Turbid Water State Research Articles

Submerged macrophyte self-recovery potential behind restoration treatments: sources of failure.

When exploring the challenges of restoring degraded lakes, we often do not observe the expected results despite executing all planned activities. Our study elucidates the reasons that impede the recovery of submerged macrophytes despite ameliorated light conditions. When prolonged lake degradation occurs, subsequent efforts to increase light availability often prove insufficient, resulting in a persistent turbid water state. In this study, we attempted to determine the reasons for these failures through a germination test and propagule bank analysis conducted in bottom sediments from a severely degraded lake, which underwent restoration. Although the bottom sediments indicate relative potential in the number of oospores and seeds, their germination efficacy remained dismally low. Based on the germination test results and factors affecting the development of submerged macrophytes (physical and chemical parameters, lake morphology), we stated that improvement of light conditions in the lake could be insufficient to recover the vegetation, especially when the potential to renew diverse plant communities from sediments naturally is low. Our findings advocate for a paradigmatic shift in lake restoration strategies. A holistic approach that includes propagule bank assessments before embarking on restoration initiatives and enabling the identification of macrophyte resurgence potentials is recommended. We also advocate for a multifaceted restoration framework, emphasizing the indispensability of augmenting natural recovery mechanisms with targeted interventions. Consequently, in some cases, macrophyte reintroduction could be the only solution. By reintroducing autochthonic species to site-specific ecological dynamics, we anticipate an increased success rate in restituting submerged vegetation, thus catalyzing ecological regeneration within degraded lake ecosystems.

Buffering capacity of submerged macrophytes against nutrient pulses increase with its coverage in shallow lakes

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.

Simulating shifting ecological states in a restored, shallow lake with multiple single-model ensembles: Lake Arreskov, Denmark

We investigated the ability of the process-based, 1-dimensional hydrodynamic-ecosystem lake model GOTM-WET to reproduce the fluctuating dynamics and shifts between turbid and clear water states following restoration in temperate, shallow Lake Arreskov, Denmark. The lake model was calibrated on a comprehensive 12-year dataset with a multiple single-model ensemble approach to address model parameter and performance metric uncertainty. Compared to earlier modelling attempts on this lake, the GOTM-WET model, which enables simulation of water column hydrodynamics such as stratification events and two zooplankton groups, improved the simulation of the maximum chlorophyll a concentrations during cyanobacteria blooms and zooplankton dynamics. However, the timing of shifts between phytoplankton and submerged macrophytes dominance following fish removal was not well reproduced, although both states were simulated. We discuss potential improvements of the model to enhance the ability to simulate the effects of restoration on the food web and ecological states in this and similar lakes.

Interactions between suspended sediments and submerged macrophytes-epiphytic biofilms under water flow in shallow lakes

Suspended sediments (SS) pollution is one of the factors affecting the transfer from turbid water state to clear water state in shallow lakes. However, the interactions between suspended sediments and submerged plants are far from clear. In this study, we investigated the settlement laws of SS in overlying water and its impact on the epiphytic biofilm of Myriophyllum verticillatum and Vallisneria natans under water flow. At least 90% of turbidity can be removed from overlying water, and the decreasing trend of water turbidity fitted the logarithmic decay model in all treatments. The size distribution of SS fit the log-normal distribution model in the first 240 min after SS addition. It should be noted that the main peak particle sizes were lower in treatments with submerged macrophytes (8.71-13.18 μm) than without plants (15.14-19.95 μm). Water flow and SS addition significantly increased the thickness of biofilms attached to M. verticillatum (p < 0.05), but they together significantly reduced the biofilm thickness on V. natans (p < 0.05). SS increased the bacterial α-diversity but decreased eukaryotic one in epiphytic biofilms. However, water flow had a more significant impact on microbial communities (especially eukaryotes) than SS and plant species. The relative abundances of dominant phylum Proteobacteria, class Alphaproteobacteria and Betaproteobacteria, and class Verrucomicrobiae increased in epiphytic biofilms after SS addition. Co-occurrence networks reveal that photosynthetic microbes in epiphytic biofilms played an important role in microbial communities under water flow and SS, and many hub microbes were increased by SS addition but reduced by water flow. These data highlight that SS decline can be predicted by the logarithmic decay model and, SS and water flow can affect the epiphytic-biofilm on submerged macrophytes.

