Temporal characteristics of algae-denitrifying bacteria co-occurrence patterns and denitrifier assembly in epiphytic biofilms on submerged macrophytes in Caohai Lake,SW China

Denitrifying bacteria in epiphytic biofilms play a crucial role in nitrogen cycle in aquatic habitats.However,little is known about the connection between algae and denitrifying bacteria and their assembly processes in epiphytic biofilms.Epiphytic biofilms were collected from submerged macrophytes(Patamogeton lucens and Najas marina L.)in the Caohai Lake,Guizhou,SW China,from July to November 2020 to:(1)investigate the impact of abiotic and biotic variables on denitrifying bacterial communities;(2)investigate the temporal variation of the algae-denitrifying bacteria co-occurrence networks;and(3)determine the contribution of deterministic and stochastic processes to the formation of denitrifying bacterial communities.Abiotic and biotic factors influenced the variation in the denitrifying bacterial community,as shown in the Mantel test.The co-occurrence network analysis unveiled intricate interactions among algae to denitrifying bacteria.Denitrifying bacterial community co-occurrence network complexity(larger average degrees representing stronger network complexity)increased continuously from July to September and decreased in October before increasing in November.The co-occurrence network complexity of the algae and nirS-encoding denitrifying bacteria tended to increase from July to November.The co-occurrence network complexity of the algal and denitrifying bacterial communities was modified by ammonia nitrogen(NH_(4)^(+)-N)and total phosphorus(TP),pH,and water temperature(WT),according to the ordinary least-squares(OLS)model.The modified stochasticity ratio(MST)results reveal that deterministic selection dominated the assembly of denitrifying bacterial communities.The influence of environmental variables to denitrifying bacterial communities,as well as characteristics of algal-bacterial co-occurrence networks and the assembly process of denitrifying bacterial communities,were discovered in epiphytic biofilms in this study.The findings could aid in the appropriate understanding and use of epiphytic biofilms denitrification function,as well as the enhancement of water quality.

Effect of light and nutrients on interspecific interactions between submerged macrophytes:implications for restoration of multispecies aquatic vegetation in eutrophic lakes

Constructing multispecies submerged vegetation systems and maintaining stable seasonal succession is crucial for restoring shallow eutrophic lakes.However,little is known about the interactions between successional and existing species of different growth forms,particularly under the low light and high nutrient conditions of eutrophic lakes.We measured the functional traits of mature Vallisneria natans(Lour.)Hara plants and Potamogeton crispus L.shoots in monoculture and mosaic patterns under different light and nutrient conditions.The effect of light on functional traits of the submerged macrophyte species was more significant than that of nutrients,but the reverse was true for P.crispus biomass allocation.Moreover,interspecific interactions affected only the submerged macrophytes under the low light condition and varied with species.Specifically,the interaction of P.crispus to V.natans was biased towards competition,while the interaction of V.natans to P.crispus was converted from facilitation to competition by eutrophication,particularly in the homogenous mosaic growth pattern.This study demonstrates that sufficient light is a prerequisite and patch planting is an effective means to form a multispecies submerged vegetation system.In addition,we emphasize that the coexistence of eutrophication and low light will likely result in a competition between submerged macrophytes thus simplifying the vegetation,even if their growth forms and growing seasons are different.These findings help explain the collapse of multispecies submerged vegetation and guide the restoration of aquatic plants in eutrophic lakes.

Phytoremediation Potential of Three Species of Macrophytes for Nitrate in Contaminated Water

