Submerged Macrophyte Communities Research Articles

The response mechanism of Hydrilla verticillata and leaf epiphytic biofilms to depth and nutrient removal.

The mechanism of morphological and physiological regulation of submerged aquatic plants (Hydrilla verticillata) is influenced by spatial and environmental changes related to water depth gradients. In the present study, changes in the aquatic microcosm were explored at the depth gradients of 0.3 m, 0.6 m, 0.9 m, 1.2 m, and 1.5 m, and the depth was recognized as a critical factor for improving water quality, especially for the removal of total phosphorus (TP) and recalcitrant protein-like molecules. At 0.9 m, the removal rates of TP and protein-like substances reached 78% and 18.67%, respectively, 1.76 times and 1.28 times the rates at 0.3 m. The maximum shoot/root growth and chlorophyll (a + b) suggest photosynthesis inhibition is minimal at 1.2 m. Fluctuations in enzyme activities imply an antioxidant response to lipid peroxidation damage under different oxidative stress. The adjusted activities of glutamine synthetase (GS) and alkaline phosphatase (APA) were an adaptive nutrient utilization strategy to different water depths. Microbiological diversity analysis of biofilms indicates that community structure changes in response to water depth. Considering the growth status and nutrient removal effects, the results indicate that the optimal planting depth for H. verticillata is 0.9-1.2 m. These findings contribute to understanding water purification mechanisms in depth gradients, and support the effective rebuilding and management of submerged macrophyte communities in natural shallow lakes.

Changes of submerged macrophytes community in Lake Fuxian since 1960s

自1960s以来，抚仙湖沿岸带沉水植物群落发展迅速，而监测频率相对不足.为了解抚仙湖沉水植物群落现状及过去60年内的变化趋势，于2016年7月，对抚仙湖全湖沉水植物进行调查，并结合以往多次调查数据进行趋势分析.本次调查设置了36条样带共41个样点.在实测数据验证后，使用卫星多光谱相机数据基于归一化植被指数（NDVI）计算全湖沉水植物分布面积.此外，计算了物种在沿岸带植被区的平均生物量、优势度和群落多样性指数.结果表明：抚仙湖沉水植物2016年夏季分布面积为5.14 km²，平均生物量（鲜重）密度为9.8 kg/m²，最高48.7 kg/m²，全湖总现存量（鲜重）5.02×10⁴ t；共采集到沉水植物13种（类），隶属于5科6属.其中，生物量最高的物种是金鱼藻（Ceratophyllum demersum），其次是黑藻（Hydrilla verticillata）和穗花狐尾藻（Myriophyllum spicatum）；出现频度最高的物种是穗花狐尾藻，其次是苦草（Vallisneria natans）和篦齿眼子菜（Potamogeton pectinatus）；物种优势度最大的物种是穗花狐尾藻，其次是金鱼藻和黑藻；抚仙湖各样点沉水植物香农-威纳多样性指数介于0.05~1.28之间，全湖平均值为0.75；除轮藻类外，沉水植物群丛的冠层在1.5~4 m之间，其中金鱼藻群丛冠层最高.丝状附着藻大量出现，附着在高大的沉水植物冠层上的生物量远远多于附着在基质上的；丝状附着藻主要附着在群落上层沉水植物100 cm以内的植冠上.在过去的60年来，抚仙湖沉水植物分布面积、全湖总生物量和物种丰富度呈增加的趋势；低矮的草甸型物种如轮藻类、苦草等优势度下降，高大的冠层型沉水植物如穗花狐尾藻、金鱼藻等成为优势种；外来物种伊乐藻在最近几年出现并成为次优势种；丝状附着藻生物量增加.以上结果表明，目前抚仙湖沉水植物群落处于生物量、分布面积和多样性最高的阶段，是维持和保护的关键时期.但相比于国外类似湖泊，抚仙湖沉水植物丰富度一直较低，目前冠层型植物占优势、外来物种快速发展和丝状附着藻增殖的态势，将会引起群落结构不稳定，如果不加以保护和管理，可能会朝着富营养化湖泊的群落结构方向发展，进而对沿岸带水质产生不利影响.除了进一步控制抚仙湖入湖营养负荷外，我们建议对群落上层高大的冠层型沉水植物进行收割，收割深度为100 cm，从而控制冠层型沉水植物以及附着在其上的丝状附着藻，为草甸型沉水植物的发展创造条件，引导抚仙湖沉水植物群落向贫营养化湖泊的群落结构方向发展，但其可行性尚需开展研究.;The submerged macrophyte communities have developed rapidly in the littoral zone of Lake Fuxian since 1960s, however, the monitoring frequency is relatively insufficient. To understand the distribution status and community changes of submerged macrophytes in Lake Fuxian during the past 60 years, we investigated and sampled the whole-lake submerged macrophytes and epiphytic filamentous algae from 41 sample sites in July 2016. The distribution of submerged macrophytes in the whole lake was determined by CBERS 04-based normalized difference vegetation index (NDVI) of August 18, 2016. In addition, the average biomass density, dominance index and the Shannon-Wiener diversity index in the coastal vegetation area were calculated. The results showed that the distribution area of submerged macrophytes in the summer of 2016 was 5.14 km². The average biomass (fresh weight) density was 9.8 kg/m², and the highest value was 48.7 kg/m². The total biomass (fresh weight) of the submerged macrophytes in the whole littoral zone was 5.02×10⁴ t. Totally 13 species of submerged macrophytes were collected, belonging to 5 families and 6 genera. Among the species, Ceratophyllum demersum had the highest biomass density, followed by Hydrilla verticillata and Myriophyllum spicatum. The species with the highest frequency of occurrence were M. spicatum, followed by Vallisneria natans and Potamogeton pectinatus. In addition, the species with the greatest dominance were M. spicatum, followed by C. demersum and H. verticillata. The Shannon-Wiener diversity index of submerged macrophytes in Lake Fuxian was between 0.05 and 1.28, having an average value of 0.75. Except for charophytes, the canopy of higher plant groups ranged from 1.5 m to 4 m, with the highest canopy of C. demersum. The biomass of epiphytic filamentous algae attached to the canopy of submerged macrophytes was much higher than that attached to the substrates, and filamentous algae mainly attached to the canopy of submerged plants within 100 cm along apical height. The historical data showed that the distribution area, total biomass and species richness of submerged macrophytes in Lake Fuxian increased sharply since 1957. The dominance of low meadow-forming species, such as charophytes and V. natans declined in recent years. In contrast, the tall canopy-forming submerged macrophytes such as M. spicatum and C. demersum dominated the community. The exotic species Elodea nuttallii emerged in recent years and became subdominant species. The epiphytic filamentous algae boosted, and most of them attached on the canopy of submerged macrophytes. At present, the submerged plant community in Lake Fuxian is at the highest stage of biomass, distribution area and diversity, which is the key period for protecting and maintaining the submerged macrophyte bed in the littoral zone. While, the dominance of tall canopy-forming species, the fast development of the exotic species and the bloom of the epiphytic filamentous algae may lead the submerged macrophytes community toward a structure composition which is common in eutrophic lakes. Therefore, it is urgent to control the nutrient loadings in Lake Fuxian and to harvest the upper crown within 100 cm of the tall species to control both the canopy-forming submerged macrophytes and the filamentous algae attached to them. These measures will create conditions for the development of the meadow-forming species and maintain the macrophytes community composition in oligotrophic lakes.

