Restoration effects of submerged macrophytes on methane production and oxidation potential of lake sediments

Authors

Jianglong Zhu, Yahua Li, Minghui Huang, Dong Xu, Yi Zhang, Qiaohong Zhou, Zhenbin Wu, Chuan Wang

The restoration of submerged macrophytes is an important step in lake ecosystem restoration, during which artificially assisted measures have been widely used for macrophyte recolonization. Compared with natural restoration, the impact of artificially assisted methods on methane (CH₄) production and oxidation of lake sediments remains unclear. Therefore, after the restoration of submerged macrophytes in some parts of West Lake (Hangzhou, China), sediment samples from West Lake were collected according to restoration methods and plant coverage. The CH₄ production potential, oxidation potential, and microbial community structure in the sediment were discussed through whole-lake sample analysis and resampling verification from typical lake areas. From the analysis of the whole lake, the average daily CH₄ production potential (ADP) of artificially restored lake areas (0.12 μg g⁻¹ d⁻¹) was significantly lower than that of the naturally restored lake areas (0.52 μg g⁻¹ d⁻¹). From the resampling analysis of typical lake areas, the ADP of naturally restored lake areas was 1.8 times that of artificially restored lake areas (P < 0.01). Although there was no significant difference in the CH₄ oxidation potential between the two restoration methods, the presence of submerged macrophytes significantly increased the abundance of the dominant methanotroph Methylocaldum in the sediment, and the rate of increase in the abundance of the dominant methanotroph Methylosinus was significantly higher in artificially assisted restoration than in natural restoration. This study revealed that the artificially assisted restoration of submerged macrophytes reduced the potential for CH₄ production and increased the abundance of dominant methanotrophs in the lake sediment, which would be beneficial for the reduction of CH₄ emissions during lake ecological restoration and environmental management.

Regulation of the Nutrient Cycle Pathway and the Microbial Loop Structure by Different Types of Dissolved Organic Matter Decomposition in Lakes

Authors

Lingling Wan, Lu Cao, Chunlei Song, Xiuyun Cao, and Yiyong Zhou

To explore the effect of different types of dissolved organic matter (DOM) decomposition on nutrient cycling pathways and the microbial loop, four lakes with different DOM sources were investigated monthly. In Lake Tangxun, Dolichospermum decay released highly labile dissolved organic nitrogen into the water column. This induced bacterial organic nitrogen decomposition, as indicated by the increased abundance of gltB, gltD, gdh, and glnA as well as aminopeptidase activity. Genes associated with dissimilatory nitrate reduction to ammonium further fueled ammonium accumulation, driving Microcystis blooms in the summer. In Lake Zhiyin, fish bait deposits (high nitrogen, similar to Dolichospermum detritus) also caused Microcystis blooms. Detritus from Microcystis decomposition then produced high levels of labile dissolved organic phosphorus, inducing phosphatase activity and increasing soluble reactive phosphorus concentrations from September to April in Lakes Tangxun and Zhiyin. The high refractory DOM from macrophytes in Lake Houguan led to insufficient nutrient availability, leading to nutrient mutualism between algae and bacteria. The high levels of labile dissolved organic carbon from terrestrial detritus in Lake Yandong increased bacterial biomass and production, resulting in low chlorophyll content due to the competitive relationship between algal and bacterial nutrient requirements. Therefore, different DOM compositions induce unique connections among available nutrients, algae, and bacteria in the microbial loop.

Ecosystem complexity explains the scale-dependence of ammonia toxicity on macroinvertebrates