Three species of aquatic plants (Scirpus validus, Phragmites australis and Acorus calamus) were used as experimental materials to study their capacity to purify contaminated water and their effects on water pH and dissolved oxygen (DO). The water was contaminated with different concentrations of nitrate (5 mg/L, 15 mg/L and 25 mg/L). The results indicated that the concentration of nitrate, species of aquatic plant and their interaction significantly impacted denitrification (P = 0.00). Under the same concentrations, the three species of aquatic plants provided varying degrees of purification. Acorus calamus provided effective purification under all three concentrations of nitrate wastewater, with removal percentages of 87.73%, 83.80% and 86.72% for nitrate concentrations of 5 mg/L, 15 mg/L and 25 mg/L, respectively. In terms of the purification ability by unit fresh weight, Acorus calamus exhibited the worst purification capacity, whereas the capacities of Scirpus validus and Phragmites australis were higher. The purification capacity of Scirpus validus for the three concentrations was as follows: 0.08 mg/(L·g FW), 0.29 mg/(L·g FW), and 0.51 mg/(L·g FW). The capacity of Phragmites australis was 0.07 mg/(L·g FW), 0.25 mg/(L·g FW), and 0.53 mg/(L·g FW). The capacity of Acorus calamus was 0.04 mg/(L·g FW), 0.12 mg/(L·g FW), and 0.21 mg/(L·g FW). Under increased concentrations of nitrate, the three species of aquatic plants exhibited various degrees of increased purification capacity. Under the different concentrations of nitrate, the three species exhibited the same trends with respect to water pH and DO, increasing first and then falling. The pH remained at approximately 7.5, and the DO fell to 4.0 mg/L. A comprehensive analysis reveals that Acorus calamus provides excellent nitrate purification, although by unit fresh weight, both Scirpus validus and Phragmites australis provide superior purification capacity.

Antioxidant systems of aquatic macrophytes in three life forms:a case study in Lake Erhai,China

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

STUDY OF THE ECOLOGICAL RESTORATION OF AQUATIC MACROPHYTES IN A EUTROPHIC SHALLOW LAKE

Investigations in 1991 to 1993 showed that a perennial submerged plant, Potamogeton maackianus A.Benn., which always dominates the submerged vegetation in the shallow lakes in the middle and lower basins of the Changjiang River, had been extinct from Donghu Lake of Wuhan, and that some other submerged plants sensitive to water contamination had also dissappeared or declined in the lake. The r-selected species, Najas marina L., Myriophyllum spicatum L. and Vallisneria sp. had superseded the K-selected one, P. maackianus, to co-dominate the submerged vegetation. Several hypereutrophic or eutrophic subregions had switched from macrophyte dominance to phytoplankton dominance, while the shrinkage of macrophytes and the deterioration of water quality had also become more and more severe in the other subregions. The emergent macrophytes were poorly developed and the share of leaf-floating plants hed increased in the lake. It was found that the existing vegetation fluctuated drastically from year to year. Macrophyte restoration experiments carried out in large enclosures at 3 subregions of different trophic state, suggested that the aquatic vegetation of less polluted sublakes,such as Niuchao, Tanglin and Houhu Lakes, could recover spontaneously after stocking of herbivorous fish stopped, that K-selected plants should be introduced into these sublakes to enhance the stability of aquatic vegetation,that a prerequisite for the recovery of macrophytes in the severely polluted basins is the reduction of external and internal nutrient loadingr coupled with feasible management measures; and that r-selected submerged species should be used as pioneer plants for the macrophyte recovery.The recently introduced exotic submerged plant, Elodea canadensis,transplanted into the Houhu Enclosure could survive but failed to surive summer in the enclosures located in the hypereutrophic Shuiguohu Bay.

Removal of Heavy Metals by Two Free Floating Aquatic Macrophytes

A concentration-dependent decrease in biomass, protein, RNA, DNA, and nutrient (NO3- and PO43-) uptake of Lemna minor and Azolla pinnata by Cr, Ni, and Zn was detected. Cr was found to exert maximum toxicity followed by Ni and Zn. Metal uptake was dependent on time and concentration of metal in the external rnedium. Both the macrophytes, however, showed preference for Zn followed by Ni and Cr. The uptake kinetics also revealed a low Vmax and high Km for Cr. L. minor was more effcient in accumulating Zn and Cr than A. pinnata in Ni. Compared to immobilized algae and bacterial capsules the test macrophytes showed a greater efficiency for metal removal

Estimation of Growth Area of Aquatic Macrophytes Expanding Spontaneously in Lake Shinji Using ASTER Data