Preliminary study on recovery and optimization of submerged macrophyte community in Lake Erhai, China

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.;With the increasing exogenous nutrients, the continued declining water quality, the year-to-year increasing algae biomass, and the sharply fluctuating water levels, the succession rate of the submerged macrophyte community in Lake Erhai increases over the past several decades. The survey in July 2016 showed that the submerged macrophyte community in Lake Erhai was seriously degraded, the number of species was significantly lower than recent (2011) historical level, and the distribution area fell by more than 70% from the historical high, and the dominant species were transformed from Ottelia acuminata, Stuckenia pectinata and Najas marina (1960s) to Ceratophyllum demersum, Potamogeton maackianus and Vallisneria natans. Submerged macrophytes mainly suffered from the stress caused by Trapa maximowiezii in shallow waters (0-2.5 m), and faced the simplification of the community structure caused by the overgrowth of C. demersum and P. maackianus in the area with medium water depth (2.5-4 m), and subjected to area shrinkage in deep waters (>4 m), which severely inhibited its positive ecological functions. By comparing the structures of submerged macrophyte community before and after the implementation of the submerged vegetation restoration and management project in the waters of Erbin Village, a typical bay in Lake Erhai, we found that artificial restoration and optimization measures positively improved the submerged plant communities in various areas with different water depths in Erbin Village. The abundance of restored species and the diversity indicators of the community was significantly improved. Therefore, we put forward suggestions and countermeasures for the restoration and optimal management of submerged vegetation in the typical eutrophic lake bays of Lake Erhai, with a view to provide a basis for the follow-up projects of the ecosystem restoration in Lake Erhai.