Alternance of alternative stable states influence the functional diversity of microcrustaceans in a shallow subtropical lake

AbstractShallow lakes in subtropical climate regions may have a clear water state with a predominance of submerged or floating macrophytes and a turbid water state with a predominance of phytoplankton. The relationship between microcrustaceans and these changes in alternative states is still incipient in the literature. And to better understand these relationships, the functional approach is an excellent tool. Thus, the present work seeks to test the hypothesis that changes in alternative stable states over time reflect the variability of composition and functional diversity of microcrustaceans. For this, samples of microcrustaceans collected in a subtropical shallow lake, which underwent alternate state changes in the period to 2000–2014, were used. Our results demonstrate a variation in composition and functional diversity among alternative states, thus supporting our hypothesis. In periods of clear water with a predominance of floating macrophytes, we found a greater functional diversity and richness of genera. This result may be associated with several factors such as greater habitat heterogeneity and lower predation pressure. The lowest values of functional diversity found in periods of turbid waters with predominance of phytoplankton and periods of clear waters with submerged macrophytes are related to the dominance of functional traits, such as scraper feeding type, herbivorous/omnivorous trophic group, littoral habitat and asexual reproduction. These traits are functional features of most Ostracoda genera, a taxonomic group abundant in these two alternative states. The predominance of this group may be related to the environmental characteristics of these periods, such as the high vegetation cover and the availability of niches unoccupied by more sensitive species, since most Ostracoda have high environmental tolerance. We conclude that the functional traits approach combined with taxonomic groups respond to temporal variations of alternative stable states and are important tools to detect environmental changes.

Eutrophication evolution trajectory influenced by human activities and climate in the shallow Lake Gehu, China

Lake eutrophication has become a global environmental pollution problem. Analysis of the eutrophication recovery trajectory of lakes can help to accurately implement regulations and reduce the cost of management. The degradation and recovery trajectories of Lake Gehu were firstly analyzed based on 34-years monitoring data of water quality. A dynamic correlation analysis of the relationship between chlorophyll-a (Chla) and the total phosphorus (TP) (total nitrogen [TN]) at an annual scale was conducted using the filtering trajectory method (FTM). The result showed that the recovery trajectory of Lake Gehu was nonlinear, and showed a short lag time in Chla decrease. The regime shift from a clear-water state to a turbid-water state occurred in Lake Gehu when the TP threshold was 120 μg/L. Combined the filtering trajectory diagram with a principal component analysis, the results revealed that the adverse climate change did not completely offset the positive effect of nutrient load reduction, which required further attention and had significance for eutrophication control. According to the dynamic changes in the algae production efficiency of total phosphorus (ETP), the result showed that the sensitivity of lake eutrophication increased and the TP carrying concentration reduced by 40%-60%. The water quality goal for controlling algae was determined as TP = 40 μg/L (Chla = 10 μg/L), which was 20% stricter than the national standard (TP = 50 μg/L). In conclusion, aquatic vegetation restoration and biological manipulation can be effective alternatives for controlling eutrophication in addition to nutrient load reduction. The eutrophication control target and adaptive strategies were determined according to the recovery trajectory of Lake Gehu in this study. The ETP has reference significance for the management of shallow lakes.

Summer temperatures and environmental dynamics during the Middle Würmian (MIS 3) in the Eastern Alps: Multi-proxy records from the Unterangerberg palaeolake, Austria

Several millennial-scale warm phases perturbed the glacial climate during Marine Isotope Stage 3 (MIS 3, ca. 57–29 ka BP). Little is known about the impact of these climatic changes on Alpine ecosystems due to the sparsity of undisturbed sediment records in the Alps and their foreland. In this study, multiple sediment-archived proxies (sediment geochemistry, stable oxygen and carbon isotopes of autochthonous carbonate, and subfossil remains of macrophytes and aquatic invertebrates) were examined in five drill cores from an ancient inner-Alpine lake at Unterangerberg (Eastern Alps) to reconstruct the palaeolake environment and to estimate summer temperature changes for the first half of MIS 3. The lacustrine sedimentation in the basin began ca. 54.6 ka, tentatively correlated with the start of Greenland Interstadial (GI) 14. We identified three distinct phases in the development of the lake. (1) A cold, oligotrophic water body influenced by snow/glacier meltwater ca. 54.6–52.2 cal ka BP. (2) A clear-water, macrophyte-dominated, productive lake ca. 52.2–44.9 cal ka BP. Submerged macrophytes were dominated by the charophyte alga Chara hispida and chironomid assemblages – by Corynocera ambigua, which is absent from the present-day fauna of the Alpine region. (3) A turbid-water, less productive lake ca. 44.9–41.5 cal ka BP. This shift towards a turbid-water state, as evidenced by the drastic reduction in the abundance of submerged macrophytes and associated invertebrates, likely occurred due to increased input of meltwater. The regime shift is tentatively correlated with the start of GI 11, for which the highest temperatures of the studied MIS 3 interval are inferred. Chironomid-based reconstructions of mean July air temperatures provide interstadial temperature estimates between ca. 11 and 12.5°C (i.e. ca. 5–6°C below present-day values), which concurs with reconstructions available from the northern Alpine foreland. Cooler July temperatures (ca. 9–10°C) are reconstructed for MIS 3 stadials. The Unterangerberg lacustrine records provide valuable new insights into MIS 3 climate dynamics inside the Eastern Alps and contribute to a better understanding of the effects of climate change on the Alpine environment.

