Macrophytes In Shallow Lakes Research Articles

Relationship between phosphorus stoichiometric homeostasis and deepwater adaptability of submerged macrophytes in Erhai Lake, China: Insights from allometric plasticity

The state transition theory suggests that the decline of submerged macrophytes in shallow lakes is closely associated with reduced stoichiometric homeostasis, particularly phosphorus homeostasis (HP). The degradation typically progresses from deeper to shallower regions, indicating a potential positive correlation between the deepwater adaptability (DA) and HP values of submerged macrophytes. Here, we investigated the distribution pattern of submerged macrophytes across different water depths of Erhai Lake to test this hypothesis. The results revealed a significant positive correlation between the DA and HP values of submerged macrophytes. Allometric analysis indicated that the morphological plasticity of submerged macrophytes was linked to their HP. Species with higher HP values, like Potamogeton maackianus, had robust plasticity strategies, particularly “real plasticity”, that enabled them to cope with deeper water stress. In contrast, species with lower HP values (Ceratophyllum demersum and Hydrilla verticillata) experienced nutrient declines, which hindered their adaptation. Additionally, species with higher HP values exhibited closer connections within the plant traits-environment network, indicating that their morphological plasticity adjustments allow better adaptation to the environmental changes caused by increasing water depth. These results confirm the relationship between DA and HP in submerged macrophytes and explain the mechanisms underlying the correlation, thus expanding regime shift theory.

Loss of submerged macrophytes in shallow lakes alters bacterial and archaeal community structures, and reduces their co-occurrence networks connectivity and complexity.

Bacteria and archaea are important components in shallow lake ecosystems and are crucial for biogeochemical cycling. While the submerged macrophyte loss is widespread in shallow lakes, the effect on the bacteria and archaea in the sediment and water is not yet widely understood. In this study, 16S rRNA gene sequencing was used to explore the bacteria and archaea in samples taken from the sediment and water in the submerged macrophyte abundant (MA) and submerged macrophyte loss (ML) areas of Caohai Lake, Guizhou, China. The results showed that the dominant bacterial phyla were Proteobacteria and Chloroflexi in the sediment; the dominant phyla were Proteobacteria, Actinobacteriota, and Bacteroidota in the water. The dominant archaea in sediment and water were the same, in the order of Crenarchaeota, Thermoplasmatota, and Halobacterota. Non-metric multidimensional scaling (NMDS) analyses showed that bacterial and archaeal community structures in the water were significantly affected by the loss of submerged macrophytes, but not by significant changes in the sediment. This suggests that the loss of submerged macrophytes has a stronger effect on the bacterial and archaeal community structures in water than in sediment. Furthermore, plant biomass (PB) was the key factor significantly influencing the bacterial community structure in water, while total nitrogen (TN) was the main factor significantly influencing the archaeal community structure in water. The loss of submerged macrophytes did not significantly affect the alpha diversity of the bacterial and archaeal communities in either the sediment or water. Based on network analyses, we found that the loss of submerged macrophytes reduced the connectivity and complexity of bacterial patterns in sediment and water. For archaea, network associations were stronger for MA network than for ML network in sediment, but network complexity for archaea in water was not significantly different between the two areas. This study assesses the impacts of submerged macrophyte loss on bacteria and archaea in lakes from microbial perspective, which can help to provide further theoretical basis for microbiological research and submerged macrophytes restoration in shallow lakes.

Distribution patterns of macrophytes in shallow lakes of the lower Paraná River floodplain: Associations with environmental conditions