In this study, we estimated the growth area of aquatic macrophytes that have expanded spontaneously in Lake Shinji, located in eastern Shimane Prefecture, Japan, using Terra satellite Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data. Visible and near infrared ASTER data from April, August, and September 2012 were used. The water depth at which ASTER can detect submersed aquatic macrophytes using in situ spectral reflectance of aquatic macrophytes and a bio-optical model was also examined. As a result, when the threshold value of a normalized difference vegetation index (NDVI) was set to 0, only aquatic macrophytes up to a depth of approximately 10 cm could be detected. The growth area of aquatic macrophytes detected by NDVI from ASTER data was in relatively good agreement with the growth area as observed by aerial photography.

Phytoremediation Potential of Some Macrophytes from a Car Wash Stream in Buea, South Western Cameroon

Phytoremediation is a cheap and environmentally friendly technique in which green plants in situ are used to clean the soil, sediments and water of heavy metals. This study investigated the phytoremediation potential of six naturally occurring macrophytes from Nange (a stream in Buea municipality where cars have been driven into and washed for over 20 years). Plant samples were collected before and after car wash, then analysed for copper, zinc, lead and cadmium accumulation. There was an increase in concentration of all the four heavy metals in water after carwash point, with Zn having the highest concentration (0.27 mg/L). Mean concentration of the heavy metals in the water showed that Zn and Pb had the highest concentrations (0.24 mg/L each) while the least concentration was obtained in Cu (0.12 mg/kg). Heavy metal concentrations in the sediments were higher after car wash point than before. Cadmium had the highest concentration (5.58 mg/kg) while Cu had the least (0.75 mg/kg). Ludwigia peruviana had the highest BAFs for all the heavy metals (22.95 for Cu, 33.41 for Zn, 21.79 for Pb and 7.85 for Cd). Species with the leasts were: Anubias barteri for Cu (7.16), Polygonum persicaria for Zn (14.28), Rorippa nasturtium-aquaticum for Pb (11.60) and Vallisneria spiralis for Cd (1.98). L. peruviana had the highest BAC values (Cu = 10.11, Zn = 14.73, Pb = 11.39, Cd = 3.85) and BCF values (Cu = 12.84, Zn = 18.67, Pb = 10.40, Cd = 4.00). A. barteri had the highest TF (Cu = 1.49, Zn = 1.27, Cd = 1.99) except for Pb where both A. barteri and L. peruviana each had a TF of 1.10. While all the six plants were found to be good accumulators of the heavy metals, L. peruviana showed remarkable efficiency indicating that the species is a good candidate for cleaning such environments.

Polycyclic Aromatic Hydrocarbon (PAHs) Concentrations in Some Aquatic Macrophytes in Hilla River, Iraq

Phragmites australis, Potamogeton pectinatus, Potamogeton perfoliatus and Ceratophyllum demersum were selected to study concentrations of PAHs in lotic ecosystems. Six sampling sites were selected along Al-Hilla River and sampling was conducted in 2010 and 2011. Sixteen PAHs listed as priority pollutants were detected in the samples collected, including Naphthalene (Nap), Acenaphthylene (Acpy), Acenaphthene (Acp), Fluorene (Flu), Phenanthrene (Phen), Anthracene (Ant), Fluoranthene (Flur), Pyrene (Py), Benzo (a) Anthracene (B(a)A), Chrysene (Chry), Benzo (b) Fluoranthene (B(b)F), Benzo (k) Fluoranthene (B(k)F), Benzo (a) Pyrene (B(a)P), Dibenzo (a, h) Anthracene (D(b)A), Benzo (ghi) Perylene (B(ghi)P) and Indeno (1,2,3-cd) Pyrene (Ind). The results of the study illustrate that the PAH concentration in macrophytes varies among their species. These variances were as follows: P. australis 0.425 to 299.424 μg/g dry weight (Dw) for B(ghi)P and B(b)F, respectively;P. perfoliatus 0.354 to 235.84 μg/g Dw for B(b)F and B(ghi)P, respectively;C. demersum 0.996 to 162.942 μg/g Dw for Ant and B(ghi)P, respectively;and P. pectinatus 0.383 to 99.87 μg/g Dw for Ant and Nap, respectively. The accumulation potential of PAHs was also investigated by calculating the Bioconcentration Factor (BCF) and Bio-sediment Accumulation Factor (BSAF). The ranges of BCF ratios were 0.05 to 5334.5, 0.08 to 1602.5, 0.01 to 536.6, 0.16 to 1882 in P. australis, P. perfoliatus, P. pectinatus and C. demersum, respectively. The range of BSAF ratios were 3.14 to 1041.6 and 1.5 to 2920.8 in P. australis and P. perfoliatus, respectively.