Authors

Miao Liu, Yan Li, Hong-Zhu Wang, Hai-Jun Wang, Rui-Ting Qiao, Erik Jeppesen

The toxic effect of unionized ammonia (NH₃) on aquatic organisms is receiving increasing attention due to the excessive nitrogen discharge to various surface waters. Researches have suggested the scale-dependence of NH₃ toxicity, being lower in field than under lab conditions. Such scale-dependence of toxicity is a big challenge to water quality criteria setting as the results solely from lab tests might not apply to natural ecosystems. Therefore, it is necessary to explore the underlying mechanism to understand the difference of toxicity across various spatial scales. In this study, we used the widely distributed gastropod Bellamya aeruginosa as the test animal and performed two 192-h microcosm experiments. Each experiment included a control and an ammonia addition treatment: N0(LC₅₀) & N+(LC₅₀), N0(LC₁₀₀) & N+(LC₁₀₀) (96-h LC₅₀ = 0.8 mg NH₃N/L, 96-h LC₁₀₀ = 18.1 mg NH₃N/L). Besides water-only, three potential key components (food, sediment, and submersed macrophytes) were included in the various treatments to mimic different complexity levels of aquatic ecosystems (Water, Water + Food, Water + Sediment, Water + Sediment + Macrophytes). The results showed that: 1) food directly improved the survival and growth of gastropods under expected lethal concentration of ammonia (96-h concentration of NH₃N = LC₂₀ of the 96-h acute test); 2) sediment and macrophyte quickly decreased the ammonia concentration, mainly by sediment adsorption and macrophyte uptake, to alleviate the ammonia stress to gastropods and permitted them to survive and grow under expected lethal concentration of ammonia (96-h concentration of NH₃N = LC₁₀∼LC₂₀ of the 96-h acute test); 3) sediment and macrophyte also provided additional food for gastropods; 4) under extremely high ammonia stress (i.e., 96-h LC₁₀₀, food was left uneaten and macrophyte died, and gastropods could, therefore, not be released from ammonia stress. Our results demonstrate that under moderate ammonia stress, the introduction of extra ecosystem elements (food, sediment, and macrophytes) significantly improved the survival and growth of gastropods, mainly by enhancing their tolerance and by quickly decreasing the NH₃ concentration and thus toxicity. However, these introduced elements had little effect at very high concentration of ammonia (i.e., 96-h LC₁₀₀). Our findings add to the understanding of the reasons behind the previous observed scale-dependent toxicity of NH₃ on aquatic organisms and contribute to better decisions on the role of NH₃ in relation to water quality management.

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

Authors

Hezhou Chen, Songhe Zhang, Xin Lv, Shaozhuang Guo, Yu Ma, Bing Han, Xiuren Hu

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.

From macrophyte to algae: Differentiated dominant processes for internal phosphorus release induced by suspended particulate matter deposition

Authors

Cheng Liu, Yiheng Du, Jicheng Zhong, Lei Zhang, Wei Huang, Chao Han, Kaining Chen, Xiaozhi Gu

In shallow lakes, eutrophication leads to a shift of the macrophyte-dominated clear state towards an algae-dominated turbid state. Phosphorus (P) is a crucial element during this shift and is usually concentrated in the suspended particulate matter (SPM) in water. However, the dominant processes controlling internal P release in the algae- (ADA) and macrophyte-dominated (MDA) areas under the influence of P-concentrated SPM remains unclear. In this study, we conducted monthly field observations of P exchange across the sediment-water interface (SWI) with the deposition of SPM in the ADA and MDA of Lake Taihu. Results revealed that both algae- and macrophyte-originated SPM led to the depletion of oxygen across the SWI during summer and autumn. Redox-sensitive P (Fe-P) and organic P (Org-P) were the dominant mobile P fractions in both areas. High fluxes of P across the SWI were observed in both areas during the summer and autumn. However, the processes controlling P release were quite different. In MDA, P release was mostly controlled by a traditional Fe-P dissolution process influenced by the coupled cycling of iron, sulfur, and P. In the ADA, Org-P control was intensified with the deterioration of algal bloom status, accompanied with the dissolution of Fe-P. Evidence from the current study revealed that the dominant process controlling the internal P release might gradually shift from Fe-P to a coupled process of Fe-P and Org-P with the shift of the macrophyte- to an algae-dominated state in shallow eutrophic lakes. The differentiated processes in the MDA and ADA should be given more attention during future research and management of internal P loadings in eutrophic lakes.

