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.

In shallow eutrophic lakes, submersed macrophytes are essential for maintaining a clear water state, and they are affected markedly by fishes directly through herbivory and indirectly by fish-invertebrate-periphyton complexity, a pathway that presently is not well understood in subtropical lakes but probably vital to lake managements. We conducted a mesocosm study involving benthic fish (Misgurnus anguillicaudatus), snails (Radix swinhoei) and submersed macrophyte (Vallisneria natans), aiming to examine whether benthic fish is detrimental to reestablishment of clear-water macrophyte-dominated state in eutrophic degraded lakes. In addition, we aimed to investigate the cascading effect that benthic fish might have on periphyton and phytoplankton and to what extent snails can alleviate this effect. Our results showed that benthic fish promoted nutrient release from the sediment and thereby facilitated the growth of phytoplankton and periphyton, leading to reduced growth of submerged macrophytes due to shading. Snails consumed the periphyton attached on the leaves of macrophytes, thereby being beneficial to the plant growth, albeit it could not fully counteract the adverse effects from benthic fish. The water quality indicators in terms of nutrients concentrations, phytoplankton biomass and light extinction coefficient along the water column was affected primarily by benthic fish, followed by macrophytes and snails. To target a clear-water condition, the water quality was best at the presence of macrophytes alone or in combination with snails, and worst at the presence of benthic fish. Our results implied that the removal of benthic fish should be a useful ecological restoration method for rehabilitation of submersed macrophytes and water quality improvement in subtropic, eutrophic, shallow lakes following external nutrient loading reduction.

实验以云南大理州洱海水生植被重度退化区(湖心平台)作为实验地点,探究极度弱光和两种底质(黏土、淤泥)环境下苦草(Vallisneria natans)在恢复过程中的形态及生理响应,并依此探讨底质改善对苦草种群恢复的作用.结果表明:(1)在极度弱光环境下,苦草部分死亡,存活数量下降,并在形态特征和生理特征上均表现出胁迫响应,其形态特征值下降,氮(N)含量和游离氨基酸含量上升,碳(C)含量和淀粉含量下降;(2)苦草不同器官对弱光环境的响应有所差异,叶片(地上部分)受到的胁迫影响大于根茎(地下部分);(3)苦草对弱光环境的响应在不同底质条件下有显著性差异,苦草在黏土底质上表现出更小的胁迫反应和更高的存活数量,两种底质相比较,黏土更适合作为苦草恢复的底质条件.研究表明在洱海当前的水质环境下有希望结合局部的底质改善来实现在南部湖心平台的沉水植物恢复.

Trapa natans is one of the main species causing the swamping in the littoral zones of Erhai Lake. It commonly forms a dense canopy on the water surface in the growing season (June–September), which hampers the local water quality and habitat of submerged macrophytes, and releases nutrients to the water after death in autumn and winter, resulting in the deterioration of local water quality. At present, there are many and positive research studies on the short-term effects of harvesting water chestnut on water quality and aquatic plants, but long-term observation results are lacking. In response to the above problems, we studied responses of water quality and aquatic plant community to the removal of Trapa in littoral zone of a northern bay in Erhai from August 2014 to January 2017. This could be the first attempt to discover the long-term effects of floating-leaved vegetation management in the freshwater ecosystem. The results showed that the artificial removal of Trapa significantly improved the local water quality in the growing season, for example, the concentrations of total nitrogen (TN), dissolved nitrogen (DN), total phosphorus (TP), and dissolved phosphorus (DP) in the non-Trapa zone (NTZ) were much lower than the concentrations of those in the adjacent Trapa zone (TZ). And the biomass of aquatic macrophyte community (BAMC) was significantly increased in the NTZ, up to the maximum value of about 21 kg/m2 in fresh weight. However, the diversity indexes of the community in the NTZ declined. Therefore, we suggested that although the removal of Trapa improved the water quality and increased the productivity of the submerged aquatic plant community, it reduced the species diversity of the aquatic plant community in the long run. This is another issue that we need to pay attention to in the later management in Erhai Lake.