Theoretical basis and Chinese practice for environmental protection of lakes with better water quality

目前，我国湖泊富营养化及蓝藻水华问题十分突出，国家高度重视湖泊的生态环境保护.自“九五”以来，国家就投入太湖、巢湖、滇池“老三湖”等重污染湖泊的治理，但成本巨大，且历经近30年才初见成效.按照湖泊污染程度，湖泊治理与保护可分为“污染治理型”“防治结合型”“生态保育型”3大类.“老三湖”的治理是典型的“先污染、后治理”的模式，水质较好湖泊主要属于生态保育型湖泊，因此，“老三湖”治理模式不适用于水质较好湖泊的保护.本文系统总结了我国水质较好湖泊优先保护理念的形成和水质较好湖泊专项实施的历程.根据水质较好湖泊的特点，及其生态系统退化与修复的一般过程，提出了水质较好湖泊保护的基本思路.从热力学角度，阐明了氮磷营养盐输入湖泊生态系统中是熵增过程，也是湖泊生态系统退化的根本原因，湖泊氮磷污染负荷源头控制是关键.湖泊流域生态安全格局是确保湖泊生态系统健康的基础，从景观生态学角度，阐明了优化湖泊流域水土资源利用、优化发展模式是减轻湖泊环境压力的重要途径.在浅水湖泊生态系统，以沉水植物占优势的“清水态”和以浮游植物占优势的“浊水态”转换过程不是沿着同一条途径，存在上临界阈值和下临界阈值，水生态修复过程表现出一种迟滞的现象.从湖泊水生态系统稳态转换理论角度，阐明了湖泊生态修复工程应在湖泊生态系统发生退化转变之前实施，才能获得较高的环境效益.通过国家财政专项对81个水质较好湖泊的支持，既能促进湖泊流域经济社会发展，又能确保湖泊水环境质量变好，湖泊水生态系统逐步改善.建议加强不同类型湖泊保护模式的总结，深入对水质较好湖泊生态系统演替理论和保护技术研究，支撑国家系统开展水质较好湖泊保护.;At present, the problems of lake eutrophication and cyanobacterial blooms in China are still very prominent and the state attaches great importance to the ecological environment protection of lakes. Since the Ninth Five-Year Plan, the government has started to control the old three lakes of Lake Taihu, Lake Chaohu and Lake Dianchi, but it has spent huge treatment costs. After nearly 30 years of governance, the trend of further deterioration of water quality in the old three lakes was initially curbed. According to the degree of lake pollution, lake control and protection can be divided into three types:pollution control type, combined prevention and control type and ecological conservation type. The governance of the old three lakes was a typical pollution first, treatment later model, and the lakes with better water quality mainly belong to ecological conservation type, so the treatment model for old three lakes is not suitable for the protection of lakes with better water quality. This paper systematically summarizes the formation of the concept of priority protection of lakes with better water quality in China and the implementation history of special lakes with better water quality. According to the characteristics of the lake with better water quality and the general process of ecosystem degradation and restoration, the basic idea for the protection of the lakes with better water quality was put forward. From a thermodynamic perspective, the entropy increase process of nitrogen and phosphorus nutrient input into the lake ecosystem was analyzed, revealing the root cause of the degradation of the lake ecosystem and the importance of source control of nitrogen and phosphorus pollution load into lakes. The ecological security pattern of lake basin is the basis for ensuring the health of lake basin ecosystem. From the perspective of landscape ecology, this paper clarifies that optimizing the use of soil and water resources in lake basin and optimizing the development pattern are important ways to reduce the environmental pressure on the lake. The transition process of the clear water state dominated by submerged plants and the turbid water state dominated by phytoplankton in shallow lake ecosystems is not along the same pathway. There are upper and lower critical thresholds, and the aquatic ecological restoration process shows a hysteresis phenomenon. From the theory of steady-state transformation of lake water ecosystem, we clarified that the lake ecological restoration project should be implemented before the degradation transformation of lake ecosystem to obtain higher protection benefits. The support of 81 lakes with better water quality through the special national finance can not only promote the economic and social development of the lake basin, but also ensure the improvement of the quality of the lake water environment and improvement of lake water ecosystem. We recommended to strengthen the summary of different types of lake protection models, conduct in-depth research on the theory of ecosystem succession and protection technology of lakes with better water quality, and support the state to carry out the protection of lakes with better water quality systematically.