Spatial and seasonal variation of water parameters, sediment properties, and submerged macrophytes after ecological restoration in a long-term (6 year) study in Hangzhou west lake in China: Submerged macrophyte distribution influenced by environmental variables

Submerged macrophyte restoration is the key stage in the reestablishment of an aquatic ecosystem. Previous studies have paid considerable attention to the effect of multiple environmental factors on submerged macrophytes. Meanwhile, few studies have been conducted regarding the spatial and seasonal characteristics of water and sediment properties and their long-term relationship with submerged macrophytes after the implementation of the submerged macrophytes restoration project. On a monthly basis, we monitored the spatial and seasonal variation in water parameters, sediment properties, and the submerged macrophyte characteristics of West Lake in Hangzhou from August 2013 to July 2019. From these measurements, we characterized the relationship between environmental factors and submerged macrophytes. Water nutrient concentrations continuously decreased with time, and the accumulation of sediment nutrients was accelerated as the submerged macrophyte communities developed on a long-term scale. The results indicated that the difference in water parameters was due to seasonal changes and land-use types in the watershed. The differences in the sediment properties were mainly attributed to seasonal changes and changes in the flow field. Redundancy analysis showed that the influence of water nutrients on the submerged macrophyte distribution was greater than that of sediment nutrients. The result also suggested that the developed root system, high stoichiometric homeostasis coefficients of P, and compensation ability of substantial leaf tissue may lead to a large distribution of Vallisneria natans in West Lake in Hangzhou. The correlation of water parameters and sediment properties with submerged macrophytes for a long time was very important as the restoration was achieved. To ensure the stability of the aquatic ecosystem after performing the submerged macrophyte restoration, a greater emphasis must be placed on reestablishing the entire ecosystem, including the restoration of aquatic animals and fish stocks. We expect these findings to serve as a reference for researchers and government agencies in the field of aquatic ecosystem restoration.

Species-specific responses of submerged macrophytes to the presence of a small omnivorous bitterling Acheilognathus macropterus

Recovery of submerged macrophytes has been considered a key factor in the restoration of shallow eutrophic lakes. However, in some subtropical restored lakes, small omnivorous fish dominate the fish assemblages and feed in part on submerged macrophytes. Knowledge of the effects of small omnivores on the growth of submerged macrophytes is scarce and their responses are potentially species-specific, i.e. the growth of some species may be hampered by fish grazing while growth of others may be promoted by the nutrients becoming available by fish excretion. We conducted mesocosm experiments to examine the effects of the small omnivorous bitterling Acheilognathus macropterus, a common species in restored subtropical lakes in China, on nutrient concentrations and the growth of four species of submerged macrophytes (Hydrilla verticillata, Vallisneria denseserrulata, Ceratophyllum demersum and Myriophyllum spicatum). We found that the bitterling significantly increased nutrient concentrations via excretion and thereby enhanced the net growth of the less grazed nuisance macrophyte M. spicatum. In contrast, the net growth of C. demersum was reduced by the bitterling, most likely due to grazing as indicated by gut content analyses. Dominance by bitterling may, therefore, pose a threat to the long-term success of lake restoration by provoking a shift in the submerged macrophyte community towards nuisance species through selective grazing. Nutrient excretion may potentially also stimulate the growth of phytoplankton and periphyton, hampering the growth of submerged macrophyte.

Environmental vs. spatial drivers of submerged macrophyte community assembly in different seasons and water depths in a mesotrophic bay of Erhai Lake, China

The relative importance of environmental and spatial drivers in shaping local plant diversity patterns remains unclear. Here, we investigated the environmental and spatial drivers regulating species composition of the submerged macrophyte community in Erhai Lake, China, over different seasons and water-depth gradients. Results showed that environmental and spatial drivers significantly affected species composition of the submerged macrophyte community at the local and fine scale, with shared environmental and spatial drivers explaining 28.3%–38.4% of the variation in species composition for all four seasons. The unique spatial drivers demonstrated a relatively stable contribution (11.8%–16.0%) over the four seasons, but the contributions from the unique environmental drivers showed seasonal variability, being high in summer and winter. In shallow water (0–3 m), spatial drivers were more important in the process of community assembly; in deep water (3–6 m), however, environmental and spatial drivers both significantly affected community assembly. The different water areas also showed strong seasonal changes in the explanatory power of environmental and spatial drivers of submerged macrophyte community variation. Thus, future studies should consider sampling macrophytes over different seasons and at different water depths to better trace the indicators regulating submerged macrophyte community assembly in lakes.

