens, and V.natans is the lowest. Meanwhile, periphyton and snails attached to P.maackianus are also the highest. Furthermore, WD had a positive effect on the biomass of two submersed macrophyte species of canopy-type P.maackianus and P.lucens, while it had a negative effect on rosette-type V.natans. Snail directly inhibited periphyton attached on V.natans and thereby increasing the biomass of aquatic plants, but the effect of snails on the biomass of the other two aquatic plants is not through inhibition of periphyton attached to their plants.DiscussionThe dominant factors affecting the biomass of submersed macrophyte in Erhai Lake were determined, as well as the direct and indirect mechanisms of WD and snails on the biomass of dominant submersed macrophyte. Understanding the mechanisms that dominate aquatic plant change will have implications for lake management and restoration.

Macrophytes with different growth forms exhibit diverse functional traits and ecological functions. In natural sub-deep lakes, there are often large differences in water quality between nearshore areas with macrophytes and open water areas. However, it remains unclear whether this phenomenon can be attributed to differences in plant growth forms. Therefore, we conducted continuous monitoring for four years, both before and after the implementation of an ecological restoration project, to explore whether the change in plant growth forms caused differences in water quality between the nearshore and open water areas. The results showed that implementing ecological restoration projects proved highly effective in improving the local environment, including water physicochemical properties and biological components, in the implementation area. First, the ecological restoration project greatly altered the plant community structure in the nearshore area before and after restoration. After restoration, there was a significant increase in the biomass and distribution area of noncanopy-forming plants (including erect and rosette-forming plants), while the opposite effect was observed for canopy-forming plants. Second, the transition of macrophyte community growth forms enhanced the stability of both macrophyte communities and water physicochemical parameters. Furthermore, the reduction in canopy-forming plants facilitated a more efficient water body exchange, resulting in greater homogeneity in water quality between the nearshore and open water areas. Overall, the presence of canopy-forming plants can hinder water body exchange due to large canopy formations on the water surface. In light of these findings, it is recommended that ecological restoration projects in natural lakes should consider the functional group composition of macrophytes.

The effect of biodiversity on ecosystem productivity has been a controversial issue in ecological research. The species richness–productivity relationship is highly variable in natural ecosystems, with a positive relationship being one of the most commonly observed relationships. Previous regional studies from terrestrial ecosystems have demonstrated that environmental gradients can regulate the species richness–productivity relationship. However, how this relationship varies in freshwater ecosystems across spatial environment gradients remains unclear. In this study, we propose that the species richness–productivity relationship can be modulated by the water depth. Here, we surveyed the submerged macrophyte community structure by establishing 24 transects and 642 quadrats in Erhai Lake, Yunnan Plateau, China. Our findings highlight that the species richness–productivity relationship gradually changed from slightly positive to strongly positive as the environment became more light-limited with the increasing water depth, supporting the stress-gradient hypothesis. The results from this study provide new insights into the biodiversity–ecosystem functioning relationships and in managing lake macrophyte communities and productivity.

Submerged macrophytes can improve water quality and buffer the effects of external nutrient loading, which helps to maintain a clear-water state in shallow lakes. We constructed 12 large enclosures with contrasting coverages (treatments) of submerged macrophytes (SMC) to elucidate their buffering capacity and resilience to nutrient pulses. We found that aquatic ecosystems with high SMC had higher buffering capacity and resilience, vice versa, i. e, the enclosures with high SMC quickly buffered the nutrient pulse and rebounded to clear-water state after a short stay in turbid-water state dominated by algae, while the treatments with low SMC could not fully buffer the pulse and rebound to clear-water state, and they slowly entered the transitional state after staying in turbid-water state. This means that the enclosures with high SMC had a better water quality than those with low SMC, i.e., the levels of nutrients and Chl-a were lower in the treatments with high plant coverage. In addition, plant coverage had a significantly positive buffering effect against nitrogen and phosphorus pulses, i.e., the nutrient concentrations in the treatments with high SMC took shorter time to return to the pre-pulse level. Overall, our results evidenced that the higher that the SMCs is, the better is the water quality and buffering capacity against nutrient pulses, i.e. the more stable is the clear-water state. However, low SMC may not be able to resist the impact of such strong nutrient pulse. Our results provide reference and guidance for water pollution control and water ecological restoration.