Green Liver Systems for Water Purification:Using the Phytoremediation Potential of Aquatic Macrophytes for the Removal of Different Cyanobacterial Toxins from Water

The protection and reasonable use of freshwater is one of the main goals for our future, as water is most important for all organisms on earth including humans. Due to pollution, not only with xenobiotics, but also with nutrients, the status of our water bodies has changed drastically. Excess nutrient load induces eutrophication processes and, as a result, massive cyanobacterial blooms during the summer times. As cyanobacteria are known to produce several toxic secondary metabolites, the so-called cyanotoxins, exhibiting hepato-, neuro- and cell-toxicity, a potential risk is given, when using this water. There is an urgent need to have a water purification system, which is able to cope with these natural toxins. Using aquatic plants as a Green Liver, the Green Liver System?, was developed, able to remove these natural pollutants. To test the ability of the Green Liver System?, several cyanobacterial toxins including artificial and natural mixtures were tested in a small-scale laboratory system. The results showed that within 7 - 14 days a combination of different aquatic macrophytes was able to remove a given toxin amount (10 μg·L-1) by 100%. The phytoremediation technology behind the Green Liver Systems? uses the simple ability of submerged aquatic plants to uptake, detoxify and store the toxins, without formation and release of further metabolites to the surrounding water.

Establishing Submersed Macrophytes via Sinking and Colonization of Shoot Fragments Clipped off Manually

In this paper, sinking and growth of apexes and mid-stems of Myriophyllum spicatum L., Hydrilla verticillata （L.f.） Royle and Ceratophyllum demersum L. in concrete ponds containing eutrophic water and sediment were investigated. Sinking rates of apexes and mid-stems reached 34.8% and 4.4% at the 6 th day and 91.1% and 66.7% at the 22 nd day for M. spicatum, 57.8% and 55.6% at the 6 th day and 100% and 97.8% at the 22 nd day for H. vertieillata, 18.9% and 86.7% at the 6 th day and 95.6% and 100% at the 22 nd day for C. demersum, respectively. Most sunken fragments established themselves successfully with significant growth. Total shoot length ofplantlets developed from apexes and mid-stems increased by 399% and 61% for M. spicatum, 593% and 256% for H. vertieillata and 114% and 104% for C. demersum, respectively. The results showed that it was feasible to establish submersed macrophytes via sinking and colonization of shoot fragments clipped off manually.

SPECIES COMPOSITION AND SEASONAL DENSITY DYNAMICS OF CILIATED PROTOZOA IN BAOAN LAKE WITH PARTICULAR REFERENCE TO THE SUBMERGED MACROPHYTES

From April 1996 to October 1997, regular samplings were carried out monthly at 8 stations in a macrophytic basin of Baoan Lake. From the water samples, 47 genera, 96 species of ciliata were identified. Stations covered with macrophytes had greater number of ciliate species and higher percentage of sessile species. The mixotroph Strombidium viride bearing algal endosymbionts dominated numerically the whole ciliate communities; most of the other dominants were bactivores. Total ciliate density in Lake Baoan was 6170-34310 ind./L. The seasonal density fluctuations of the dominant species populations were also investigated. Maximum abundances were observed in spring and winter during the decay of macrophytes and minimum densities were observed during the summer months of luxuriant macrophytes growth.

