Submerged vegetation development in two shallow, eutrophic lakes

Submerged macrophyte vegetation in two shallow lakes in the Netherlands, Lake Veluwemeer and Lake Wolderwijd, has been affected by eutrophication in the late 1960's and 1970's. Recent changes in the vegetation occurred in the period following lake restoration measures. Between 1987 and 1993, the dominance of Potamogeton pectinatus decreased, while Charophyte meadows expanded over the same time interval. The pattern of change of the dominant macrophyte species might result from changes in the underwater light climate. Seasonally persistent clear water patches associated with the Chara meadows have been observed in the last few years. The interaction between submerged macrophyte vegetation succession and water transparency in the lakes is discussed.     

Effects of submerged plants on water quality and biota in large-scale experimental ponds

We quantitatively studied the effect of submerged plants on water quality and biota under fish-free conditions for 3 weeks in four large freshwater experimental ponds (533 m³ per pond) at the Aqua Restoration Research Center, Japan. Two artificially harvested ponds with scant vegetation were used as “harvested ponds” (H1, H2), and the other two ponds, which were naturally dominated by Hydrilla verticillata, were used as “vegetated ponds” (V1, V2). The PVI (percent water volume infested with macrophytes) was employed as an index of vegetation abundance. Vegetated ponds had much clearer water than harvested ponds. The water quality in H2 (PVI 10%) was better than in H1 (PVI 3%), whereas the water quality did not differ significantly between the two vegetated ponds (V1, PVI 38% and V2, PVI 84%). Therefore, the threshold between clear water and turbidity was between 10 and 38% in PVI. Our result also showed that a turbid water state was created shortly after harvest. Green algae were abundant in the harvested ponds, and diatoms were dominant in the vegetated ponds. Rotifers were stably dominant in the harvested ponds. Aquatic worms were more abundant in the harvested ponds than in the vegetated ponds. Unexpectedly, zooplanktons were much less abundant in the vegetated ponds; therefore, zooplankton grazing was not the main mechanism behind the cleaner water in our experiment. These results are physical evidence that the presence of dense macrophytes was the main factor in the creation of a clear water state.     

Long‐term dynamics of submerged macrophytes and algae in a small and shallow,eutrophic lake: implications for the stability of macrophyte‐dominance

1. Submerged macrophyte and phytoplankton components of eutrophic, shallow lakes have frequently undergone dynamic changes in composition and abundance with important consequences for lake functioning and stability. However, because of a paucity of long‐term survey data, we know little regarding the nature, direction and sequencing of such changes over decadal–centennial or longer timescales. 2. To circumvent this problem, we analysed multiple (n = 5) chronologically correlated sediment cores for plant macro‐remains and a single core for pollen and diatoms from one small, shallow, English lake (Felbrigg Hall Lake, Norfolk, U.K.), documenting 250 years of change to macrophyte and algal communities. 3. All five cores showed broadly similar stratigraphic changes in macrophyte remains with three distinct phases of macrophyte development: Myriophyllum–Chara–Potamogeton (c. pre‐1900), to Ceratophyllum–Chara–Potamogeton (c. 1900–1960) and finally to Zannichellia–Potamogeton (c. post‐1960). Macrophyte species richness declined from at least 10 species pre‐1900 to just four species at the present day. Additionally, in the final Zannichellia–Potamogeton phase, a directional shift between epi‐benthic and phytoplankton‐based primary production was indicated by the diatom data. 4. Based on macrophyte–seasonality relationships established for the region, concomitant with the final shift to Zannichellia–Potamogeton, we infer a reduction in the seasonal duration of plant dominance (plant‐covered period). Furthermore, we hypothesise that this change in species composition resulted in a situation whereby macrophyte populations were seasonally ‘sandwiched’ between two phytoplankton peaks in spring and late summer as observed in the contemporary lake. 5. We suggest that eutrophication‐induced reductions in macrophyte species richness, especially if the number of plant‐seasonal strategies is reduced, may constrict the plant growing season. In turn, this may render a shallow lake increasingly vulnerable to seasonal invasions of phytoplankton resulting in further species losses in the plant community. Thus, as part of a slow (over perhaps 10–100s of years) and self‐perpetuating process, macrophytes may be gradually pushed out by phytoplankton without the need for a perturbation as required in the alternative stable states model of plant loss.     