Introduction Macrophytes are essential for maintaining the health of shallow lake ecosystems, however, the driving and responsive relationship between ecological factors (such as seasonal changes and nutrition, etc.) and plant communities is not yet clear. Methods In this study, we conducted seasonal surveys of macrophyte community composition in lakes with different nutrient states, aiming to understand the incidence relation between macrophyte community diversity, seasonal changes and environmental factors. Results According to the classification criteria of comprehensive nutritional index, there were significant differences in the trophic status of the three lakes. Among them, the Xihu Lake has reached mild eutrophication with a TLI value of 56.33, both Cibi Lake and Haixihai Lake are mesotrophic with TLI value of 36.03 and 33.48, respectively. The results of diversity analysis showed a significant negative correlation between α-diversity (include Species richness, Shannon-Wiener index, Simpson index and Pielou index) and lake nutrient status. Among them, Xihu Lake showed the lowest α-diversity in all seasons, Haixihai Lake exhibited the middle α-diversity, Cibi Lake indicated the highest α-diversity. Non-metric multidimensional ordination showed that there were obvious spatial structures differences among the macrophyte communities in the three lakes. Macrophyte community composition in the three lakes was more similar in summer and autumn, but there was a wider gap in spring and winter. The redundancy analysis indicated distinct differences between diversity index and ecological factors, the eigenvalues of Axis 1 and Axis 2 being, respectively, 36.13% and 8.15%. Environmental factors could explain 44.8% of the total variation in macrophyte communities structure. Among these, nitrogen, phosphorus, water transparency and water temperature contributed 50.2%, 3.5%, 3.8% and 27.5%, respectively. Conclusions In summary, the community structure of macrophytes in plateau shallow lakes is co-regulated by seasons and nutrients.

Aquatic plants in lakeshore zone play an important role in maintaining the health of lake ecosystem.In order to understand the current status of aquatic plants in the lakeshore zone of Lake Erhai,this study investigated the aquatic plants quarterly in the lakeshore zone from 2020 to 2021.The results showed that there were 206 species of aquatic plants belonging to 56 families and 156 genera in the lakeshore zone of Lake Erhai,including 149 species of hygrophytes,24 species of emergent plants,21 species of submerged plants,7 species of floating plants and 5 species of floating leaf plants.Among all these species,Cynodon dactylon,Zizania latifolia,Vallisneria natans and Trapa bispinosa,etc.were common species,Lonicera japonica,Elymus dahuricus,etc.were occasional species.From the distribution of floristic regions,the species in the lakeshore zone of Lake Erhai were mainly world distribution（83 species） and tropical distribution（55 species）,accounting for 40.28% and 26.71% of the total species,respectively.In terms of plant communities,there were 18 main plant community types in the lakeshore zone of Lake Erhai,including 4 types of hyophytes communities,3 types of emergent plant communities,9 types of submerged plant communities and 2 types of floating leaf plant communities.Among all the communities,the main dominant communities were Cynodon dactylon communities,Zizania latifolia communities,Vallisneria natans communities and Trapa bispinosa communities.Compared with previous investigations,it was concluded that the diversity of aquatic plants in the lakeshore zone of Lake Erhai had been significantly improved in recent years.However,there are some problems such as the small area of lakeshore zone and the single of emergent plant community.Therefore,the construction of lakeshore zone should be further strengthened to improve the habitats and increase species diversity.

为研究沉水植物光合特性与其分布水深的关系,选取黑藻(Hydrilla verticillata)、苦草(Vallisneria na-tans)、水蕴草(Egeria densa)、大茨藻(Najas marina)、微齿眼子菜(Potamogeton maackianus)、光叶眼子菜(Potamogeton lucens)和穿叶眼子菜(Potamogeton perfoliatus)等15种洱海常见沉水植物,测定其光合作用参数.结果表明:光合速率为2.8—18.1μmol O2/(g DW·h)、暗呼吸速率为0.3—2.0μmol O2/(g DW·h)、光补偿点为6.3—63.8μE/(m2·s)、光饱和点为55.6—441.5μE/(m2·s),不同沉水植物间光合作用参数存在显著差异.结合洱海全湖沉水植物分布水深调查结果,沉水植物的光补偿点和光饱和点与分布水深呈显著负相关;苦草与其他物种比较具有更低的光补偿点6.3μE/(m2·s)、光饱和点55.6μE/(m2·s),更适宜在深水或弱光条件下生长,可作为沉水植被修复的先锋物种.