Abstract Macrophytes play key functional and structural roles in floodplain shallow lakes (SL), determining the maintenance of ecosystem functions and sustaining several productive activities. We assessed the relationship between macrophyte community structure (composition, species abundance, and plant diversity) and environmental features (SL dynamics, connectivity, morphometry, flood frequency, and water and sediment physical and chemical variables) in the Lower Paraná River floodplain. Our main hypothesis was that SL macrophyte distribution depends on direct and indirect environmental controls operating at multiple scales. The SL dynamics during the April–November 2017 Paraná River pulse were described according to their coverage changes (vegetation, and turbid or clear open water), which were assessed with supervised classifications on Landsat 8‐OLI scenes. Twenty‐five SL of the Lower Paraná River floodplain were selected to represent all the SL dynamics identified. Ten SL were permanently connected and 15 were disconnected from watercourses. Water and sediment physical and chemical characteristics, macrophyte assemblage composition, and plant species abundance were surveyed at each site in the 2018 summer. Also, SL morphometric parameters and flood frequency were estimated from ancillary geographic data. The direct controlling factors of macrophyte assemblages were water ammonium and water nitrate concentrations, turbidity, and flood frequency. No clear effects of environmental conditions on macrophyte diversity were found. The connectivity of SL with watercourses affected their water and sediment physical and chemical features. Also, SL dynamics during the 2017 pulse were related to flood frequency. As connectivity to watercourses affects SL chemical and physical variables, and SL dynamics were associated with flood frequency, connectivity and SL dynamics indirectly influenced macrophyte assemblage composition. This study improved our understanding of macrophyte distribution patterns in the study area, which was related to several environmental variables operating at different scales. Our results highlight the importance of the variability of hydrological connectivity in promoting environmental heterogeneity in river floodplain systems, which is central to their functioning, and the high biodiversity that characterises these environments.

Seasonality and Species Specificity of Submerged Macrophyte Biomass in Shallow Lakes Under the Influence of Climate Warming and Eutrophication.

Climate warming and eutrophication caused by anthropogenic activities strongly affect aquatic ecosystems. Submerged macrophytes usually play a key role in shallow lakes and can maintain a stable clear state. It is extremely important to study the effects of climate warming and eutrophication on the growth of submerged macrophytes in shallow lakes. However, the responses of submerged macrophytes to climate warming and eutrophication are still controversial. Additionally, the understanding of the main pathways impacting submerged macrophytes remains to be clarified. In addition, the influence of seasonality on the growth responses of submerged macrophytes to climate warming and eutrophication requires further elucidation. In this study, we conducted a series of mesocosm experiments with four replicates across four seasons to study the effects of rising temperature and nutrient enrichment on the biomass of two submerged macrophytes, Potamogeton crispus and Elodea canadensis. Our results demonstrated the seasonality and species specificity of plant biomass under the influence of climate warming and eutrophication, as well as the main explanatory factors in each season. Consistent with the seasonal results, the overall results showed that E. canadensis biomass was directly increased by rising temperature rather than by nutrient enrichment. Conversely, the overall results showed that P. crispus biomass was indirectly reduced by phosphorus enrichment via the strengthening of competition among primary producers. Distinct physiological and morphological traits may induce species-specific responses of submerged macrophytes to climate warming and eutrophication, indicating that further research should take interspecies differences into account.

Effects of Sediments Phosphorus Inactivation on the Life Strategies of Myriophyllum spicatum: Implications for Lake Restoration

Eutrophication often results in the loss of submerged vegetation in shallow lakes and turns the lake to be a turbid state. Recovery of submerged macrophytes is the key in the restoration of shallow eutrophic lakes to create a clear water state. However, internal loading control was considered as the critical process for the recovery of submerged macrophytes in shallow lakes after the external nutrient reduction. Phoslock® (Lanthanum modified bentonite) is a useful passivation material in controlling the internal loadings (release of phosphorus from the sediments), which was applied to restore the eutrophic lakes. However, the effects of Phoslock® on the growth and life strategies of submerged macrophytes are less focused so far. In the present study, we studied the responses in the growth and morphological characteristics of Myriophyllum spicatum to the addition of Phoslock® to the sediments. Our results showed that the addition of Phoslock® significantly decreased the contents of bioavailable forms of phosphorus in the sediments, such as redox-sensitive phosphorus bound to Fe and Mn compounds (BD–P), phosphorus bound to aluminum (Al–P) and organic phosphorus (Org–P). However, the concentration of the non-bioavailable forms of phosphorus in the sediments, such as calcium bound phosphorus (Ca–P), increased significantly in the Phoslock® treatments compared with the controls. At the end of the experiments, the total biomass, aboveground biomass and relative growth rate (RGR) of M. spicatum decreased significantly in the Phoslock® mesocosms compared with the controls. In contrast, the wet root biomass, root–shoot biomass ratio, root numbers and root length of M. spicatum were significantly higher in the Phoslock® treatments than that in the controls. Our results indicated that the growth of M. spicatum was suppressed by the addition of Phoslock®, and thus the biomass was decreased; however, the increase of root biomass might be beneficial to the inhibition of phosphorus release and resuspension of sediments and to the restoration of the lake ecosystem.