沉水植物重建对富营养水体氮磷营养水平的影响

利用富营养浅水湖泊(武汉东湖)中所建立的大型实验围隔系统,研究了沉水植物对水体N、P营养水平的影响．结果表明,沉水植物重建后N、P营养水平显著降低．在研究期间,水生植物围隔总N和总P水平均显著低于对照围隔和大湖水体,而且水生植物围隔的总P含量一般维持在0．1mg·L^-1左右。季节性波动远低于对照围隔和大湖水体．水生植物围隔水体中氨态氮和亚硝态氮含量较低．而硝态氮含量与对照围隔和和大湖水体差别不大．由此可见。恢复以沉水植物为主的水生植被,可以有效地降低N、P营养循环速度,控制浮游植物过度增长,是重建富营养湖泊生态系统的重要措施．     

Submerged macrophyte vegetation and the European Water Framework Directive: assessment of status and trends in shallow,alkaline lakes in the Netherlands

The submerged macrophyte vegetation of lakes created after enclosing former estuaries, situated in the central and south-western part of the Netherlands, has been monitored annually from 1992 onwards. Between 1992 and 2004, pronounced changes in overall cover and species composition of the submerged vegetation have occurred, resulting from changes of water quality and morphology in the lakes. In most cases vegetation cover and species diversity increased or remained stable, with the exception of two lakes in the southwest part of the country. Abundance and species composition were assessed according to the requirements of the EU Water Framework Directive, using the assessment procedure proposed to assess macrophytes in natural water bodies in the Netherlands. The assessment procedure included calculation of the ‘ecological quality ratio’ (EQR) for each of eleven water bodies in each of 13 years, based on transect monitoring data. The EQR indicating Good Ecological Status for Macrophytes was achieved in only three of the lakes. The consequences of hydromorphological modifications, and measures necessary to achieve the desired condition are discussed. Nutrient concentrations should be reduced further, while additional management measures are necessary to improve conditions for macrophytes.     

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 (0.1 km²) in Lake Tai (Wuxi, China) in August 2006. The whole bay was fully covered by more than 1 cm cyanobacterial bloom since June, which caused massive killing of fish and aquatic vegetations. Some 4 tons 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 0 cm to 30 cm, 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.     

Plant community structure determines primary productivity in shallow,eutrophic lakes

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Predicting the effect of climate change on temperate shallow lakes with the ecosystem model PCLake

Global average surface temperatures are expected to rise by about 1.4–5.8°C from the present until the year 2100. This temperature increase will affect all ecosystems on earth. For shallow lakes—which can be either in a clear water or a turbid state—this climate change will expectedly negatively affect water transparency though the prediction is far from conclusive and experimental investigations elucidating the potential climatic effects on shallow lakes are still rare. The aim of this study was to further shape and sharpens hypotheses on the impact of climate change on shallow lakes by applying an existing and well-calibrated ecosystem model, PCLake. We focused on asymptotic model behaviour for a range of temperature and loading scenarios in a factorial design. We conclude that climate change will likely lead to decreased critical nutrient loadings. Combined with an expected increase in the external nutrient loading, this will increase the probability of a shift from a clear to a turbid state. As the model predicts a higher summer chlorophyll-a concentration, a stronger dominance of cyanobacteria during summer and a reduced zooplankton abundance due to climate change, the turbid state itself is likely to become even more severe.     