Reproductive phenology of submerged macrophytes: A tracker of year‐to‐year environmental variations

AbstractAimsPhenological shifts are useful trackers of climate change effects on species and communities, but few datasets focus on the climate‐driven phenological responses of aquatic ecosystem vegetation. We aimed to analyse the reproductive seasonality of a submerged macrophyte community, the synchrony among coexisting species, and their responses to several environmental factors.LocationA Mediterranean coastal pond in Valencia, Spain.MethodsWe analysed three years of data on a submerged macrophyte community. Plants were collected biweekly or monthly (57 collection dates) to quantify the intensity of flowering/fruiting per species. The reproductive seasonality of the community was tested with circular statistics. The reproductive synchrony of species was calculated with the Augspurger and the Freitas and Bolmgren indexes. The environmental factors driving reproduction in macrophytes were analysed with redundancy analyses.ResultsThe reproductive activity of the community was seasonal (peaking in spring–summer) or non‐seasonal (peaking in spring–summer and autumn) depending on the year‐to‐year variations of precipitations after the summer drought. The synchronization among species in flowering and fruiting (overlap in time and intensity) was low, because the species‐specific reproductive patterns were spread throughout the long Mediterranean growing season. Temperature, water level, and conductivity were the variables that best explained flowering variability in the community, indicating that these factors affected each species differently. Fruiting was temporally correlated to flowering, with different lags depending on the species life cycle.ConclusionsThe species‐specific responses suggest that a warming climate may act as an environmental filter, gradually changing the community structure. The reproductive phenology of submerged macrophytes could act as an early indicator of a changing climate, with charophytes (Characeae) as suitable tracker candidates. Further studies concerning their phenology are needed to assess the resilience of submerged macrophyte communities to drought and assure the long‐term functioning of Mediterranean aquatic ecosystems.

Estimated light compensation depth explains growth of Stuckenia pectinata in Te Waihora

Loss of a long-term stable submerged macrophyte community reverses shallow lakes from a clear into a turbid water state. Stuckenia pectinata densely colonised the marginal area of a shallow coastal lagoon Te Waihora (Lake Ellesmere) before its removal by the 1968 “Wahine” storm. This study aims to understand the current light availability for S. pectinata growth in Te Waihora with high turbidity and water level fluctuations. In a greenhouse experiment, acclimations in S. pectinata to lower light intensities involved elongation in stems and leaves, and an increase in photosynthetic efficiency per unit dry weight for both leaves and stems. However, these acclimations did not reduce the daily compensation irradiance (under which daily net photosynthesis equals zero) that was identified at 8.8 ± 0.9 μmol photons m−2 s-1. Subsequently, we estimated light compensation depth (LCD), based on the derived daily compensation irradiance, incident irradiance at the water surface, and light attenuation coefficients for the lake. In areas shallower than LCD, there is no light limitation for the growth of S. pectinata. The LCD ranged from 0.23 to 0.54 m and well explained the growth range of S. pectinata in Te Waihora during the 2016–2017 ecological survey. The estimated LCD also suggests low turbidity levels in early spring are critical for the growth of S. pectinata with the current water level fluctuation regimes. LCD is a useful parameter for understanding the growth of submerged macrophytes in shallow turbid lakes from the perspective of light availability.

Hydrologic and anthropogenic influences on aquatic macrophyte development in a large, shallow lake in China

AbstractMacrophyte composition and abundance affect the overall ecological structure and function of lakes, and information on the nature and timing of changes in macrophyte assemblages is therefore crucial for understanding their impact on the lake ecosystem.We examined plant macrofossils, diatoms, cladocerans, and physical and geochemical proxies in two sediment cores from Lake Liangzi, a large shallow lake located on the Yangtze floodplain in south‐east China, in order to assess recent and historical trajectories of ecological and environmental change. We also used archival records of environmental changes (i.e. hydrological, climatic, fishery, and social factors) in the lake and its catchment. Lake Liangzi is of particular interest as it, unlike most other lakes in the region, has maintained a macrophyte‐dominated state.The results revealed a shift in the lake during the pastca. 160 years from a clear water, low‐growing submerged macrophyte community (e.g.Najas minor, charophyte species), dominated by planktonic diatoms and planktonic cladocerans, towards a more pollutant‐resistant, tall‐growing macrophyte community (Potamogetonspp.,Ceratophyllum demersumandMyriophyllum spicatum) and dominance by benthic and epiphytic diatoms and littoral cladocerans. However, a pronounced increase in the abundance of planktonicBosminaspp. at the expense of true littoral cladoceran species and a decline in plant macrofossil assemblages in the surface samples suggest that the lake recently entered into a new unprecedented state.Redundancy analysis indicated major changes in hydrology and moderate inputs of anthropogenic pollutants as well as climate warming as the main drivers of ecological and environmental changes in the lake.Our study demonstrated that plant macrofossils from radionuclide‐dated short sediment cores provide reliable information on the nature and timing of changes in the macrophyte community in Lake Liangzi. Furthermore, our study provided information on the composition of the plant community before recent strong perturbations and also showed the trajectories and suggested the drivers of ecological and environmental change, thereby offering valuable information for lake managers in the Yangtze region and elsewhere.Our study identifies a relatively resistant community of submerged macrophytes, which may be an initial restoration target for managers when restoring the many lakes in the region where submerged vegetation coverage has declined during the last few decades.