Effects of origin and water depth on morphology and reproductive modes of the submerged plant Vallisneria natans

• The decline of submerged macrophytes in shallow lakes and wetlands has cascade effects on regional and global biodiversity. In the last two decades, the rapid disappear of macrophyte beds in the floodplain lakes of the middle lower Yangtze River has become a great concern for the ecological security of the region.• This study aims to quantify the effects of water depth on biomass production and clonal (i.e. stolons and winter buds) and sexual reproduction (i.e. fruits) of Vallisneria natans, a cosmopolitan, perennial submergent species.• Results of the research clearly demonstrated the unimodal relationship between biomass production and water depth, and there was an optimal range of 35–50 cm, beyond which total biomass decreased. Also, both clonal and sexual reproductive outputs decreased with water depth. Furthermore, for the first time, the study revealed the distinct reproduction patterns of the individuals originated from seeds and clonal propagules. While seed-generated plants produced significantly more fruits (and no winter buds), plants developed from clonal propagules produced more stolons and winter buds.• There findings plead further researches on V. natans seedling establishment and reproductive ecology because of the importance of winter buds as food for wintering waterbirds in the middle Yangtze floodplains.

Interactions between bacteria and fungi in macrophyte leaf litter decomposition.

Microbes play an important role in decomposition of macrophytes in shallow lakes, and the process can be greatly affected by bacteria-fungi interactions in response to material composition and environmental conditions. In this study, microbes involved in the decomposition of leaf litter from three macrophyte species, Zizania latifolia, Hydrilla verticillata and Nymphoides peltata, were analysed at temperatures of 5, 15 and 25 °C. Results indicate that the decomposition rate was affected by temperature. Bacterial alpha diversity increased significantly along the time, while both temperature and plant species had a significant impact on the bacterial community, and plant type was shown to be the most important driving factor for the fungal community. The cosmopolitan bacterial taxa affiliated with Gammaproteobacteria, Bacteroidetes, Deltaproteobacteria, Firmicutes and Spirochaetes were key species in the investigated ecological networks, demonstrating significant co-occurrence or co-exclusion relationships with Basidiomycota and Ascomycota, according to different macrophyte species. This study indicates that bacteria involved in the decomposition of macrophyte leaf litter are more sensitive to temperature variance, and that fungi have a higher specificity to the composition of plant materials. The nutrient content of Hydrilla verticillata promoted a positive bacteria-fungi interaction, thereby accelerating the decomposition and re-circulation of leaf litter.

Interaction among bacterioplankton and macrophytes in shallow lakes with high macrophyte cover

Growth of submerged and emergent macrophytes was studied together with heterotrophic bacterioplankton abundance and production in two Hungarian shallow lakes with dominant macrophyte covers. It was expected that bacterioplankton numbers and activity would have an effect on macrophyte biomass accumulation. Bacterial production and abundance showed a strong seasonal pattern with maximum in the warmest months (July, August). It was found that macrophyte biomass increased with heterotrophic bacterial production and abundance up to 5.6 µg C l− 1 h− 1 and 5.30*106 cells, respectively, while over that value was negatively associated with macrophyte growth. It was also shown that the relationship between heterotrophic bacteria and macrophytes also varied seasonally, showing a multifaceted relationship. It was demonstrated that macrophytes are not only the most significant carbon and energy source for the bacteria in shallow, macrophyte-dominated lakes, but are also competing organisms that could be supressed by excessive bacterial activity. These findings could help better understand the interaction between macrophytes and bacterioplankton, and assist wetland managers in quantifying what may be a primary cause of reed die-back.

Effects of juvenile crucian carp (Carassius carassius) removal on submerged macrophyte growth-implications for subtropical shallow lake restoration.