Decreased underwater light availability is one of the most important environmental factors leading to the decline in submersed macrophytes in lakes. However, previous studies mostly focused on a relatively short time scale and lack data on the continuous monitoring of plant life history traits under different light conditions. The present experiment studied the growth, morphology and C/N metabolism of a representative submersed macrophyte, Vallisneria natans, in response to various light regimes (2.8%, 7.1%, 17.1%, and 39.5% ambient light intensity) over a period of 12 months. The results showed that the total biomass and ramet number increased with increased light intensity; in contrast, individual biomass, leaf number and maximum leaf length decreased with increased light intensity. The C/N metabolism indices of V. natans indicated that leaves were the most sensitive to light availability, stems were moderately sensitive, and roots were the least sensitive. V. natans grown in an extremely low-light environment exhibited decreased soluble carbohydrate (SC) and starch and increased free amino acid (FAA) and total nitrogen (TN) levels. The increasing function of the ramet number under different light regimes was fitted. The obtained maximum environmental capacity of the ramet number (K) and the days that the ramet number reached K/2 in each treatment are useful for lake V. natans restoration and seedling cultivation in similar limited-resource environments. Harvesting partial ramets properly and maintaining the ramet number near K/2 could ensure the maximum increase rate of the population to satisfy the demand of V. natans seedlings for lake restoration. The results obtained in this study can be used for lake water level management to achieve specific purposes during submerged macrophyte recovery, such as plant height elongation or plant population quantity expansion.

Submerged plants are crucial for maintaining a clear water state in lakes, and their distribution area is usually determined by the lowest distribution boundary (LDB). However, LDBs of submerged macrophytes often exhibit species differences and vary with seasonal water level fluctuations. In this study, we carried out a three-year field investigation to compare differences in the LDBs of three submerged macrophyte species (i.e., Vallisneria natans, Ceratophyllum demersum, and Potamogeton maackianus) and explore their distinct determinants. We assumed that LDB differences would exist among the three species and that water level (WL) would be a key environmental driver of LDB dynamics, with an association between the LDBs of submerged macrophytes and water level fluctuations. Indeed, our results showed significant seasonal differences between C. demersum and V. natans in the period of September - May, with mean annual values in the order (low to high) V. natans, C. demersum, and P. maackianus. Furthermore, water level was a major driving force of the LDB dynamics of the three species. Changes in the LDBs of the three species in response to water level fluctuations could be divided into three periods: i.e., recovery growth period (April to June), stress tolerance period (July to September), and recession period (October to following March). This study not only clarifies the impact of water level on submerged macrophyte growth and distribution, but also has larger practical implications for the ecological restoration and management of large deep lakes.

The impact of biodiversity on ecosystem functions and services remains a hot topic in ecology, especially under increasing biodiversity loss. Studies on the relationship between biodiversity and ecosystem functioning have identified two regulatory mechanisms: i.e., “niche complementarity” and “selection effect”. However, the relative importance of these mechanisms within communities and how they change along ecological gradients remain unclear. Moreover, most studies have focused on relatively stable ecosystems (e.g., forest, grassland), with limited attention paid to ecosystems with obvious seasonal changes, e.g., freshwater ecosystems. In this study we conducted a seasonal survey of submerged macrophyte communities to clarify the mechanisms of biodiversity on biomass productivity accounting for seasons and water depth. Our results showed that (1) seasonal variations of community indicators exhibited different trends along the water depth gradient. Biomass productivity showed high seasonal variation at intermediate depth while temporal beta diversity showed the opposite pattern. (2) Water depth not only inhibited biomass productivity directly but also indirectly by reducing species richness and enhancing temporal beta diversity. (3) The positive effect of species richness on biomass productivity was lower than the negative effect of evenness (path coefficients: 0.13 vs. 0.27), indicating that “selection effect” played a more important role in the relationship. (4) In shallow areas, the mechanism underlying the effects of biodiversity on biomass productivity was more from “selection effect”, whereas, in deep areas, the effects were related to both “niche complementarity” and “selection effect”. Our findings suggest that “niche complementarity” and “selection effect” are two complementary mechanisms underpinning the relationship between biodiversity and biomass productivity, and their relative importance varies with specific ecological gradients. Our results also provide a reference for studies on other freshwater organisms with analogous distribution patterns as submerged macrophytes with water depth. Scale dependence of the above relationship should be considered in future studies.