Effects of common carp on water quality and submerged vegetation: results from a short-term mesocosm experiment in an artificial wetland

Bioturbation by non-native common carp (carp) can facilitate an ecosystem phase shift from clear to turbid water in shallow lakes and ponds, with negative effects on abundance of aquatic animals and plants. Management of carp often involves reducing populations below a threshold biomass at which important components of the aquatic ecosystem, like submerged aquatic vegetation (SAV), recover. To evaluate control targets for a nuisance carp population in Malheur Lake in SE Oregon, USA, we conducted a mesocosm experiment in a small wetland within the lake’s drainage basin. We stocked 200-m2 enclosures with individual or multiple carp of different age classes at target biomasses of 50, 100, and 300 kg ha−1. We measured turbidity, total suspended sediments, nutrients, chlorophyll-a, and surface coverage of SAV four times within a single growing season. Turbidity increased through time and soluble P was higher in 300 kg ha−1 carp treatments compared to the control, but few changes were observed for any of the other variables at any biomass. Results suggest that with well established SAV, a maintenance carp biomass threshold of &lt;200 kg ha−1 will not result in a phase shift from a clear- to turbid-water state in the short-term.

Adaptive Evolution Can Both Prevent Ecosystem Collapse and Delay Ecosystem Recovery.

AbstractThere is growing concern about the dire socioecological consequences of abrupt transitions between alternative ecosystem states in response to environmental changes. At the same time, environmental change can trigger evolutionary responses that could stabilize or destabilize ecosystem dynamics. However, we know little about how coupled ecological and evolutionary processes affect the risk of transition between alternative ecosystem states. Using shallow lakes as a model ecosystem, we investigate how trait evolution of a key species affects ecosystem resilience under environmental stress. We find that adaptive evolution of macrophytes can increase ecosystem resilience by shifting the critical threshold, which marks the transition from a clear-water state to a turbid-water state to a higher level of environmental stress. However, following the transition, adaptation to the turbid-water state can delay the ecosystem recovery back to the clear-water state. This implies that restoration could be more effective when implemented early enough after a transition occurs and before organisms adapt to the alternative state. Our findings provide new insights into how to prevent and mitigate the occurrence of regime shifts in ecosystems and highlight the need to understand ecosystem responses to environmental change in the context of coupled ecological and evolutionary processes.

Algal migration and nutrient enrichment contribute to patterns in phytoplankton versus epiphyton communities

Algal dominance between phytoplankton and epiphyton plays an essential role in predicting shallow lake shifts between clear-water and turbid-water states. However, compared to resources competition, studies on algal life-form shifts between phytoplankton and epiphyton have traditionally received less interest, as few studies have focused on algal communities in both habitats concurrently. We conducted a 4 × 3 factorial design microcosm experiment to explore the mutual feedback relationship between phytoplankton and epiphyton. The initial algal life-form (epiphytic algae and phytoplanktonic algae alone or together) and nutrients enrichment (ambient, enrichment with N and P alone or together) were manipulated. After 28 days of incubation, the results suggested that the nutrient effects on the phytoplankton and epiphyton communities differed among the three different initial algal life-forms. A significant competitive advantage of phytoplankton was found even in treatments containing only epiphytic algae as the initial algal community. The contribution of nutrient enrichment to phytoplankton abundance (13%) was similar to that of epiphyton abundance (11%). In the mutual influence between two algal communities, epiphyton was likely to be a beneficiary as the phytoplankton community contributed 15% of the variance in epiphyton abundance. In addition, significant algal life-form shifts between phytoplankton and epiphyton only occurred in treatments containing one algal life-form, but not in treatments containing both algal life-forms at the beginning of the experiment. Our results emphasized the competitive advantage of phytoplankton in utilizing nutrient resources in the water column of shallow lakes. Moreover, we demonstrated that algal life-form shift was an adaptive behavior closely correlated with environmental variation. These results will provide broader insights to explore algal succession between phytoplankton and epiphyton in shallow lakes. To better understand the mutual influence mechanism between two algal life-forms under different nutrient conditions, research on multiple short time-scales based on algal migration is needed in the future.