Rapid expansion of juvenile fish after biomanipulation can delay the successful restoration of submerged macrophytes, leading to a turbid water status in subtropical shallow lakes. Aimed to test the effects of direct removal of juvenile crucian carp (Carassius carassius) on water quality and growth of two submerged macrophytes Vallisneria natans and Hydrilla verticillate, a short-term outdoor mesocosm experiment was conducted in the Taihu Laboratory for Lake Ecosystem Research (TLLER). The results indicated that the concentrations of TN, TP, suspended solids, and chlorophyll a decreased significantly with increasing removal density of juvenile crucian carp, thus resulting in a clear status of the water. Additionally, the mean relative growth rate of V. natans and H. verticillata in the low- and high-density removal treatments were higher than that in the controls. Moreover, the removal of juvenile crucian carp also significantly increased the stem length of V. natans, but no significant effect on that of H. verticillata. Meanwhile, the total number of V. natans and H. verticillata in the low- and high-density removal treatments was higher than that in the controls, but all of H. verticillata were lower than the initials. Our results indicated that removing juvenile crucian carp could improve the water quality, increasing relative growth rate, height, and reproduction of V. natans, and improving the survival rate of H. verticillata. The promotion of fish removal on the V. natans growth was greater than H. verticillata. The results also implied that it was necessary to continuously remove the juvenile benthivorous fish several times for restoring the submerged macrophytes in shallow lakes.

Long-term succession of aquatic plants reconstructed from palynological records in a shallow freshwater lake

Aquatic plants in shallow freshwater lakes play a key role in stabilizing ecological function and providing valuable ecosystem services, yet they are severely degraded worldwide. An improved understanding of long-term aquatic plant succession is critical to investigate the potential driving mechanisms and to facilitate ecological restoration. In this paper, we reconstructed changes in the aquatic plant community over the past century based on palynological records from Changdang Lake, Middle and Lower Yangtze River Basin (MLYB), China. Our results reveal that aquatic plants in Changdang Lake have undergone three clear phases: emergent macrophytes dominated the aquatic vegetation in the 1900s–1970s, submerged macrophytes in the 1970s–1990s, and floating macrophytes increasingly after the 1990s. Significant changes in the aquatic plant communities were caused by increasing anthropogenic pressures, such as damming and nutrient loading from agriculture, aquaculture, and urbanization after the Chinese economic reform. We argue that Changdang Lake is currently in a transition phase between a macrophyte-dominated state and an algae-dominated state. Our palynological record is different from many contemporary studies, which suggest submerged plants dominated most lakes in this region before the 1950s. We suggest that the return of the aquatic plants to their 1970s–1980s state would be a realistic target for lake restoration. Our results show that palynological records can reveal long-term dynamics of macrophytes in shallow lakes for sustainable lake restoration and management.

Mowing Submerged Macrophytes in Shallow Lakes with Alternative Stable States: Battling the Good Guys?

Submerged macrophytes play an important role in maintaining good water quality in shallow lakes. Yet extensive stands easily interfere with various services provided by these lakes, and harvesting is increasingly applied as a management measure. Because shallow lakes may possess alternative stable states over a wide range of environmental conditions, designing a successful mowing strategy is challenging, given the important role of macrophytes in stabilizing the clear water state. In this study, the integrated ecosystem model PCLake is used to explore the consequences of mowing, in terms of reducing nuisance and ecosystem stability, for a wide range of external nutrient loadings, mowing intensities and timings. Elodea is used as a model species. Additionally, we use PCLake to estimate how much phosphorus is removed with the harvested biomass, and evaluate the long-term effect of harvesting. Our model indicates that mowing can temporarily reduce nuisance caused by submerged plants in the first weeks after cutting, particularly when external nutrient loading is fairly low. The risk of instigating a regime shift can be tempered by mowing halfway the growing season when the resilience of the system is highest, as our model showed. Up to half of the phosphorus entering the system can potentially be removed along with the harvested biomass. As a result, prolonged mowing can prevent an oligo—to mesotrophic lake from becoming eutrophic to a certain extent, as our model shows that the critical nutrient loading, where the lake shifts to the turbid phytoplankton-dominated state, can be slightly increased.