Increasing eutrophication poses a considerable threat to freshwater ecosystems, which are closely associated with human well-being. As important functional entities for freshwater ecosystems, submerged macrophytes have suffered rapidly decline with eutrophication. However, it is unclear whether and how submerged macrophytes maintain their ecological functions under increasing eutrophication stress and the underlying patterns in the process. In the current study, we conducted an extensive survey of submerged macrophytes in 49 lakes and reservoirs (67% of them are eutrophic) on the Yunnan-Guizhou Plateau of southwestern China to reveal the relationship between submerged macrophyte biodiversity and ecosystem functioning (BEF) under eutrophication stress. Results showed that submerged macrophytes species richness, functional diversity (FD), and β diversity had positive effects on ecosystem functioning, even under eutrophication. Functional diversity was a stronger predictor of community biomass than species richness and β diversity, while species richness explained higher coverage variability than FD and β diversity. This suggests that species richness was a reliable indicator when valid functional traits cannot be collected in considering specific ecological process. With increasing eutrophication in water bodies, the mechanisms underlying biodiversity-ecosystem functioning evolved from “niche complementarity” to “selection effects”, as evidenced by decreased species turnover and increased nestedness. Furthermore, the relative growth rate, specific leaf area, and ramet size in trade-off of community functional composition became smaller along eutrophication while flowering duration and shoot height became longer. This study contributes to a better understanding of positive BEF in freshwater ecosystems, despite increasing anthropogenic impacts. Protecting the environment remained the effective way to protect biodiversity and corresponding ecological functions and services. It will be important to consider different facets of biodiversity on ecosystem functioning in future studies to improve effective management plans.

Adaptations to low light and water depth stresses are crucial for the survival of submerged macrophytes. To determine the phenotypic responses of Vallisneria natans to such stresses, we combined a field investigation, a light control experiment and an in situ response experiment to evaluate adaptive variations in fresh weight and morphological and physiological characteristics. In the field investigation and the light control experiment, water depth and light intensity were the main environmental factors affecting the fresh weight and morphological characteristics of V. natans. Fresh weight and leaf length were the most significant responding variables, and they were positively correlated with water depth and negatively correlated with underwater light intensity, although the root length and leaf number exhibited no variation. The leaf length elongated more rapidly at sites with intermediate water depth and low light intensity, and the allometric slopes were steeper. Furthermore, the in situ response experiment results showed a unimodal distribution of the chlorophyll-a concentrations of V. natans along with increasing water depths (from 0.5 m to 8.5 m). The turning point of the chlorophyll-a concentration occurred at 5.5 m, which is almost the maximum water depth at which V. natans occurs in Lake Erhai. Overall, our results evidenced that light availability is an important driving factor that controls the status of V. natans by affecting not only its morphology and physiology but also its biomass allocation and ramet production. An alternative resource allocation pattern of V. natans could be a shift between light acquisition and clonal reproduction.

为构建种群动态模型以指导沉水植被修复工程实践,研究采用同质园实验方法对6种常见沉水植物(竹叶眼子菜(Potamogeton wrightii)、眼子菜(P.distinctus)、光叶眼子菜(P.lucens)、穿叶眼子菜(P.perfoliatus)、扭叶眼子菜(P.intortifolius)和苦草(Vallisneria natans)的克隆生长模式进行了连续观测研究,获取了分株形成速率、空间扩张速率、株高增加速率等种群扩张动态参数,及分株数、间隔子长度、分株高度等克隆构件特征参数.结果表明,6种沉水植物的分株数从28d开始增长,其中苦草的分株形成速率最高,平均为1.09株/d,分株形成最大速率出现在55d之后;穿叶眼子菜和扭叶眼子菜的分株形成速率低于苦草,但是高于竹叶眼子菜、眼子菜和光叶眼子菜,最大速率出现在41d之后.虽然苦草的分株最多,但是分株的株高最低,其株高增长速率均值为0.2 cm/d.眼子菜属物种中竹叶眼子菜和眼子菜株高增长速率最高,光叶眼子菜的株高增长速率和分株形成速率都最低.克隆系占据面积随时间的扩张速率为穿叶眼子菜(113.22 cm2/d)＞扭叶眼子菜(71.70 cm2/d)＞苦草(35.48 cm2/d)＞竹叶眼子菜(12.09cm2/d)＞眼子菜(3.07cm2/d)＞光叶眼子菜(0.53 cm2/d).此外,研究还发现眼子菜属植物普遍表现出匍匐茎上"节"的形成,而苦草则不具备这种特性,匍匐茎"节"的形成及随之形成的不定根在眼子菜属植物空间扩张过程中具有重要的生态功能,并在种群构建方面与苦草等其他物种发生分异.基于眼子菜属植物匍匐茎上的"节"可以形成跳跃性的分株,在种群面积扩张方面更具优势;而苦草形成分株的数量更多、速度更快,在提高种群密度保障种群稳定方面更有优势.