水下弱光环境是水生态系统沉水植被恢复面临的常见问题,因此研究沉水植物在弱光胁迫下的适应性是必要的.本研究以云南大理州洱海南部重度退化的湖区(惯称湖心平台,曾分布有大面积沉水植物)作为实验地点,选取4种沉水植物——苦草(Vallisneria natans)、轮叶黑藻(Hydrilla verticillata)、穗花狐尾藻(Myriophyllum spicatum)、竹叶眼子菜(Potamogeton wrightii)作为研究对象,开展原位盆栽实验,通过比较4种沉水植物的生长情况来选取潜在的目标物种,为洱海湖心平台沉水植被恢复提供科学依据.结果 表明:1)水下弱光环境对4种沉水植物生长均造成胁迫,生物量和株高显著性降低,物种间的相对生物量差异表现为:苦草和竹叶眼子菜＞轮叶黑藻和穗花狐尾藻,相对株高差异表现为:苦草＞竹叶眼子菜＞穗花狐尾藻＞轮叶黑藻;2)生理特征方面,苦草、穗花狐尾藻和竹叶眼子菜有相似的胁迫反应,均表现为:氮(N)和游离氨基酸含量均升高,碳氮比(C∶N比)、可溶性糖含量与淀粉含量均下降,仅苦草的叶绿素含量升高,而轮叶黑藻的生理胁迫反应表现为:氨基酸含量下降,可溶性糖和淀粉含量升高;3)四者最终存活率为:苦草＞竹叶眼子菜＞穗花狐尾藻＞轮叶黑藻,且苦草相对生物量和相对株高高于其他3种植物.以上原位实验结果表明,与其他3种植物相比,苦草更能适应湖心平台的水下弱光环境,可考虑作为湖心平台沉水植被恢复的目标物种.

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.

近几十年来,在入湖营养盐增加,水质持续下降,藻类生物量逐年升高及水位大幅波动下,洱海沉水植物群落演替速度加剧.2016年7月调查显示:洱海沉水植被退化严重,物种数量显著低于近期（2011年）历史水平,分布面积较有记录的历史最高水平（1980s）下降超过70%,优势物种由1960s的海菜花（Ottelia acuminata）、蓖齿眼子菜（Stuckenia pectinata）和大茨藻（Najas marina）等转变为微齿眼子菜（Potamogeton maackianus）、金鱼藻（Ceratophyllum demersum）以及苦草（Vallisneria natans）.沉水植物群落在浅水区（0～2.5 m）主要遭受了浮叶植物细果野菱（Trapa maximowiezii）过度生长带来的生境胁迫,在中等水深区（2.5～4 m）面临微齿眼子菜和金鱼藻的过度生长导致的群落结构的单一化影响,而在深水区（＞4 m）面临着面积萎缩的风险,这些严重抑制了沉水植物清水稳态效应的发挥.通过对比洱海典型湖湾洱滨村水域沉水植物群落结构在恢复与优化前后的变化,发现洱滨村水域各水深区间沉水植物群落各项多样性指标与恢复物种的丰度均显著提升.因此,我们总结提出了针对洱海典型富营养湖湾沉水植被恢复和管理的建议和对策,以期为后续进一步开展洱海生态系统恢复相关工作提供依据.

AbstractSpatiotemporal variation in community composition is of considerable interest in ecology. However, few studies have focused on seasonal variation patterns in taxonomic and functional community composition at the fine scale. As such, we conducted seasonal high‐density sampling of the submerged macrophyte community in Hongshan Bay of Erhai Lake in China and used the generalized dissimilarity model (GDM) to evaluate the effects of environmental factors and geographic distance on taxonomic and functional beta diversity as well as corresponding turnover and nestedness components. At the fine scale, taxonomic turnover and nestedness as well as functional turnover and nestedness showed comparable contributions to corresponding taxonomic and functional beta diversity, with different importance across seasons. All taxonomic and functional dissimilarity metrics showed seasonal variation. Of note, taxonomic beta diversity was highest in summer and lowest in winter, while functional beta diversity showed the opposite pattern. Taxonomic and functional turnover showed similar change patterns as taxonomic and functional beta diversity. Taxonomic nestedness was low in summer and high in winter. Functional nestedness was also lower in summer. These results suggest that under extreme environmental conditions, both turnover and nestedness can exist at the fine scale and seasonal community composition patterns in submerged macrophytes should be considered. Future investigations on community assembly mechanisms should pay greater attention to long‐term dynamic characteristics and functional information.