Response of Vallisneria natans to Increasing Nitrogen Loading Depends on Sediment Nutrient Characteristics

High nitrogen (N) loading may contribute to recession of submerged macrophytes in shallow lakes; yet, its influences vary depending on environmental conditions. In August 2013, we conducted a 28-day factorial-designed field mesocosm experiment in Lake Taihu at the Taihu Laboratory for Lake Ecosystem Research (TLLER) to examine the effects of high N loading on the growth of Vallisneria natans in systems with contrasting sediment types. We ran the experiments with two levels of nutrient loading—present-day external nutrient loading (average P: 5 μg·L−1·day−1, N: 130 μg·L−1·day−1) and P: 5 μg·L−1·day−1, and with three times higher N loading (N: 390 μg·L−1·day−1) and used sediment with two contrasting nutrient levels. V. natans growth decreased significantly with increasing N loading, the effect being dependent, however, on the nutrient status of the sediment. In low nutrient sediment, relative growth rates, leaf biomass and root biomass decreased by 11.9%, 18.2% and 23.3%, respectively, at high rather than low N loading, while the decline was larger (44.0%, 32.7% and 41.8%, respectively) when using high nutrient sediment. The larger effect in the nutrient-rich sediment may reflect an observed higher shading of phytoplankton and excess nutrient accumulation in plant tissue, though potential toxic effects of the high-nutrient sediment may also have contributed. Our study confirms the occurrence of a negative effect of increasing N loading on submerged plant growth in shallow nutrient-enriched lakes and further shows that this effect is augmented when the plants grow in nutrient-rich sediment. External N control may, therefore, help to protect or restore submerged macrophytes, especially when the sediment is enriched with nutrients and organic matter.

Emergent Macrophytes Alter the Sediment Composition in a Small, Shallow Subtropical Lake: Implications for Methane Emission

Aquatic macrophytes in shallow lakes emit high levels of methane. We hypothesize that the presence of emergent aquatic macrophytes in an artificial shallow lake promotes important input of autochthonous organic matter (OM) in sediment and higher levels of methane emission via bubbles. Samplings were performed at three sites in a small, shallow subtropical lake: (1) one station in the limnetic region and (2) - (3) two stations in the littoral region ((2) inside and (3) outside aquatic macrophyte stands). A higher concentration of OM was observed at the macrophyte station, and within this site, a higher methane concentration was observed in the sediment. These results could explain the methane ebullition values at macrophyte sites. At the macrophyte station, methane emission via bubbles contributed 17% to 56% of the total methane emission; however, at the other stations, its contribution via bubbles, was lower than 1%. This research confirmed the importance of emergent macrophytes at Polegar Lake as a source of OM in sediment and methane emission via bubbles. Further, we could confirm the positive effects of temperature on methane emission, mainly by bubbles.

Plant community structure determines primary productivity in shallow, eutrophic lakes

Summary Regime shifts are commonly associated with the loss of submerged macrophytes in shallow lakes; yet, the effects of this on whole‐lake primary productivity remain poorly understood. This study compares the annual gross primary production (GPP) of two shallow, eutrophic lakes with different plant community structures but similar nutrient concentrations. Daily GPP rates were substantially higher in the lake containing submerged macrophytes (586 ± 23 g C m−2 year−1) than in the lake featuring only phytoplankton and periphyton (408 ± 23 g C m−2 year−1; P < 0.0001). Comparing lake‐centre diel oxygen curves to compartmental estimates of GPP confirmed that single‐site oxygen curves may provide unreliable estimates of whole‐lake GPP. The discrepancy between approaches was greatest in the macrophyte‐dominated lake during the summer, with a high proportion of GPP occurring in the littoral zone. Our empirical results were used to construct a simple conceptual model relating GPP to nutrient availability for these alternative ecological regimes. This model predicted that lakes featuring submerged macrophytes may commonly support higher rates of GPP than phytoplankton‐dominated lakes, but only within a moderate range of nutrient availability (total phosphorus ranging from 30 to 100 μg L−1) and with mean lake depths shallower than 3 or 4 m. We conclude that shallow lakes with a submerged macrophyte–epiphyton complex may frequently support a higher annual primary production than comparable lakes that contain only phytoplankton and periphyton. We thus suggest that a regime shift involving the loss of submerged macrophytes may decrease the primary productivity of many lakes, with potential consequences for the entire food webs of these ecosystems.