Indirect effects of fish community structure on submerged vegetation in shallow,eutrophic lakes: an alternative mechanism

The loss of submerged macrophytes during eutrophication of shallow lakes is a commonly observed phenomenon. The proximate reason for this decline is a reduction of available light due to increasing phytoplankton and/or epiphyton biomass. Here we argue that the ultimate cause for the transition from a macrophyte-dominated state to a phytoplankton-dominated state is a change in fish community structure. A catastrophic disturbance event (e.g. winterkill) acting selectively on piscivores, cascades down food chains, eventually reducing macrophyte growth through shading by epiphyton, an effect that is reinforced by increasing phytoplankton biomass. The transition back from the phytoplankton to the macrophyte state depends on an increase in piscivore standing stock and a reduction of planktivores. A conceptual model of these mechanisms is presented and supported by literature data and preliminary observations from a field experiment.     

Modeling nutrients,oxygen and critical phosphorus loading in a shallow reservoir in China with a coupled water quality – Macrophytes model

Many macrophyte-dominated clear lakes switch to a phytoplankton-dominated turbid state when the lake becomes eutrophic. An existing Yuqiao Reservoir Water Quality Model (YRWQM) and the macrophyte submodel were coupled to simulate the effect of submerged macrophytes on nutrients and dissolve oxygen cycles in a shallow reservoir in China. The level of phosphorus loading in a transition from a clear to turbid state was addressed using the integrated model. The model runs from seedling establishment until dying out, from March 1 to July 18 in 2009. The simulations were performed for a contingent range of P loadings, starting from three different initial conditions. The results indicated that the integrated model improves accuracy of predictions compared to YRWQM. The concentrations of nutrients declined slightly during the macrophyte growth period in the reservoir and dissolved oxygen increased slightly. Although nutrient concentrations increased by submerged macrophyte release during the extinction period, the effect on the nutrients was less than that of transfer with nutrient-rich water. More released nutrients may enhance increases in substantial abundance. The critical phosphorus loading level during a switch from the clear to turbid state was estimated by these scenarios. The threshold for the switch is ∼6.1 mgP m⁻² d⁻¹ with an initial total phosphorus concentration of 160 μg l⁻¹. Moreover, the results demonstrated that the switch was also dependent on the initial total phosphorus concentration. These results suggest that the reservoir in a clear water state is at risk of a switch as nutrient levels are close to the critical levels.     

Variation in vegetation and seed banks of freshwater lakes with contrasting intensity of aquaculture along the Yangtze River, China

We examined species composition and richness in the seed banks and established vegetation in lakes along the Yangtze River with two contrasting aquaculture types: pen-culture lakes, characterized by pen-culture of commercial fish and crab, and lake-culture lake with more intensive stocking of the entire lake. The mean live biomass and species richness of submerged vegetation was significantly reduced or absent in lake-culture lakes (0-159 g m⁻² and 0-0.3 species per sample, respectively), compared to those with pen-culture lakes (1552-2971 g m⁻² and 1.5-3.2 species per sample, respectively) in all three study years. Also mean seedling density and species richness of seed banks were significantly lower in these lake-culture lakes (133 m⁻² vs. 265 m⁻² and 0.6 species vs. 0.9 species per sample, respectively). These results suggest that intensive aquaculture in these lakes has had serious negative effects on submerged vegetation and the associated seed banks. Vegetation history was partly reflected by distribution patterns of seed banks across sediment depth strata. A principal component analysis produced a very clear separation of lakes from pen-culture and lake-culture on the basis of their species composition. Moreover, the principal component analysis also indicated that the variation in the soil seed bank corresponded poorly with vegetation data. This probably reflects species-specific strategies for seed production.     