Climate change as the dominant driver of recent ecological changes in a semi-arid alpine lake from the Chinese Loess Plateau

Semi-arid areas of northern China are under increasing pressures from anthropogenic activities and climate change. Although wetland areas in these drylands have experienced dramatic, unidirectional shifts in their ecological status in recent centuries, fundamental driving forces are poorly quantified. Here, we examine changes in sedimentary proxies (diatoms, spectrally-inferred chlorophyll-a, stable isotopes) preserved in a radiometrically-dated core from Tianchi Lake, an alpine lake within the margin of the East Asian Summer Monsoon (EASM) limit in China’s southwestern Loess Plateau. Our algal trends were compared with regional instrumental records, changes in EASM intensity, and with previously published paleolimnological data from this same lake to determine the principal drivers of regional ecological changes. We found no clear evidence that geochemical and biological proxies were strongly affected by deforestation and other human activities. Major environmental changes during the past ~ 200 years were found to be predominantly driven by climatic fluctuations, extreme precipitation events, and changes in EASM intensity. Prior to ~ 1965 CE, diatom assemblages indicate an oligotrophic, clear water state. Shifts in dominance between benthic Staurosirella pinnata and planktonic Lindavia comensis were likely controlled by ice-cover dynamics. Between ~ 1965 and 1980 CE an abrupt shift to a turbid water state during a period of extreme precipitation events was caused by excessive nutrient-laden soil erosion in the already susceptible deforested catchment. This turbid period was evidenced by a rapid increase to dominance of Achnanthidium minutissimum, a sharp decline in oligotrophic Lindavia comensis, increased primary production, and peaks in sediment grain size and SiO2 content. Post- ~ 1980 CE, we provide evidence that a shift towards planktonic diatom dominance can best be explained by changes in climate and EASM intensity, despite substantial nitrogen deposition in the region during the past few decades. Specifically, a drier and warmer climate together with weakened EASM wind strength resulted in decreased erosion and a return to a clear water state, coupled with enhanced thermal stability. Collectively, these observations expand our understanding of how changes in climate, extreme precipitation events, and fluctuations in EASM intensity influence semi-arid alpine lakes in northern China, as well as climate’s leading role in driving ecological change over the past two centuries, despite the intensification of human disturbances during recent decades.

Short-term decisions in lake restoration have long-term consequences for water quality

Ecological regime shifts from clear to turbid water states in shallow temperate lakes are quite well-investigated phenomena but critical time lags from human interaction with the lake and restoration activities are much less understood. This is a complex challenge for institutions who manage lakes but are usually less familiar with non-linear dynamics, slow and fast influences on water quality and how to manage those from a social-ecological perspective. We extend a well-known minimal model of shallow lake regime shifts to enable simulations over time with short- and long-term management measures (nutrient reduction, trawling, planting of aquatic vegetation). While we explore the mathematical conditions for ecological bistability, we also identify the necessary and sufficient extent of measures to restore the clear water state. Restoration scenarios evaluated by trajectories in the state space demonstrate the increased effectiveness from combined measures even when considering countereffective activities such as pike fishing. But, single measures alone may delay or even miss the overall restoration target. Our analysis demonstrates the importance of understanding transient dynamics where stable state analyses alone remain elusive about alternative ways to interact with bistability. We conclude that successful management of bistable systems, and particularly shallow temperate lakes, needs careful balancing between short-term improvements and long-term influence on the systems state.

How many submerged macrophyte species are needed to improve water clarity and quality in Yangtze floodplain lakes?