The response of structure and nitrogen removal function of the biofilm on submerged macrophytes to high ammonium in constructed wetlands

The ammonium exceedance discharge from sewage treatment plants has a great risk to the stable operation of subsequent constructed wetlands(CWs).The effects of high ammonium shocks on submerged macrophytes and epiphytic biofilms on the leaves of submerged macrophytes in CWs were rarely mentioned in previous studies.In this paper,the 16S rRNA sequencing method was used to investigate the variation of the microbial communities in biofilms on the leaves of Vallisneria natans plants while the growth characteristics of V.natans plants were measured at different initial ammonium concentrations.The results demonstrated that the total chlorophyll and soluble sugar synthesis of V.natans plants decreased by 51.45%and 57.16%,respectively,and malondialdehyde content increased threefold after8 days if the initial NH_(4)^(+)-N concentration was more than 5 mg/L.Algal density,bacterial quantity,dissolved oxygen,and pH increased with high ammonium shocks.The average removal efficiencies of total nitrogen and NH_(4)^(+)-N reached 73.26%and 83.94%,respectively.The heat map and relative abundance analysis represented that the relative abundances of phyla Proteobacteria,Cyanobacteria,and Bacteroidetes increased.The numbers of autotrophic nitrifiers and heterotrophic nitrification aerobic denitrification(HNAD)bacteria expanded in biofilms.In particular,HNAD bacteria of Flavobacterium,Hydrogenophaga,Acidovorax,Acinetobacter,Pseudomonas,Aeromonas,and Azospira had higher abundances than autotrophic nitrifiers because there were organic matters secreted from declining leaves of V.natans plants.The analysis of the nitrogen metabolic pathway showed aerobic denitrification was the main nitrogen removal pathway.Thus,the nitrification and denitrification bacterial communities increased in epiphytic biofilms on submerged macrophytes in constructed wetlands while submerged macrophytes declined under ammonium shock loading.

Effects of previous drying of sediment on root functional traits and rhizoperformance of emerged macrophytes

Purpose of the current study was to investigate the effects of constantly wet and dried-rewetted sediments on root functional traits of emerged macrophytes and their nutrients removal abilities.It is based on the hypothesis that root characteristics and nutrients removal abilities of plants will be altered in the course of sediment desiccation.Four emerged macrophytes including two fibrous-root plants(Canna indica and Acorus calamus)and two thick-root plants(Alocasia cucullata and Aglaonema commutatum)were investigated for their root functional traits and rhizoperformance in both wet and dried-rewetted sediments.Results showed that sediment desiccation followed by rewetting substantially altered the root functional traits(root surface area,radial oxygen loss,and root activity)of plants due to adverse changes in morphological characteristics(porosity,bulk density,particle density)of dried-rewetted sediments than by wet sediments.Consequently,limited plants growth and removal of nitrogen(N),phosphorus(P)and dissolved organic carbon(DOC)were recorded in driedrewetted sediments and their pore water than in wet sediments.Radial oxygen loss from plant roots correlated positively with root functional traits,plants growth,and removal of N,P and DOC from pore water and sediment in both sediment types.Among the macrophyte species,the fibrous-root plants having advantages root functional traits,greatly influenced the rhizospheric conditions(pH,dissolved oxygen and redox potential),and demonstrated higher N,P and DOC reduction from both sediment types.While,the thick-rooted plants with thick diameter roots(D>1 mm)and higher rhizome exhibited longer life-span in both sediment types.

EFFECTS OF SUBMERGED MACROPHYTES ON SEDIMENT SUSPENSION AND NH_4-N RELEASE UNDER HYDRODYNAMIC CONDITIONS

Aquatic vegetation affects sediment suspension and nutrient release by changing the flow structure. Experiments on the influence of submerged vegetation on flow structure, sediment suspension, and NH4-N release were carried in a flume with natural submerged vegetation. Turbulence characteristics in the vegetation section were measured using a three-dimensional acoustic Doppler velocimeter. The effects of submerged vegetation on bed shear stress （ τb ）, sediment suspension, and NH4-N release were analyzed. Results show that with vegetation, bed shear stress is reduced by about 20% - 80%, which, in turn, reduces sediment suspension. The impact of submerged vegetation on sediment suspension and NH4-N release should be considered along with flow intensity. When the flow Reynolds number is relatively small, the submerged vegetation is quite capable of inhibiting sediment suspension and reducing NH4-N release, but when the Reynolds number reaches a certain value, the presence of aquatic plants exacerbates sediment suspension and promotes NH4-N release. Results also reveal that a highly significant positive correlation exists between NH4-N concentration and water turbidity in both vegetated and non-vegetated channels.