Periphyton is an ecological essential in freshwater lakes and rivers. Its abundance and biomass are very dynamic in various habitats and subject to various factors, for example, nutrient and light. Following flooding events, the transitional area adjacent to a river inlet and the shallow lake generates diverse habitats for periphyton with gradients in current velocity, suspended matters, nutrients, and light, which would strongly shape the growth and community of periphyton. In this study, three sampling sites were established around a river inlet in Erhai Lake, China, and a field survey was conducted in the sites from April to August (flooding seasons) in 2019 to investigate the abundance and biomass of periphyton and explore influential factors. The results showed that three study areas have different gradients of current velocity depending on the distance to the river inlet, thereby regulating the concentrations of nutrients and suspended matters, which strongly affected the periphyton community; to be specific, the biomass of periphyton was inhibited by the concentration of suspended matters and high concentrations of silicate mainly reduced the diversity of periphyton. Our results imply that the study on the driving factors of periphyton could help to understand its community assembly mechanism and biomass and species composition of periphyton can provide some reference for trophic state of the lake.

Information on temporal dynamics of phytoplankton communities and their responses to environmental factors can provide insights into mechanisms driving succession of phytoplankton communities that is useful in programs to manage and or remediate undesirable assemblages. Populations of phytoplankton can be controlled by bottom-up factors such as nutrients and temperature or top-down such as predation by zooplankton. Traditionally, taxonomic diversity based on morphologies has been the measure used for analysis of responses to environmental factors. Recently, according to functional groupings, including functional groups (FG), morpho-FG (MFG), and morphology-based FG (MBFG), functional diversity has been used to represent functional aspects of phytoplankton communities. However, to what extent these taxonomic and functional groupings are congruent at seasonal time-scales and the main environmental factors, which drive succession, have remained less studied. Here, we analyzed absolute and relative proportions of a phytoplankton community during a 3-year period in Lake Erhai, a eutrophic highland lake in China. Alpha diversity and beta diversity, as measured by Shannon-Wiener and Bray-Curtis indices of taxonomic grouping and three functional groupings (FG, MFG, and MBFG) were applied to investigate environmental factors determining diversity. Significant, positive relationships were observed between taxonomic diversity and functional diversity that were strongly linked through seasons. In order to exclude the influence of dominant species' tolerance to extreme environments, the dominant species were excluded one by one, and the results showed that residual communities still exhibited similar patterns of succession. This synchronous temporal pattern was not principally driven by the dominant genera (Microcystis, Psephonema, and Mougeotia). Instead, the entire phytoplankton community assemblages were important in the pattern. Most diversity indices of taxonomic and functional groupings were significantly correlated with solar irradiance, but not nutrient concentrations. Because the lake is eutrophic and there were already sufficient nutrients available, additional nutrients had little effect on seasonal taxonomic and functional diversity of phytoplankton in Lake Erhai.