Zooplankton, phytoplankton and the microbial food web in two turbid and two clearwater shallow lakes in Belgium

Components of the pelagic food web in four eutrophic shallow lakes in two wetland reserves in Belgium (Blankaart and De Maten) were monitored during the course of 1998–1999. In each wetland reserve, a clearwater and a turbid lake were sampled. The two lakes in each wetland reserve had similar nutrient loadings and occurred in close proximity of each other. In accordance with the alternative stable states theory, food web structure differed strongly between the clearwater and turbid lakes. Phytoplankton biomass was higher in the turbid than the clearwater lakes. Whereas chlorophytes dominated the phytoplankton in the turbid lakes, cryptophytes were the most important phytoplankton group in the clearwater lakes. The biomass of microheterotrophs (bacteria, heterotrophic nanoflagellates and ciliates) was higher in the turbid than the clearwater lakes. Biomass and community composition of micro- and macrozooplankton was not clearly related to water clarity. The ratio of macrozooplankton to phytoplankton biomass – an indicator of zooplankton grazing pressure on phytoplankton – was higher in the clearwater when compared to the turbid lakes. The factors potentially regulating water clarity, phytoplankton, microheterotrophs and macrozooplankton are discussed. Implications for the management of these lakes are discussed.     

Abrupt shift from clear to turbid state in a shallow eutrophic, biomanipulated lake

Monitoring data were used to assess causes behind a recent shift from a clear-water to a turbid-water state in Lake Major, a 10 ha shallow lake in Hungary. In 1999–2000, fish manipulation was conducted in this hypertrophic lake. Reduced fish stock resulted in clearing water and the development of a dense (>80% coverage) submerged vegetation in 2005. During the recent abrupt shift, which occurred in 2007, submerged vegetation subsequently declined after a two-year period of clear water and abundant vegetation. An intense decay of macrophytes within the lake produced a rapid transition between the clear- and turbid-water states. During the clear-water state in 2005–2006, the most important variables predominantly correlating with macrophyte cover were Secchi transparency, temperature and TN, while TN, temperature, Secchi depth and chlorophyll-a were the most significant variables during the turbid-water state in 2007. Nitrogen may play a significant role in the cover of submerged macrophytes when TP is moderate. We argue that several factors in concert are necessary to initiate a shift. Water temperature likely has contributed to triggering shift through inter-year-dependent changes in cover of macrophytes, with fish recruitment having key roles in the dynamics of shallow lakes. Handling editor: Luigi Naselli-Flores     

Relations between trophic state indicators and plant biomass in Florida lakes

We collected quantitative data on macrophyte abundance and water quality in 319 mostly shallow, polymictic, Florida lakes to look for relationships between trophic state indicators and the biomasses of plankton algae, periphyton, and macrophytes. The lakes ranged from oligotrophic to hypereutrophic with total algal chlorophylls ranging from 1 to 241 mg m^–3. There were strong positive correlations between planktonic chlorophylls and total phosphorus and total nitrogen, but there were weak inverse relationships between the densities of periphyton and the trophic state indicators total phosphorus, total nitrogen and algal chlorophyll and a positive relationship with Secchi depth. There was no predictable relationship between the abundance of emergent, floating-leaved, and submersed aquatic vegetation and the trophic state indicators. It was only at the highest levels of nutrient concentrations that submersed macrophytes were predictably absent and the lakes were algal dominated. Below these levels, macrophyte abundance could be high or low. The phosphorus–chlorophyll and phosphorus–Secchi depth relationships were not influenced by the amounts of aquatic vegetation present indicating that the role of macrophytes in clearing lakes may be primarily to reduce nutrient concentrations for a given level of loading. Rather than nutrient concentrations controlling macrophyte abundance, it seems that macrophytes acted to modify nutrient concentrations.     