Phytotoxic effects of microcystins, anatoxin-a and cylindrospermopsin to aquatic plants: A meta-analysis

Authors

Yanyan Zhang, Sung Vo Duy, Gabriel Munoz, Sébastien Sauvé

Global warming and eutrophication may lead to increased incidence of harmful algal blooms and related production of cyanotoxins that can be toxic to aquatic plants. Previous studies have evaluated the phytotoxic effects of cyanotoxins on aquatic plants. However, most studies have evaluated only a limited number of plant species and cyanotoxins; there is also considerable variability between studies, which obscures general patterns and hinders understanding of the phytotoxic effects of cyanotoxins. Here, we conducted a comprehensive meta-analysis by compiling 41 published papers to estimate the phytotoxic effects of anatoxin-a, cylindrospermopsin, and microcystins in 34 species of aquatic plants, with the aim of 1) investigating the phytotoxicity of different cyanotoxins to aquatic plants; 2) determining the aquatic plant species most sensitive to the phytotoxic effects of cyanotoxins; and 3) evaluating the bioaccumulation potential of cyanotoxins in aquatic plants. Most aquatic plants were negatively affected by cyanotoxin exposure and their response was dose-dependent; however, morphological indicators and photosynthesis of certain aquatic plants were marginally stimulated under low concentrations of anatoxin-a and cylindrospermopsin. Anatoxin-a showed the greatest bioaccumulation capacity in aquatic plants compared to cylindrospermopsin and microcystin variants. Bioaccumulation factors of cyanotoxins in aquatic plants generally decreased with increasing water exposure concentrations. Our study supports the One Health goal to manage the risk of public exposure to toxic substances, and indicates that cyanotoxins warrant further investigations in aquatic plants. Environmental managers and public health authorities need to be alert to the long-term exposure and chronic toxicity of cyanotoxins, and the potential trophic transfer of cyanotoxins from aquatic plants to higher-order organisms.

The shift from macrophytic to algal particulate organic matter favours dissimilatory nitrate reduction to ammonium over denitrification in a eutrophic lake

Authors

Xingyu Jiang,Guang Gao,Yang Hu,Keqiang Shao,Xiangming Tang,Hua Cheng,Jing Li

1. In eutrophic lakes, the shift from a macrophyte-dominated state to an algae-dominated state changes the particulate organic matter (POM) sources, which in turn alters organic matter quality that is released and that sinks to the benthos. However, the influences of this shift on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are unclear in eutrophic lakes.
2. Here, we elucidated how various POM sources influenced these nitrate reduction pathways in a eutrophic lake. The lake sediments from August were incubated under algae, macrophyte, and soil treatments in aquarium tanks. Potential denitrification and DNRA rates and related functional gene abundances were measured periodically. Meanwhile, the n-alkanes composition of POM and fluorescence excitation-emission matrices of dissolved organic matter were measured before and during incubations, respectively, to aid in the analysis of the potential mechanisms by which organic matter quality affect nitrate reduction.
3. The results indicated that algal detritus was more labile and decomposed faster than other POM, coupled with higher DNRA rates and nrfA gene abundances. Macrophyte treatments resulted in nitrate accumulation and the increase of gene abundances related to denitrification, but no increase in denitrification rates was observed. In soil treatments, nitrate reduction processes were not significantly influenced by the addition of POM. Overall, compared with other sources, algae-derived POM can produce high-quality organic carbon, low dissolved oxygen and nitrate concentrations, which is more conducive to DNRA than denitrification.
4. A loss of macrophytes and increase in algal biomass will induce changes in autochthonous POM quality and potentially drive nitrogen recycling in eutrophic lakes.

Habitat coupling mediated by the multi-interaction network linked to macrophyte meadows: ponds versus lakes