Effects of lake restoration on breeding abundance of globally declining common pochard (Aythya ferina L.)

Common pochard Aythya ferina are rapidly declining globally, partly due to water quality change at breeding habitats. Lake restoration at two southern Danish lakes (external nutrient loading reduction, bream Abramis brama and roach Rutilus rutilus removal and stocking of pike Esox lucius) improved water clarity and submerged macrophyte cover. Nesting pochard on one lake increased from 2.3 females per annum pre-treatment to 13.9 afterwards and from 22.7 females to 99.6 post-treatment at a second. Numbers fell from 27.7 to 11.3 at a third untreated lake with consistently high water clarity, but which was colonised and became dominated by holly-leaved naiad Najas marina, which provides no food resources for breeding pochard. Linear modelling (controlling for serial autocorrelation) showed statistically significant effects of annual summer measurements of suspended solids (negative) and Secchi depth (positive) on pochard abundance at both restored lakes and chlorophyll (negative) at one of those lakes, but no effects at the third. Breeding pochard numbers also correlated positively with submerged macrophyte cover at one restored lake with adequate data. Results support the hypothesis that lake restoration to improve water quality enhances conditions for locally breeding pochard, as long as restored conditions secure appropriate submerged macrophyte communities for nesting pochard.

Turbidity, Waterfowl Herbivory, and Propagule Banks Shape Submerged Aquatic Vegetation in Ponds.

The aquatic vegetation in nutrient-rich shallow lakes and ponds is structured by the interplay of multiple biotic and abiotic drivers. We tested the contribution of the macrophyte propagule bank and the delayed as well as direct impact of waterbirds on submerged aquatic vegetation in a peri-urban pond system. To clarify the functional hierarchy of predictor variables, effects of herbivorous waterfowl and propagule bank potential were ranked relative to environmental, phytoplankton, and zooplankton indicators. Two aspects of the aquatic vegetation – community composition and total pond-scale cover – were discriminated. Within vegetation communities, phytoplankton biovolume and waterfowl herbivory during summer were linked to low macrophyte abundance, whereas propagule density of angiosperms was positively associated with specific assemblages of submerged macrophytes. High algal biovolume and summer waterfowl grazing seemed to affect maximal pond-scale cover of submerged aquatic vegetation. The presence of waterfowl in cold and spring periods was unrelated to vegetation structure in the consecutive main growth season. In addition, availability of propagules in the sediment did not automatically prompt pond-wide vegetation cover (especially when overruled by high waterfowl densities), nor did it guarantee a position in the submerged macrophyte community. Nonetheless, propagule bank potential was related to the waterbody’s general ecological status, since turbid ponds exhibited impoverished propagule reserves compared to ponds residing in a clear, macrophyte-dominated state. Inadequate recruitment therefore represents a plausible bottleneck for macrophyte establishment. We conclude that phytoplankton-caused turbidity and high waterfowl biomass densities greatly restrict submerged macrophyte abundance. Propagule banks also participate in structuring submerged aquatic vegetation, though a stronger role is reserved for herbivorous waterfowl.

Effect of Nutrient Loadings on the Regulation of Water Nitrogen and Phosphorus by Vallisneria natans and Its Photosynthetic Fluorescence Characteristics