Successful re-establishment and maintenance of submerged macrophytes is a pivotal problem for the restoration of shallow eutrophic lakes, since submerged macrophytes can facilitate a shift of lake ecosystems from a turbid water state to a clear water state through their structuring roles. However, little is known about the optimal number of macrophyte species (species richness) needed for the recovery of shallow eutrophic lakes. Here, we investigated 19 shallow eutrophic lakes along the mid-lower reaches of the Yangtze River to reveal the underlying interrelations among water clarity, water quality and species richness. Our results showed positive correlations of water nutrients (TN and TP) with Kd and Red/Blue light ratio, suggesting that bad water quality can significantly lower water clarity. The results of path analysis indicated that the underwater light climate directly affects morphological and physiological traits and species richness of submerged macrophytes. Changes in functional traits also affected significantly the species richness of submerged macrophytes. Moreover, the presence of submerged macrophytes not only had positive effects on the underwater light climate (water clarity), but also on water quality. Most importantly, the assemblage of three or more submerged macrophyte species can significantly improve water clarity in these sampled eutrophic lakes, but not water quality. Additionally, the basic species assemblage of Myriophyllum spicatum and Vallisneria natans can be used for restoration of submerged macrophytes in shallow eutrophic Yangtze floodplain lakes. It is concluded that species richness and species assemblage are critical for recovery of submerged macrophytes in shallow eutrophic lakes.

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.

Experimental test of abiotic and biotic factors driving restoration success of Vallisneria americana in the Lower Bay of Green Bay

Abstract The eutrophication of aquatic habitats is a primary driver of ecosystem degradation, often culminating in a switch from a macrophyte-dominated clear water state to a phytoplankton-dominated turbid water state. While numerous studies have documented the ecological implications of this switch, subsequent reductions in nutrient and sediment loading do not consistently result in predictable reversals to macrophyte dominance. Re-introduction of rooted aquatic macrophytes at appropriate scales and species combinations may disrupt these negative feedbacks, although our current limited understanding of these complex mechanisms hinders the development of effective, targeted restoration strategies. We evaluated the potential for restoration of Vallisneria americana (wild celery) in the Lower Bay of Green Bay by altering restoration size and co-planting with the emergent species Schoenoplectus acutus (hardstem bulrush). Wild celery survival among all sites exceeded 90% in 2015 and 110% in 2016. However, in contrast to our expectations the effect of restoration size and interspecific facilitation on survival was either marginal or non-significant. Instead, various environmental effects focused largely on the interaction of water depth, substrate characteristics, and the abundance of unrestored floating macrophytes (i.e. Lemna, Ceratophyllum, and Utricularia spp.) drove variability in restoration success, suggesting that future work focus on identifying restoration methods that can withstand a highly dynamic environment. Our results provide insight into the factors continuing to limit the re-establishment of aquatic macrophytes in degraded systems, suggesting a more limited role of water quality and greater role of interspecific competition and propagule limitation or seedling establishment than previously recognized.

Determining suitable submerged macrophyte biomass in terms of dissolved oxygen concentration and biodiversity in the South Basin of Lake Biwa, Japan

After a regime shift in eutrophic lakes from a turbid water state with phytoplankton blooms to a clear water state, troublesome issues caused by excessive growths of aquatic plants occurred. This study investigated the relationships between submerged macrophyte biomass, and associated fauna and flora, and environmental factors at 52 stations in the South Basin of Lake Biwa. Polynomial regression analysis gave 5 significant regression equations (P 6000 g m−2 or PVI was > 60%, DO concentrations became too low for the survival of fish and invertebrates (< 4.3 mg l−1). In addition, the cubic regressions of periphyton and epifauna taxa richness showed inflection points were at 3000 g m−2 of submerged macrophyte biomass. Epifaunal biomass, submerged macrophyte species richness, periphyton and epifaunal taxa richness showed inflection points at 30% PVI. Previous studies suggested that regime shifts occur at particular thresholds, and community structure and biodiversity change significantly. Therefore, we assumed regression inflection points were threshold indicators. We recommend an appropriate submerged macrophyte biomass of 3000–6000 g m−2 or a PVI of 30–60% to maintain the ecosystem in a clear water state with fewer troublesome issues — starting at these levels macrophyte growth should be controlled in Lake Biwa.