Response of community composition and biomass of submerged macrophytes to variation in underwater light, wind and trophic status in a large eutrophic shallow lake

Light climate is of key importance for the growth, community composition of submerged macrophytes in lakes and, they, in turn, are affected by lake depth and the degree of eutrophication. To test the relationships between submerged macrophyte presence and the ratio of Secchi disk depth(SDD) to water depth, i.e. SDD/depth, nutrients and wind, we conducted an extensive sampling campaign in a macrophyte-dominated area of the eastern region( n = 36) in 2016 in Lake Taihu, China, and combined the data gathered with results from extensive physico-chemical monitoring data from the entire lake. We confirmed that SDD/Depth is the primary factor controlling the community composition of macrophytes and showed that plant abundance increased with increasing SDD/Depth ratio( p < 0.01), but that only SDD/Depth > 0.4 ensured growth of submerged macrophytes. Total phosphorus and total nitrogen also influenced the growth and community composition of macrophytes( p < 0.01), while Chl a was an indirectly affecting factor by reducing underwater light penetration. Wave height significantly influenced plant abundance( p < 0.01), whereas it had little effect on the biomass( p > 0.05). The key to restore the macrophyte beds in the lake is to reduce the nutrient loading. A decrease of the water level may contribute as well in the shallow bays but will not bring plants back in the main part of the lake. As the tolerance of shade and nutrients varied among the species studied, this should be taken into account in the restoration of lakes by addition of plants.

Algal-bloom control by allelopathy of aquatic macrophytes—A review

Algal-bloom control is an important issue for water environment protection as it induces several negative impacts on the lives of aquatic organisms,aquaculture,landscaping,and human health.The development of an environment-friendly,cost-effective,and convenient alternative for controlling algal bloom has gained much concern.Using the allelopathy of aquatic macrophytes as a novel and safe method for algal-bloom control is a promising alternative.This paper reviews the development and potential application about allelopathy of aquatic plants on algae,including the allelopathic research history,the potential research problems,the research methodology,and the reported aquatic macrophytes and their inhibitory allelochemicals.Potential modes of inhibition action of allelochemicals on algae,possible ways for application,and future development directions of research on algal-bloom control by aquatic macrophytes were also presented.

The effects of biodiversity gradient on plant mass and metabolism of individual submerged macrophytes

Background:The effects of biodiversity on community function and services are frequently studied in the history of ecology,while the response of individual species to biodiversity remains great elusive.In this study,we determined the biodiversity effects on community productivity as well as species level plant mass and carbon(C)and nitrogen(N)metabolism of eight submerged plants.These macrophytes in Lake Erhai were selected and planted in a water depth of one meter along a diversity gradient of 1,2,4 or 8 species.Then,the community productivity or species level plant mass,soluble protein,free amino acid and soluble carbohydrate were correlated to species richness to determine the biodiversity effects on community and single species.Results:The results showed that the community level biomass was positively correlated to plant species richness although the species level plant mass of individual species responded differently to the overall plant species richness.Namely,only one plant mass positively correlated to species richness and the others decreased or showed no significant correlation with the increase of species richness.The soluble proteins of most macrophytes were positively correlated to species richness;however,both the free amino acid and soluble carbohydrate of the plants were negatively or not significantly correlated to species richness.Conclusions:These results indicated that the selection effects might dominate in our aquatic communities and the negative impacts of biodiversity on C and N metabolism of the macrophytes increased with the increase of species richness,which might result from the strong competition among the studied species.The biodiversity effects on the plant mass,and C and N metabolism of individual submerged species were first reported in this study,while more such field and control experiments deserve further research.