Authors

Eric Puche, María A. Rodrigo, Matilde Segura & Carmen Rojo

Morphometric differences between ponds and lakes have implications in habitat-dimensioning and coupling. The prevalence of pelagic over benthic habitats in lakes differs from ponds, where macrophytes dominate, offering both within-meadow free water and support for benthic organisms. We assessed four Mediterranean waterbodies (two ponds and two lakes) situated along an environmental–morphometric gradient, combining a model based on taxonomic composition with a multi-interaction network perspective of habitat coupling. The communities’ composition (both regarding taxa and their corresponding nodes) fits to this gradient. The composition of benthic and within-meadow habitats was similar in the ponds, while in the lakes, the highest similarity occurred between planktonic habitats (pelagic and within-meadow), and the benthic habitat had unshared populations. The network approach suggests two contrasting patterns of habitat coupling between ponds and lakes. Three functional modules, coupled by macrophytes, herbivores, and mixotrophs, emerged in the ponds: a microbial loop, an autotrophic food chain, and macrophytes hosting benthic microalgae. In the lakes, two disconnected modules appeared: the pelagic plankton plus the within-meadow connector herbivores, and the benthos plus the within-meadow primary producers. Within-meadow herbivores and small phytoplankton nodes were central in the pond and lake networks. Furthermore, benthic nodes showed high functional redundancy and were highly influential in spreading the disturbances’ effects. These results demonstrate: (1) the diversity of functional structures in ponds emerging from a mixed composition; (2) the importance of within-meadow organisms as connectors, and (3) the relevance of benthos which has the greatest diversity and redundancy, as well as the most influential network’s elements.

Phytoremediation of CYN, MC-LR and ANTX-a from Water by the Submerged Macrophyte Lemna trisulca

Authors

Małgorzata Kucała, Michał Saładyga and Ariel Kaminski

Cyanotoxins are harmful to aquatic and water-related organisms. In this study, Lemna trisulca was tested as a phytoremediation agent for three common cyanotoxins produced by bloom-forming cyanobacteria. Cocultivation of L. trisulca with Dolichospermum flos-aquae in BG11 medium caused a release of the intracellular pool of anatoxin-a into the medium and the adsorption of 92% of the toxin by the plant—after 14 days, the total amount of toxin decreased 3.17 times. Cocultivation with Raphidopsis raciborskii caused a 2.77-time reduction in the concentration of cylindrospermopsin (CYN) in comparison to the control (62% of the total pool of CYN was associated with the plant). The greatest toxin limitation was noted for cocultivation with Microcystis aeruginosa. After two weeks, the microcystin-LR (MC-LR) concentration decreased more than 310 times. The macrophyte also influenced the growth and development of cyanobacteria cells. Overall, 14 days of cocultivation reduced the biomass of D. flos-aquae, M. aeruginosa, and R. raciborskii by 8, 12, and 3 times, and chlorophyll a concentration in comparison to the control decreased by 17.5, 4.3, and 32.6 times, respectively. Additionally, the macrophyte stabilized the electrical conductivity (EC) and pH values of the water and affected the even uptake of cations and anions from the medium. The obtained results indicate the biotechnological potential of L. trisulca for limiting the development of harmful cyanobacterial blooms and their toxicity.

Decades needed for ecosystem components to respond to a sharp and drastic phosphorus load reduction

Authors

Ingrid Chorus, Antje Köhler, Camilla Beulker, Jutta Fastner, Klaus van de Weyer, Tilo Hegewald & Michael Hupfer

Lake Tegel is an extreme case of restoration: inflow treatment reduced its main external phosphorus (TP) load 40-fold, sharply focused in time, and low-P water flushed the lake volume ≈ 4 times per year. We analysed 35 years of data for the time TP concentrations took to decline from ≈ 700 to 20–30 µg/l, biota to respond and cyanobacteria to become negligible. The internal load proved of minor relevance. After 10 years, TP reached 35–40 µg/l, phytoplankton biomass abruptly declined by 50% and cyanobacteria no longer dominated; yet 10 years later at TP < 20–30 µg/l they were below quantifiable levels. 20–25 years after load reduction, the lake was stably mesotrophic, macrophytes had returned down to 6–8 m, and vivianite now forms, binding P insolubly in the sediment. Bottom-up control of phytoplankton through TP proved decisive. Five intermittent years with a higher external P load caused some ‘re-eutrophication’, delaying recovery by 5 years. While some restoration responses required undercutting thresholds, particularly that of phytoplankton biomass to TP, resilience and hysteresis proved irrelevant. Future research needs to focus on the littoral zone, and for predicting time spans for recovery more generally, meta-analyses should address P load reduction in combination with flushing rates.