Predicting total phosphorus levels as indicators for shallow lake management

Total phosphorus (TP) is commonly used to assess water quality in shallow lakes and other surface waters. Shallow lakes require special consideration because they can transition between two alternative stable states: (1) a clear-water state that typically supports abundant submerged vegetation and provides high quality wildlife habitat, and (2) a turbid-water state with frequent algal blooms and poor habitat quality. A shallow lake’s TP level in relation to critical TP tipping points determines whether the lake is in a highly resilient clear state, a highly resilient turbid state, or whether the lake is in a dynamic region where either state is possible. Further, management options differ for highly resilient clear lakes versus lakes that may be in the turbid state. For example, resilient clear lakes may be assigned special watershed and shoreline protection to preserve their healthy conditions. On the other hand, managers may plan to allocate resources for assessment and possible in-lake management for lakes that have TP levels where the turbid state is possible. Managers would benefit from models that can predict whether a lake is in a resilient clear state or may be in the turbid state without physically visiting a lake to collect water samples or other in-lake data.We used TP data from 118 shallow lakes in Minnesota and previously estimated TP tipping points to classify lakes as either highly resilient clear lakes (“stable-clear”) or possibly turbid (“bistable-turbid”). We used random forest methodology to build models for predicting these two TP classes using both remotely sensed watershed-scale predictors and in-lake variables, and we performed recursive feature elimination to find the most parsimonious model. We demonstrate that TP class can be predicted using only watershed-scale remotely sensed variables and that land cover and use, soil texture, and ecoregion are key predictor variables for TP class. We highlight how our model predictions can be used as indicators to help make management decisions for a set of shallow lakes.

Responses of Wetland Ecosystems to Drought-Induced Fish Eliminations

Landscape alterations can alter fish occurrence and abundance in wetlands. By increasing turbidity in wetland ponds, fishes can induce a shift from a clear- to a turbid-water state, resulting in reduced plant and amphibian abundance. In 2012, a drought reduced water levels and eliminated fishes from many wetlands of the prairie-pothole region (PPR) in the Midwestern United States. Our study objective was to evaluate if fish eliminations were associated with changes in wetland condition. A one-way ANOVA was used to compare the magnitude of change in parameters indicative of wetland condition (turbidity, plant cover and taxon richness, and tiger-salamander (Ambystoma tigrinum) biomass and numerical abundance) from 29 Iowa PPR wetlands between 2010 and 2011 (pre-drought) and 2014–2015 (post-drought) among wetlands where 1) all fishes were eliminated 2) large-bodied fishes were eliminated, 3) small-bodied fishes were eliminated, 4) fishes were retained, and 5) fishes were never detected. Turbidity decreased and plant cover increased more where large-bodied fishes were eliminated compared to the other groups. Changes in tiger-salamander biomass and abundance and plant-taxon richness were not different among groups. Results indicate that wetlands can shift from a turbid- to a clear-water state following drought and elimination of large-bodied fishes.

Phytoplankton response to fish-induced environmental changes in a temperate shallow pond-type lake

AbstractSince 1967, the temperate, shallow, pond-type Lake Warniak has been subjected to different biomanipulation methods including the introduction of common carp,Cyprinus carpioL., grass carp,Ctenopharyngodon idella(Val.), silver carp,Hypophthalmichthys molitrix(Val.), and bighead carp,Hypophthalmichthys nobilis(Richardson) and then their removal in an effort to control macrophytes and phytoplankton. Recently, pilot stocking with predatory fish, particularly pike,Esox luciusL., has also been conducted. Hence, an examination of the long-term response patterns of phytoplankton to multiple fish-induced stressors was undertaken. In recent years,Charadomination (2000-2004) has helped to stabilize a clear-water state, high/good ecological status, and meso-eutrophic conditions. After the disappearance of Charales in 2004, the rapid, unstable changes in phytoplankton biomass, structure, and biodiversity suggested a shift toward a turbid-water state. As a result, the phytoplankton assemblages changed from those dominated by cryptophytes Y+X2+X1+LO(2000-2004) through those dominated by cyanobacteria K (2005-2008), dinoflagellates LO+Y (2009-2011), and cryptophytes Y+LO+F+X2 (2012), to those dominated by diatoms D+K+P+A (2013-2014) with representative taxa that occur in nutrient-rich and/or nutrient-poor water bodies. The 1967-2014 changes indicated that four periods, two with clear-water state and two with turbid-water state, alternately, one after the other, resulted from different fish pressure. Higher autochthonous fish biomass was usually accompanied by lower phytoplankton biomass. In contrast, the introduction of Cyprinidae fish had a stimulating effect on summer phytoplankton dominated by cyanobateria. Among the nutrients, only phosphorus played an important role.