Macrophytes in shallow lakes: Relationships with water, sediment and watershed characteristics

We examined macrophyte–environment relationships in shallow lakes located within the Prairie Parkland and Laurentian Mixed Forest provinces of Minnesota. Environmental variables included land cover within lake watersheds, and within-lake, water and sediment characteristics. CCA indicated that sediment fraction smaller than 63μm (f<63), open water area, turbidity, and percent woodland and agricultural cover in watersheds were significant environmental variables explaining 36.6% of variation in macrophyte cover. When Province was added to the analysis as a spatial covariate, these environmental variables explained 30.8% of the variation in macrophyte cover. CCA also indicated that pH, f<63, percent woodland cover in watersheds, open water area, emergent vegetation area, and organic matter content were significant environmental variables explaining 43.5% of the variation in macrophyte biomass. When Province was added to the analysis as a spatial covariate, these environmental variables explained 39.1% of the variation in macrophyte biomass. The f<63 was the most important environmental variable explaining variation for both measures of macrophyte abundance (cover and biomass) when Province was added as a spatial covariate to the models. Percent woodland in watersheds, turbidity, open water area, and Ca+Mg explained 34.5% of the variation in macrophyte community composition. Most species showed a negative relationship with turbidity and open water area except for Potamogeton richardsonii, Stuckenia pectinata, and filamentous algae. Our study further demonstrates the extent to which macrophyte abundance and community composition are related to site- and watershed-scale variables including lake morphology, water and sediment characteristics, and percent land cover of adjacent uplands.

The impact of phosphorus enrichment on a Nitella sp. from sand-mine void wetlands, Capel, Western Australia

The disappearance of submerged macrophytes in shallow lakes is one of the most critical problems caused by eutrophication. The impact of increased phosphorus concentration on the establishment of a Nitella species as a suitable macrophyte for the rehabilitation of sandmine void wetlands at Capel in Western Australia, in the context of an imminent proposal to discharge effluent water from a waste processing plant into the wetlands was studied in the laboratory. The Nitella species was cultured in aquarium tanks in eutrophic and mesotrophic concentrations of phosphorus with a control (oligotrophic) treatment and its growth and lifecycle were monitored. It was observed that eutrophication had a significant impact on growth, morphology and lifecycle of Nitella. In addition to showing profuse vegetative growth, Nitella species produced no reproductive organs in the eutrophic medium. An approximate ratio of 1:1 male to female reproductive organs was recorded in the mesotrophic and control treatments but with reduced vegetative growth. Higher biomass of Nitella and filamentous algae including blue green algae was also recorded in the eutrophic treatment. In the eutrophic medium, the height, number of nodes, internode distance and the biomass were also found to be significantly higher than those in the other treatments. It was concluded that with the projected discharge of waste water into the Capel Wetlands, the Nitella meadows would display an initial accelerated growth followed by excessive growth of filamentous algae including cyanobacteria leading to increase in turbidity and eventual disappearance of Nitella species.

Biomass and nutrient stock of submersed and floating macrophytes in shallow lakes formed by rewetting of degraded fens

Ecosystem restoration by rewetting of degraded fens led to the new formation of large-scale shallow lakes in the catchment of the River Peene in NE Germany. We analyzed the biomass and the nutrient stock of the submersed (Ceratophyllum demersum) and the floating macrophytes (Lemna minor and Spirodela polyrhiza) in order to assess their influence on temporal nutrient storage in water bodies compared to other freshwater systems. Ceratophyllum demersum displayed a significantly higher biomass production (0.86–1.19 t DM = dry matter ha−1) than the Lemnaceae (0.64–0.71 t DM ha−1). The nutrient stock of submersed macrophytes ranged between 28–44 kg N ha−1 and 8–12 kg P ha−1 and that of floating macrophytes between 14–19 kg N ha−1 and 4–5 kg P ha−1 which is in the range of waste water treatment plants. We found the N and P stock in the biomass of aquatic macrophytes being 20–900 times and up to eight times higher compared to the nutrient amount of the open water body in the shallow lakes of rewetted fens (average depth: 0.5 m). Thereafter, submersed and floating macrophytes accumulate substantial amounts of dissolved nutrients released from highly decomposed surface peat layers, moderating the nutrient load of the shallow lakes during the growing season from April to October. In addition, the risk of nutrient loss to adjacent surface waters becomes reduced during this period. The removal of submersed macrophytes in rewetted fens to accelerate the restoration of the low nutrient status is discussed.