Phenotypic plasticity is an important adaptation to spatial and temporal environmental variations. For submerged macrophytes, adaptation to water depth and light variation is particularly important. To determine the morphological and physiological adaptive strategies of Vallisneria natans at different water depths and light conditions, we combined field investigation, light control experiment and in situ physiological response experiment. In the field investigation and the light control experiment, both water depth and light intensity had prominent effects on the morphological of V. natans , especially in fresh weight and leaf length. The leaf length elongated more rapidly at intermediate water depth sites with lower light intensity. In the in situ experiment, the survival boundary of V. natans is 5.5 m in Lake Erhai. Below this depth, the chlorophyll-a content increased gradually with increasing water depth. Our results demonstrated that V. natans can adapt to water depth and light availability by changing morphological, physiological and resource allocation. At low light condition, V. natans invested more resource for light acquisition, simultaneously, changing the photosynthetic pigment content to compensate for light attenuation; conversely, more resource was directed towards reproduction. These results will provide new insight for species selection when conducting aquatic plants restoration in freshwater ecosystem.HIGHLIGHTS

Antioxidant systems are vital in life activities of macrophytes. Species with different life forms need to cope with distinct environments by modifying physiological characters, especially antioxidant systems. In order to find differences among life forms and consequence of lake eutrophication, we studied three antioxidant enzymes activity (superoxide dismutase SOD, ascorbate oxidase APX and catalase CAT) and total soluble phenolics (TP) content in leaves of 26 macrophyte species in September 2013 in Lake Erhai, China. We found that antioxidation varied accordingly with life forms. The activities of SOD and APX in emergent macrophytes (EM) and floating-leaved macrophytes (FM) were much lower than those of submerged macrophytes (SM). On the contrary, TP content was much higher in EM and FM species. There was a negative correlation between TP and antioxidant enzyme activities (CAT and APX). The results suggested that EM and FM species rely on phenolics might to adapt to adverse environments (higher herbivores predation pressure and UV radiation intensity), while SM species more rely on antioxidant enzymes possibly due to lower demand for antioxidation and/or lack of light and inorganic C availability for phenolics synthesis. We also found FM species represent highest fitness in term of antioxidant system, which would lead to overgrowth of FM species and littoral zone bogginess during lake eutrophication. Finally, it is necessary to carry out the verification experiment under the control condition in the later stage, especially for the dominant ones in eutrophic lakes, to understand the exact adaptive mechanisms of them.

Eutrophication and hydrodynamics determine the final distribution patterns of aquatic macrophytes; however, there is limited available knowledge regarding their interactive effects. Morphological and biomechanical responses to eutrophication and hydrodynamic stresses were assessed by sampling five abundant and dominant species, Potamogeton maackianus, P. pectinatus, P. lucens, Ceratophyllum demersum and Myriophyllum spicatum, in three macrophyte beds in Lake Erhai, Yunnan Province, China: one exposed to eutrophication and moderate southeast (SE) wind; one with mesotrophication, but sheltered by the lakeshore, with weak wind disturbance; and one with meso-eutrophication and strong SE wind. The results showed significant interactive effects of eutrophication and hydrodynamics on most biomechanical traits and some morphological traits, suggesting that aquatic macrophytes preferentially undergo biomechanical adjustments to resist the coexisting eutrophication and hydrodynamic stresses. In particular, hydrodynamics increased both the tensile force and tensile strain of P. maackianus under meso-eutrophication and dramatically decreased them in eutrophic areas, suggesting that eutrophication triggers mechanical failure in this species. Additionally, P. pectinatus, C. demersum and M. spicatum showed the lowest and highest values for the biomechanical variables (greater values for M. spicatum) in the most eutrophic and hydrodynamic areas, respectively, implying that increases in hydrodynamics primarily induce mechanical damage in eutrophic species. The plants generally exhibited greater tensile strain in both shallow and deep waters and the greatest tensile force at moderate depths. The stem cross-sectional area, plant height, stem length, internode length, and branch traits were all responsible for determining the biomechanical variables. This study reveals that hydrodynamic changes primarily induce mechanical damage in eutrophic species, whereas eutrophication triggers mechanical damage in sensitive species.