Climate-related differences in the dominance of submerged macrophytes in shallow lakes

It has been suggested that shallow lakes in warm climates have a higher probability of being turbid, rather than macrophyte dominated, compared with lakes in cooler climates, but little field evidence exists to evaluate this hypothesis. We analyzed data from 782 lake years in different climate zones in North America, South America, and Europe. We tested if systematic differences exist in the relationship between the abundance of submerged macrophytes and environmental factors such as lake depth and nutrient levels. In the pooled dataset the proportion of lakes with substantial submerged macrophyte coverage (> 30% of the lake area) decreased in a sigmoidal way with increasing total phosphorus (TP) concentration, falling most steeply between 0.05 and 0.2 mg L⁻¹. Substantial submerged macrophyte coverage was also rare in lakes with total nitrogen (TN) concentrations above 1–2 mg L⁻¹, except for lakes with very low TP concentrations where macrophytes remain abundant until higher TN concentrations. The deviance reduction of logistic regression models predicting macrophyte coverage from nutrients and water depth was generally low, and notably lowest in tropical and subtropical regions (Brazil, Uruguay, and Florida), suggesting that macrophyte coverage was strongly influenced by other factors. The maximum TP concentration allowing substantial submerged macrophyte coverage was clearly higher in cold regions with more frost days. This is in agreement with other studies which found a large influence of ice cover duration on shallow lakes' ecology through partial fish kills that may improve light conditions for submerged macrophytes by cascading effects on periphyton and phytoplankton. Our findings suggest that, in regions where climatic warming is projected to lead to fewer frost days, macrophyte cover will decrease unless the nutrient levels are lowered.     

Reduction of benthic macroinvertebrates due to waterfowl foraging on submerged vegetation during autumn migration

If present in large numbers, as during migration, herbivorous waterfowlmay reduce the amount of submerged vegetation. Because the vegetation is a keyfactor in shallow eutrophic lakes, removal of the green biomass can be expectedto affect also other biota that depend on the vegetation. We conducted anexperiment to determine how the abundance of chironomids andPisidium sp. were affected by intense foraging ofwaterfowlon the submerged plant Potamogeton pectinatus. This wasdone in Lake Ringsjön in southern Sweden, during the autumn migration ofthe birds. Three treatments, replicated six times, were used: (i) closed cagesthat excluded all waterfowl, (ii) semi-open cages that excluded only largewaterfowl (geese and swans), and (iii) open plots where all waterfowl couldfreely enter. Waterfowl densities were monitored during the experiment. Theresults suggest that the foraging of large waterfowl (swans) had a clearlynegative effect on macroinvertebrate abundance and aboveground biomass ofP. pectinatus. At the end of the experiment, the densityofchironomids was about 46% lower in the open than in the closed cages. Ingeneral, the density of Pisidium sp. tended to be lower inthe open plots. Small waterfowl alone did not seem to affect either thevegetation or macroinvertebrates. We suggest that thePisidium sp. was influenced at an early stage of grazing,when waterfowl foraged on aboveground biomass, whereas chironomids wereaffectedat a later stage, when swans were digging for below-ground tubers.     

Ambiguous climate impacts on competition between submerged macrophytes and phytoplankton in shallow lakes

1. Shallow lakes may switch from a state dominated by submerged macrophytes to a phytoplankton‐dominated state when a critical nutrient concentration is exceeded. We explore how climate change may affect this critical nutrient concentration by linking a graphical model to data from 83 lakes along a large climate gradient in South America. 2. The data indicate that in warmer climates, submerged macrophytes may tolerate more underwater shade than in cooler lakes. By contrast, the relationship between phytoplankton biomass [approximated by chlorophyll‐a (chl‐a) or biovolume] and nutrient concentrations did not change consistently along the climate gradient. In warmer climates, the correlation between phytoplankton biomass and nutrient concentrations was overall weak, especially at low total phosphorus (TP) concentrations where the chl‐a/ TP ratio could be either low or high. 3. Although the enhanced shade tolerance of submerged plants in warmer lakes might promote the stability of their dominance, the potentially high phytoplankton biomass at low nutrient concentrations suggests an overall low predictability of climate effects. 4. We found that near‐bottom oxygen concentrations are lower in warm lakes than in cooler lakes, implying that anoxic P release from eutrophic sediment in warm lakes likely causes higher TP concentrations in the water column. Subsequently, this may lead to a higher phytoplankton biomass in warmer lakes than in cooler lakes with similar external nutrient loadings. 5. Our results indicate that climate effects on the competitive balance between submerged macrophytes and phytoplankton are not straightforward.     