Submerged macrophytes are an important component of aquatic ecosystems. During the growing period, submerged macrophytes can absorb nitrogen and phosphorus nutrients to reduce pollution loadings. Shoots of submerged macrophytes can also promote the adhesion of suspended substances in water, reducing the turbidity. The release of nutrients in sediments can be suppressed by its root system, and the resuspension of sediments caused by disturbance of winds and waves can also be resisted. The role of submerged macrophytes in ecological restoration of eutrophic lakes has attracted widespread attention. In 1960, the submerged plants Vallisneria natans and Potamogeton malaianus had been the dominant species in East Taihu. However after 2002, Nymphoides peltatum, Elodea nattalii, P. malaianus, etc. have gradually taken over the dominant roles along with significant elevations of nitrogen and phosphorus levels. Nutrients in water are not the only key factors causing eutrophication of water bodies; the nutrient source for submerged plant growth affect both the purification efficiency and the photosynthetic characteristics of submerged macrophytes. Excessive nitrogen and phosphorus concentrations can inhibit the photosynthetic physiological activities of submerged macrophytes, affecting the succession of aquatic vegetation. In addition, under high nutrient conditions, the competition from periphytic algae and planktonic algae may also directly poison submerged macrophytes, leading to its degradation and disappearance. Systematic studies on the regulation and photosynthetic fluorescence response mechanism of submerged macrophytes to varied nutrient loadings are helpful in revealing their relationships. The seedlings of submerged macrophyte V. natans were transplanted in a laboratory mesocosm to study the effect of nutrient loadings on its regulation of water nitrogen and phosphorus. Three nitrogen and phosphorus loadings from low, medium, and high levels derived from nitrate, ammonium, and phosphate were setup as the aquatic medium for the plant growth. Twelve harvests were carried out to determine the evolution of nutrient removal performance of V. natans. Its photosynthetic fluorescence characteristics were measured by a pulse-amplitude modulated fluorometer (Diving-PAM). Results showed that the nitrogen and phosphorus adsorption abilities of V. natans were gradually enhanced with the increase of nutrient concentrations in the range of TN ≤ 12 mg·L-1 and TP ≤ 1.0 mg·L-1. In the treatment of high nutrient concentrations (TN=12 mg·L-1 and TP=1.0 mg·L-1), the removal rates of nitrogen and phosphorus reached more than 95%. V. natans preferentially absorbed ammonium nitrogen when its concentration was high. The medium nutrient concentrations (TN:8-12 mg·L-1 and TP:0.6-1.0 mg·L-1) did not significantly affect the Fv/Fm ratio of leaves. However, the low nutrient concentrations (TN=3 mg·L-1 and TP=0.3 mg·L-1) could improve the Fv/Fm ratio of leaves and were beneficial for the growth of V. natans. The inhibition of photosynthetic activity and light tolerance were enhanced with the increase in nutrient concentrations. The photosynthetic activity of V. natans gradually recovered with no significant changes in the capacity for light harvesting, when the nutrient concentrations gradually decreased in the water. Our results indicate that the high nitrogen and phosphorus loadings indeed hamper the photosynthetic capacity, which may subsequently restrain the maintenance of the dominance of V. natans in the submerged macrophyte communities.

Response of Submerged Macrophyte Communities to External and Internal Restoration Measures in North Temperate Shallow Lakes.

Submerged macrophytes play a key role in north temperate shallow lakes by stabilizing clear-water conditions. Eutrophication has resulted in macrophyte loss and shifts to turbid conditions in many lakes. Considerable efforts have been devoted to shallow lake restoration in many countries, but long-term success depends on a stable recovery of submerged macrophytes. However, recovery patterns vary widely and remain to be fully understood. We hypothesize that reduced external nutrient loading leads to an intermediate recovery state with clear spring and turbid summer conditions similar to the pattern described for eutrophication. In contrast, lake internal restoration measures can result in transient clear-water conditions both in spring and summer and reversals to turbid conditions. Furthermore, we hypothesize that these contrasting restoration measures result in different macrophyte species composition, with added implications for seasonal dynamics due to differences in plant traits. To test these hypotheses, we analyzed data on water quality and submerged macrophytes from 49 north temperate shallow lakes that were in a turbid state and subjected to restoration measures. To study the dynamics of macrophytes during nutrient load reduction, we adapted the ecosystem model PCLake. Our survey and model simulations revealed the existence of an intermediate recovery state upon reduced external nutrient loading, characterized by spring clear-water phases and turbid summers, whereas internal lake restoration measures often resulted in clear-water conditions in spring and summer with returns to turbid conditions after some years. External and internal lake restoration measures resulted in different macrophyte communities. The intermediate recovery state following reduced nutrient loading is characterized by a few macrophyte species (mainly pondweeds) that can resist wave action allowing survival in shallow areas, germinate early in spring, have energy-rich vegetative propagules facilitating rapid initial growth and that can complete their life cycle by early summer. Later in the growing season these plants are, according to our simulations, outcompeted by periphyton, leading to late-summer phytoplankton blooms. Internal lake restoration measures often coincide with a rapid but transient colonization by hornworts, waterweeds or charophytes. Stable clear-water conditions and a diverse macrophyte flora only occurred decades after external nutrient load reduction or when measures were combined.