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.

Removal of planktivorous fish is used extensively in northern temperate lakes to reduce phytoplankton abundance via enhanced zooplankton grazing. However, whether this method would work also in large subtropical highland lakes to alleviate cyanobacterial blooms is unknown. We conducted a one-year pilot in situ experiment where we removed a substantial biomass of fish in a fenced-in area, followed by a 3-year whole-lake experiment where the dominant fish species (Japanese smelt) was removed in Lake Erhai in southwest China. The fencing experiments showed that between July and November, when the biomass of the removed stock reached 4 g/m(2), the zooplankton biomass inside the fence increased significantly compared to a control fence. In the full-lake experiment, we found that sustained removal of Japanese smelt led to an increase in the biomass of cladocerans (Daphnia spp. but especially of Bosmina spp.) and a significant decrease in the biomass of Cyanobacteria and Chlorophyta. Additionally, a marked increase in the ratio of zooplankton to phytoplankton biomass, as well as an increase in the body size of cladocerans, emphasising the importance of enhanced top-down control for mitigating cyanobacterial blooms following extensive fish removal. Our results reveal that removal of small fish (here Japanese smelt) can lead to a reduction of the phytoplankton and cyanobacteria biomass through a trophic cascade in highland deep subtropical lakes. Thus fish removal may be a feasible additional restoration tool to external nutrient loading reduction in such lakes.

Introduction of planktivorous fish or eutrophication can alter lake food webs, especially the zooplankton communities are susceptible to changes in top–down and bottom-up controls, and eventually lead to the dominance of harmful cyanobacteria. Hence, for the recovery of large-size zooplankton abundance that graze on cyanobacteria, there is an urgency to understand the relative roles of top–down and bottom-up effects. Although much is known about these two effects in temperate lakes, little knowledge about their relative importance in subtropical highland lakes exists, where eutrophication and stocked planktivorous fish are of particular concern. Thus, we conducted research in Lake Erhai in Yunnan plateau, China, to examine the drivers affecting the dynamics of crustacean zooplankton. Redundancy analysis (RDA) was used to analyze the effects of environmental variables, phytoplankton biomass, and planktivorous fish on the biomass and body weight of zooplankton taxa in the lake, and variance decomposition analysis was applied to examine the relative roles of the bottom–up and top–down controlling factors on zooplankton biomass. The results of RDA and Pearson correction analysis showed that total nitrogen (TN) in the water column affected phytoplankton and altered the biomass of cladocerans, while water temperature directly affects the biomass of cladocerans. These findings indicate a pronounced bottom–up control link exists from nutrients to phytoplankton, and then to zooplankton. The abundance of Japanese smelt was negatively correlated both with the biomass and body weight of cladocerans. This finding suggests the pronounced top–down control link exists from fish to zooplankton. The results of variance decomposition analysis showed that TN and zooplanktivorous predation were more important in driving total zooplankton biomass, while TN, water temperature and fish predation were more essential in the variation of zooplankton biomass. Our study provides a reference for the recovery of large-size zooplankton populations in eutrophicated lakes.

Introductions or alien species invasions will induce changes in aquatic ecosystems but are rarely reported in Chinese highland lakes. The Japanese smelt (Hypomesus nipponensis) invaded and has become a dominant fish species in Lake Erhai, a highland lake in southwestern China, since 2016. Here, we engineered Ecopath models for two different periods, 2008–2009 (preinvasion) and 2016–2018 (postinvasion), in Lake Erhai to model ecosystem impacts from the Japanese smelt invasion. In the dynamic Ecosim model based on the 2016–2018 Ecopath model, we ran three 50-year scenarios to simulate the potential effects of Japanese smelts on the system. Our results showed competition between invasive and native species as well as changes in trophic structures, highlighting the impacts of the invasive species over time. The lake ecosystem additionally experienced significant degradation after invasion, mainly reflected in several related indicators, such as total biomass/total system throughput (TB/TST), total primary production/total biomass (TPP/TB), total primary production/total respiration (TPP/TR), Finn's mean path length (FML), Finn's cycling index (FCI) and the Connectance Index (CI). The simulation results indicated that the relative biomass of icefish (Neosalanx taihuensis), bighead carp (Hypophthalmichthys nobilis), sharpbelly (Hemiculter leucisculus), and zooplankton were significantly affected by increasing the strength of the top-down control of the Japanese smelt on its prey. It is also important to do ecological regulation of planktivorous fishes in the studied Lake Erhai, especially the Japanese smelt.