In-lake algal bloom removal and submerged vegetation restoration using modified local soils

A “modified local soil induced ecological restoration” (MLS-IER) technology was developed for the restoration of degraded shallow lakes. Modified local soils that mixed with macrophyte seeds were used to flocculate the algal blooms and sink them down to the bottom of the lake. The increased water clarity and the improved sediment quality due to the covering of clean modified local soils make it possible for a quick restoration of submerged macrophytes in eutrophic shallow lakes. The MLS-IER technology was tested in the whole bay of Liaoyangyuan (0.1km2) in Lake Tai (Wuxi, China) in August 2006. The whole bay was fully covered by more than 1cm cyanobacterial bloom since June, which caused massive killing of fish and aquatic vegetations. Some 4tons of chitosan-modified local soils were sprayed over the whole bay and the severe bloom was successfully removed within one day. The secchi depth was increased from 0cm to 30cm, and the chlorophyll-a, total-P, and total-N were all reduced by more than 86% within one day's time. Four months after the treatment, submerged macrophytes were successfully restored within the whole bay. Cyanotoxin microcystins RR and LR were reduced by 50% and 40%, respectively, compared to those outside the bay 4 months later. The biodiversity index of zoobenthos and that of phytoplankton inside the bay became higher than that outside the bay, while zooplankton diversity index remained relatively unchanged. This field trial study indicated that restoration of submerged macrophytes in shallow lakes could be significantly accelerated by using MLS-IER technology. The long-term ecological response and the transition mechanism between algal cells and submerged macrophytes in the sediment need to be further studied in controlled whole lake experiments.

Habitat structure and juvenile fish ontogeny shape zooplankton spring dynamics

Macrophytes in shallow lakes have the potential to alter fish-zooplankton interactions considerably. How far predation effects by newly hatched fish (0+ fish) on zooplankton are influenced by different types of aquatic vegetation, and how effects change during the first weeks of fish ontogeny remains, however, less clear. In order to address these issues, we examined the predation effects of 0+ fish on zooplankton in three different habitats during spring and summer in a shallow, eutrophic lake in Sweden. Zooplankton and fish samples were taken along the reed vegetation, in a shallow, unvegetated part of the lake and above dense, submersed vegetation to relate 0+ fish predation effects to vegetation complexity. All the size classes of zooplankton decreased when 0+ fish started to feed on them in all the different habitats. The magnitude of predation effects depended, however, on both the size of zooplankton and the complexity of the vegetation. While small cladocerans could maintain stable populations in the dense Chara vegetation after 0+ fish had started to feed on them, medium and large-sized zooplankton disappeared from all the habitats. Our results suggest that only small cladocerans can use dense vegetation as a refuge against 0+ fish predation, while medium and large zooplankton are not safe from 0+ fish predation in any habitat.

Quantitative Assessment of Ammonia-Oxidizing Bacterial Communities in the Epiphyton of Submerged Macrophytes in Shallow Lakes

In addition to the benthic and pelagic habitats, the epiphytic compartment of submerged macrophytes in shallow freshwater lakes offers a niche to bacterial ammonia-oxidizing communities. However, the diversity, numbers, and activity of epiphytic ammonia-oxidizing bacteria have long been overlooked. In the present study, we analyzed quantitatively the epiphytic communities of three shallow lakes by a potential nitrification assay and by quantitative PCR of 16S rRNA genes. On the basis of the m(2) of the lake surface, the gene copy numbers of epiphytic ammonia oxidizers were not significantly different from those in the benthic and pelagic compartments. The potential ammonia-oxidizing activities measured in the epiphytic compartment were also not significantly different from the activities determined in the benthic compartment. No potential ammonia-oxidizing activities were observed in the pelagic compartment. No activity was detected in the epiphyton of Chara aspera, the dominant submerged macrophyte in Lake Nuldernauw in The Netherlands. The presence of ammonia-oxidizing bacterial cells in the epiphyton of Potamogeton pectinatus was also demonstrated by fluorescent in situ hybridization microscopy images. By comparing the community composition as assessed by the 16S rRNA gene PCR-denaturing gradient gel electrophoresis approach, it was concluded that the epiphytic ammonia-oxidizing communities consisted of cells that were also present in the benthic and pelagic compartments. Of the environmental parameters examined, only the water retention time, the Kjeldahl nitrogen content, and the total phosphorus content correlated with potential ammonia-oxidizing activities. None of these parameters correlated with the numbers of gene copies related to ammonia-oxidizing betaproteobacteria.