Sedimentary macrofossil records reveal ecological change in English lakes: implications for conservation

Aquatic macrophytes play a key role in providing habitat, refuge and food for a range of biota in shallow lakes. However, many shallow lakes have experienced declines in macrophyte vegetation in recent decades, principally due to eutrophication. As changes in macrophyte composition and abundance can affect overall ecological structure and function of a lake, an assessment of the timing and nature of such changes is crucial to our understanding of the wider lake ecosystem. In the typical absence of historical plant records, the macro-remains of macrophytes preserved in lake sediments can be used to assess long-term changes in aquatic vegetation. We generated recent (150–200 years) plant macrofossil records for six English lakes subject to conservation protection to define past macrophyte communities, assess trajectories of ecological change and consider the implications of our findings for conservation targets and strategies. The data for all six lakes reveal a diverse submerged macrophyte community, with charophytes as a key component, in the early part of the sedimentary records. The stratigraphies indicate considerable change to the aquatic vegetation over the last two centuries with a general shift towards species more typically associated with eutrophic conditions. A common feature is the decline in abundance of low-growing charophytes and an increase in tall canopy-forming angiosperms such as fine-leaved Potamogeton species, Zannichellia palustris and Callitriche species. We hypothesise, based on findings from long-term datasets and palaeoecological records from enriched shallow lakes where plants are now absent, that the observed shifts provide a warning to managers that the lakes are on a pathway to complete macrophyte loss such that nutrient load reduction is urgently needed. It is the sound understanding of present-day plant ecology that affords such reliable interpretation of the fossil data which, in turn, provide valuable context for current conservation decisions.

Decade‐long time delays in nutrient and plant species dynamics during eutrophication and re‐oligotrophication of Lake Fure 1900–2015

Summary A central topic in nature conservation and ecological restoration is the potential of ecosystems to recover after the reduction of negative anthropogenic impacts. Often, protracted delays in biotic response to abiotic change have been observed. We analysed a unique long‐term data series in Lake Fure, Denmark, spanning the transformation from pristine environmental conditions in the early 1900s through a period (1920–1970) of eutrophication – from accelerating sewage input of phosphorus (P) – and subsequent re‐oligotrophication after sewage cleaning (1970–2015). We examine time delays between P inputs, in‐lake P concentrations and the richness and composition of submerged macrophyte communities. Lake P concentration exhibited decade‐long delays in response to periods of increasing or decreasing P inputs. It took 40 years and a 25‐fold increase in P input before P concentrations suddenly took off in the 1960s, reflecting profound sediment accumulation. Following reduced P input from c. 1970, it took 5 years before P concentrations showed the first signs of a decline. In 2014, water P concentration was still markedly higher than in 1931, despite much lower P inputs, because of elevated sediment release. Fifty years of eutrophication led to a reduction in aquatic macrophyte richness from 36 species to 12. Species’ responses were closely related to their growth strategy and depth distribution. Deep‐growing mosses, charophytes and short angiosperms disappeared, while tall angiosperms survived and pollution‐tolerant macroalgae colonized and spread. Subsequently, 45 years of oligotrophication led to clearer waters, macrophyte richness recovering to 28 species and some charophytes and short angiosperms reappearing. Dominance of pollution‐tolerant macroalgae persisted, however. Change in species dominance takes longer than colonization by new species. Synthesis. Time delays of P concentrations, water clarity and macrophyte richness and composition were long and complex. Neglecting growth strategies of species makes application of extinction debt and colonization credit concepts dubious, because numbers of oligotrophic species decrease and eutrophic species increase concomitantly during eutrophication and vice versa during oligotrophication. Although the original high species richness may be attained, it is unlikely that the original species composition is restored because many oligotrophic species have vanished from the regional species pool.

Impacts of sediment type on the performance and composition of submerged macrophyte communities

To restore deteriorated lake ecosystems, it is important to identify environmental factors that influence submerged macrophyte communities. While sediment is a critical environmental factor for submerged macrophytes and many studies have examined effects of sediment type on the growth of individual submerged macrophytes, very few have tested how sediment type affects the growth and species composition of submerged macrophyte communities. We constructed submerged macrophyte communities containing four co-occurring submerged macrophytes (Hydrilla verticillata, Myriophyllum spicatum, Ceratophyllum demersum and Chara fragilis) and subjected them to three sediment treatments, i.e., clay, a mixture of clay and quartz sand at a volume ratio of 1:1 and a mixture at a volume ratio of 1:4. Compared to the clay, the 1:1 mixture treatment greatly increased overall biomass, number of shoot nodes and shoot length of the community, but decreased its diversity. This was because it substantially promoted the growth of H. verticillata within the community, making it the most abundant species in the mixture sediment, but decreased that of M. spicatum and C. demersum. The sediment type had no significant effects on the growth of C. fragilis. As a primary nutrient source for plant growth, sediment type can have differential effects on various submerged macrophyte species and 1:1 mixture treatment could enhance the performance of the communities, increasing the overall biomass, number of shoot nodes and shoot length by 39.03%, 150.13% and 9.94%, respectively, compared to the clay treatment. Thus, measures should be taken to mediate the sediment condition to restore submerged macrophyte communities with different dominant species.