Phytoplankton and primary production in clear-vegetated,inorganic-turbid,and algal-turbid shallow lakes from the pampa plain (Argentina)

Shallow lakes often alternate between two possible states: one clear with submerged macrophytes, and another one turbid, dominated by phytoplankton. A third type of shallow lakes, the inorganic turbid, result from high contents of suspended inorganic material, and is characterized by low phytoplankton biomass and macrophytes absence. In our survey, the structure and photosynthetic properties (based on ¹⁴C method) of phytoplankton were related to environmental conditions in these three types of lakes in the Pampa Plain. The underwater light climate was characterized. Clear-vegetated lakes were more transparent (K _d 4.5–7.7 m⁻¹), had high DOC concentrations (>45 mg l⁻¹), low phytoplankton Chl a (1.6–2.7 μg l⁻¹) dominated by nanoflagellates. Phytoplankton productivity and photosynthetic efficiency (α ~ 0.03 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²) were relatively low. Inorganic-turbid lakes showed highest K _d values (59.8–61.4 m⁻¹), lowest phytoplankton densities (dominated by Bacillariophyta), and Chl a ranged from 14.6 to 18.3 μg l⁻¹. Phytoplankton-turbid lakes showed, in general, high K _d (4.9–58.5 m⁻¹) due to their high phytoplankton abundances. These lakes exhibited the highest Chl a values (14.2–125.7 μg l⁻¹), and the highest productivities and efficiencies (maximum 0.56 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²). Autotrophic picoplankton abundance, dominated by ficocianine-rich picocyanobacteria, differed among the shallow lakes independently of their type (0.086 × 10⁵–41.7 × 10⁵ cells ml⁻¹). This article provides a complete characterization of phytoplankton structure (all size fractions), and primary production of the three types of lakes from the Pampa Plain, one of the richest areas in shallow lakes from South America. Handling editor: J. Padisak     

The structuring role of free-floating versus submerged plants in a subtropical shallow lake

In shallow temperate lakes many ecological processes depend on submerged macrophytes. In subtropical and tropical lakes, free-floating macrophytes may be equally or more important. We tested the hypothesis that different macrophyte growth forms would be linked with different bottom-up and top-down mechanisms in out-competing phytoplankton. We compared experimentally the effects of submerged and free-floating plants on water chemistry, phytoplankton biomass, zooplankton and fish community structure in a shallow hypertrophic lake (Lake Rodó, 34°55S 56°10W, Uruguay). Except for the retention of suspended solids, we found no other significant bottom-up process connected with either Eichhornia crassipes or Potamogeton pectinatus. Free-floating plants had a lower abundance of medium-sized zooplankton than any other microhabitat and submerged plants were apparently preferred by microcrustaceans. Fish showed a differential habitat use according to species, size-class and feeding habits. Dominant omnivore-planktivores, particularly the smallest size classes, preferred submerged plants. In contrast, omnivore-piscivores were significantly associated with free-floating plants. The density of omnivorous-planktivorous fish, by size class, significantly explained the distribution of medium-sized zooplankton, the high number of size 0 fish being the main factor. The abiotic environment and the structure of the zooplankton community explained little of the fish distribution pattern. Our results suggest that bottom-up effects of free-floating plants are weak when cover is low or intermediate. Top-down effects are complex, as effects on zooplankton and fish communities seem contradictory. The low piscivores:planktivores ratio in all microhabitats suggests, however, that cascading effects on phytoplankton through free-floating plant impacts on piscivorous fish are unlikely to be strong.