植物体内的C、N和总酚是影响其被牧食和凋落物降解的重要因素,从而影响C和N在食物链的传递和生物地化循环.本研究测定了云南洱海3种生活型(挺水、沉水和浮叶)26种水生植物叶片中C、N和总酚含量以及C/N比率,并分析了它们与总酚的关系.结果表明:挺水植物叶片的C、N和总酚平均含量为448.08、39.30和24.70 mg/g,浮叶植物叶片的C、N和总酚平均含量为433.32、35.16和28.05 mg/g,沉水植物叶片的C、N和总酚平均含量为378.36、27.31和10.28 mg/g;总体上看,植物叶片C和N含量:挺水植物＞浮叶植物＞沉水植物;挺水和浮叶植物叶片的总酚含量差异不显著,且均远高于沉水植物叶片的总酚含量;26种植物叶片中C和N与总酚含量均呈显著正相关.在富营养化条件下,3种生活型植物所处生境的光照和CO2供给均差异显著,会对这些植物叶片的C、N和总酚含量有一定影响,此外由于应对生境中的胁迫(草食性昆虫、病原体及紫外线辐射等)压力以及自身的生长策略的不同也可能导致C、N和酚在三者中形成差异;通过对13种沉水植物叶片总酚含量比较,推测光叶眼子菜和微齿眼子菜较为适合作为洱海耐牧食恢复先锋物种;N与总酚正相关的关系可能在富营养化进程中加速湖泊沼泽化.

This study evaluated the effects of sediment capping with local unpolluted soil and re-vegetation with Vallisneria natans on the water quality in a two-month in situ experiment in a mesotrophic lake. Water quality was improved substantially by sediment capping whether alone or along with re-vegetating using V. natans, although varied greatly with time. Sediment capping was an effective technique to reduce nitrogen (N) and phosphorus (P) release from fertile sediment, which thus inhibited phytoplankton growth in the water column. Re-vegetation using V. natans helped to improve water quality slightly more. Sediment capping alone or along with re-vegetating also changed interactions of the water quality parameters substantially. Compared to the control, sediment capping alone enforced the relationships between Chl-a and both SD and K, but along with re-vegetation weakened the relationships. In addition, sediment capping alone enforced the relationship between Chl-a and N-NH4, but with or without along with the re-vegetation the relationships between Chl-a and both TN and N-NO3 decreased.

Artificial enhancement of lakes using filter-feeding silver carp is a technique that is widely used to manipulate and/or promote fisheries' production in Chinese lakes. To evaluate the impact of this enhancement on nutrient cycling in the water column, we analyzed two silver carp biomass gradient testing sites by stocking experimental enclosures within Lake Taihu (eutrophic) with 0, 35, 70 and 150gm−3 of silver carp biomass and those within Lake Erhai (mesotrophic) with 0, 20, 50 and 100gm−3 of silver carp biomass. Total phosphorus and dissolved nitrogen were obviously influenced by the fish density. At relatively low densities, the silver carp were able to graze for particles of food directly, resulting in a decline in phosphorus and chlorophyll-a levels within the Lake Taihu enclosures. When fish were introduced to the Lake Erhai enclosures, chlorophyll-a levels increased. Increased dissolved nitrogen levels were recorded in the Lake Taihu enclosures during the experiment, while the enhanced uptake of nutrients by increased algae resulted in decreased dissolved nitrogen levels in the Lake Erhai enclosures. We hypothesize that although silver carp could potentially reduce cyanobacteria in water bodies with large algal blooms, the relatively small change in fish in Lake Taihu is still too minor to control severe algal blooms. In relatively clean water bodies such as Lake Erhai, artificial ecological enhancement using silver carp should be evaluated carefully before action is taken in order to assess the risk of increasing small algal blooms and nutrients.

对湖北省京山县绿林镇菊科(Compositae)植物区系资源进行了调查,结果表明该区共有菊科植物55属80种,具我国种子植物属15个地理分布区类型中的12个.从起源来看,绿林镇地区菊科植物以温带起源为主,温带属占63.6％(35属),热带属占21.8％(12属),世界分布属占12.7％(7属),有1个中国特有属,说明该地区菊科植物区系有以温带成分为主多类成分共存的特点.该地区菊科资源植物十分丰富,按其性质和用途可分为野生蔬菜类、蜜源植物类、观赏植物类、药用植物类、香料植物类、杀虫植物类等,具有较大开发利用价值.