Effects of water level fluctuation on the growth of submerged macrophyte communities

Aquatic plant communities are frequently subjected to water level fluctuation. While many experimental studies have examined effects of water level fluctuation on the growth of individual aquatic plant species, very few have tested those on aquatic plant communities. We constructed aquatic communities consisting of four submerged macrophytes (Potamogeton perfoliatus, Myriophyllum spicatum, Chara fragilis and Ceratophyllum demersum), and subjected them to three static water depths (30, 90 and 150cm) and three water level fluctuations with low, medium and high frequency (two, four and eight cycles of water depth change between 30cm to 150cm). Water depth in each of the three fluctuation treatments was on average 90cm, which was comparable to the static water depth of 90cm. Increasing water depth significantly decreased the growth of the communities in terms of biomass, number of shoot nodes and shoot length because it decreased the growth of most species (P. perfoliatus, M. spicatum and C. fragilis). Compared to the static water depth of 90cm, water level fluctuation increased number of shoot nodes and shoot length of the communities, whereas fluctuation frequency had little impact. We conclude that water level fluctuation may potentially increase the vegetative spread of submerged macrophyte communities and managing water level fluctuation may be helpful for the restoration of submerged macrophyte communities in degraded wetlands.

Do Amplitudes of Water Level Fluctuations Affect the Growth and Community Structure of Submerged Macrophytes?

Submerged macrophytes are subjected to potential mechanical stresses associated with fluctuating water levels in natural conditions. However, few experimental studies have been conducted to further understand the effects of water level fluctuating amplitude on submerged macrophyte species and their assemblages or communities. We designed a controlled experiment to investigate the responses of three submerged macrophyte species (Hydrilla verticillata, Ceratophyllum demersum and Elodea nuttallii) and their combinations in communities to three amplitudes (static, ± 30 cm, ± 60 cm) of water level fluctuations. Results showed that water level fluctuating amplitude had little effects on the community performance and the three tested species responded differently. H. verticillata exhibited more growth in static water and it was negatively affected by either of the water level fluctuations amplitude, however, growth parameters of H. verticillata in two fluctuating water level treatments (i.e., ± 30 cm, ± 60 cm) were not significantly different. On the other hand, the growth of C. demersum was not significantly correlated with different amplitude treatments. However, it became more abundant when water levels fluctuated. E. nuttallii was inhibited by the two fluctuating water level treatments, and was less in growth parameters compared to the other species especially in water level fluctuating conditions. The inherent differences in the adaptive capabilities of the tested species indicate that C. demersum or other species with similar responses may be dominant species to restore submerged macrophyte communities with great fluctuating water levels. Otherwise, H. verticillata, E. nuttallii or other species with similar responses could be considered for constructing the community in static water conditions.

Dominance ofMyriophyllum spicatumin submerged macrophyte communities associated with grass carp

Re-establishment of macrophyte communities is a key to restore eutrophic shallow lakes. The species composition of the plant communities may change rapidly during the recovery period. A basin in subtropical Lake Qinhu (China) was restored by biomanipulation including fish removal followed by planting of submerged macrophytes in 2011. In September-December 2011, dominance of Vallisneria spinulosa and Ceratophyllum demersum shifted to dominance of Myriophyllum spicatum. Meanwhile, the CPUE (catch per unit effort) showed that the number and biomass of grass carp (Ctenopharyngodon idella ) demonstrated a marked increase. Stable isotope analyses revealed that grass carp preferred V. spinulosa and C. demersum to M. spicatum . We propose that grazing by grass carp was responsible for the observed shift in dominance towards M. spicatum but as alternative explanations are possible, further tests by controlled experiments are needed to draw firm conclusions.

Vegetative Propagule Pressure and Water Depth Affect Biomass and Evenness of Submerged Macrophyte Communities.

Vegetative propagule pressure may affect the establishment and structure of aquatic plant communities that are commonly dominated by plants capable of clonal growth. We experimentally constructed aquatic communities consisting of four submerged macrophytes (Hydrilla verticillata, Ceratophyllum demersum, Elodea nuttallii and Myriophyllum spicatum) with three levels of vegetative propagule pressure (4, 8 and 16 shoot fragments for communities in each pot) and two levels of water depth (30 cm and 70 cm). Increasing vegetative propagule pressure and decreasing water level significantly increased the growth of the submerged macrophyte communities, suggesting that propagule pressure and water depth should be considered when utilizing vegetative propagules to re-establish submerged macrophyte communities in degraded aquatic ecosystems. However, increasing vegetative propagule pressure and decreasing water level significantly decreased evenness of the submerged macrophyte communities because they markedly increased the dominance of H. verticillata and E. nuttallii, but had little impact on that of C. demersum and M. spicatum. Thus, effects of vegetative propagule pressure and water depth are species-specific and increasing vegetative propagule pressure under lower water level can facilitate the establishment success of submerged macrophyte communities.