Do alternative stable states exist in large shallow Taihu Lake,China?

Regime shifts from submersed macrophyte dominance to phytoplankton dominance have been widely reported in small-to medium-sized shallow lakes.However,alternative stable states in large shallow lakes(surface area>500 km^(2))remain unconfirmed.To understand the alternative stable states and the main influencing factors of submersed macrophytes in large lakes,the ecosystem states from monitoring data from 1959 to 2019 in large shallow Taihu Lake(2338 km^(2)in average depth of 2.12 m)in China were examined.Changes in submersed macrophyte coverage(C_(Mac))and phytoplankton chlorophyll a(Chl a)in the time series and their relationships with environmental factors were analyzed.During the field investigation from August 2018 to May 2019,nutrients and Chl a showed obvious heterogeneity across the lake,being generally higher in the western and northern areas and lower in the southeast area,while C_(Mac)was only observed in the eastern areas,e.g.,East Taihu Lake,Xukou Bay,and Gonghu Bay.During the long-term monitoring from 1959 to 2019 in the Central Region,Meiliang Bay,and East Taihu Lake,Chl a increased significantly in the time series.C_(Mac)varied slightly among different subareas,always at low levels(<10%)in the Central Region and Meiliang Bay but at relatively high levels in East Taihu Lake(10%–90%).Frequency distributions of response variables had no multimodality except for C_(Mac)in East Taihu Lake,with two peaks between 15%and 20%and between 55%and 60%.A dual relationship was found between Chl a and total phosphorus(TP)in the areas with and without macrophytes,while C_(Mac)showed no relationship with TP,and submersed macrophytes did not flourish in the Central Region and Meiliang Bay even when TP was at very low levels(≈10 mg/m3).Taihu Lake had similar algal turbidity(TurbAlg)as small-to mediumsized lakes but generally presented with higher values of nonalgal turbidity(TurbNonAlg),as did their contribution to total turbidity as a percentage.This study suggested that large shallow Taihu Lake may have no alternative stable states,but more evidence is needed for East Taihu Lake,which was dominated by macrophytes,as it remains unknown whether hysteresis occurs between the processes of eutrophication and oligotrophication.Unfavorable conditions caused by wind might be the main reason due to the absence of submersed macrophytes in Taihu Lake.These results demonstrate that stricter nutrient control is needed to maintain a healthy state or to recover from a decayed state for large lakes.

Assessment of the Ecological Status and Threats of Welala and Shesher Wetlands, Lake Tana Sub-Basin (Ethiopia)

The ecological status of the Welala and Shesher Wetlands, on the eastern side of Lake Tana, were studied during pre-rainy, main-rainy, post-rainy and dry seasons from May 2009 to January 2010. Species composition, diversity and abundance of macrophytes, benthic macro-invertebrates and birds were assessed and physico-chemical parameters were measured. Phosphate and silicate concentrations, conductivity, TDS and turbidity varied significantly (p 0.05) between the two wetlands. The values for dissolved oxygen and pH ranged between 4.8 - 7.8 mg/l and 6.7 - 7.6, respectively. The values for nitrate and phosphate ranged between 0.2 - 3.1 mg/l and 0.2 - 3.3 mg/l, respectively. A total of 274 benthic macro-invertebrate individuals belonging to 5 families were collected, 32, 699 individual birds belonging to 62 species were enumerated and 13 species of macrophytes were identified. The two wetlands are shrinking at an alarming rate, mainly because of unsustainable farming practices and a huge irrigation project on Ribb River which is at presently under construction. Farming practices include draining and pumping of water for irrigation and expansion of farmland at the cost of the wetlands. The construction of a dam in Ribb River prevents overflow from Ribb River into the wetlands and disrupts the connection with Lake Tana which is vital for the survival of these two wetlands. We conclude that the Welala and Shesher Wetlands are valuable wetlands which need urgently protection.