Phytoplankton-dominated State Research Articles

Regime shifts in shallow lakes explained by critical turbidity

Worldwide, water quality managers target a clear, macrophyte-dominated state over a turbid, phytoplankton-dominated state in shallow lakes. The competition mechanisms underlying these ecological states were explored in the 1990s, but the concept of critical turbidity seems neglected in contemporary water quality models. In particular, a simple mechanistic model of alternative stable states in shallow lakes accounting for resource competition mechanisms and critical turbidity is lacking. To this end, we combined Scheffer's theory on critical turbidity with insights from nutrient and light competition theory founded by Tilman, Huisman and Weissing. This resulted in a novel graphical and mathematical model, GPLake-M, that is relatively simple and mechanistically understandable and yet captures the essential mechanisms leading to alternative stable states in shallow lakes. The process-based PCLake model was used to parameterize the model parameters and to test GPLake-M using a pattern-oriented strategy. GPLake-M's application range and position in the model spectrum are discussed. We believe that our results support the fundamental understanding of regime shifts in shallow lakes and provide a starting point for further mechanistic and management-focused explorations and model development. Furthermore, the concept of critical turbidity and the relation between light-limited submerged macrophytes and nutrient-limited phytoplankton might provide a new focus for empirical aquatic ecological research and water quality monitoring programs.

A new technique for quantifying algal bloom, floating/emergent and submerged vegetation in eutrophic shallow lakes using Landsat imagery

With the increasing problem of eutrophication, lacustrine ecosystem can undergo complex changes, often resulting in a shift from a clear macrophyte-dominated state to a turbid phytoplankton-dominated state. However, it's not clear how lake transitions occur at regional and global scales. This is due to a lack of long-term monitoring and an effective algorithm that can distinguish floating/emergent aquatic vegetation (FEAV), submerged aquatic vegetation (SAV) and algal bloom (AB). This paper presents a novel three-step classification algorithm based on Landsat imagery, (ie., vegetation and bloom indices (VBI) algorithm): (1) to distinguish between aquatic vegetation (AV) and non-aquatic vegetation (non-AV) extents by an aquatic vegetation index (AVI) derived from tasseled cap transformation; (2) to identify FEAV and SAV within AV extent by using the normalized difference vegetation index (NDVI), and (3) to extract AB from non-AV extent with floating algae index (FAI). The performance of the VBI algorithm was validated using 1307 field samples collected across 22 lakes in the middle and lower reaches of the Yangtze River (MLY) in China, with a pooled overall accuracy of 84.49%. Finally, the VBI algorithm was applied to Landsat data from 1985 to 2021 to quantify the temporal changes of FEAV, SAV and AB in large lakes (i.e., > 50 km2) along the MLY. Within these lakes, AV showed a significant decrease over the past 37 years, mainly due to the decrease of SAV; while AB occurred with higher frequency and in more lakes. The transition from a macrophyte-dominated state to a phytoplankton-dominated aquatic systems is still ongoing in the MLY area. This study shows outstanding innovation in the construction of a new classification index (i.e., AVI), and the method of calculating the adaptive threshold, which makes VBI algorithm have great potential in mapping long-term changes and capturing transformation between lake communities on a global scale.

Warming lowers critical thresholds for multiple stressor–induced shifts between aquatic primary producers

In aquatic ecosystems, excessive nutrient loading is a global problem that can induce regime shifts from macrophyte- to phytoplankton-dominated states with severe consequences for ecosystem functions. Most agricultural landscapes are sites of nutrient and pesticide loading, which can interact with other stressors (e.g., warming) in additive, antagonistic, synergistic or reversed forms. The effects of multiple stressors on the resilience of macrophyte-dominated states and on critical thresholds for regime shifts are, however, unknown. We test the effects of individual and combined stressors of warming, nitrate, and various pesticides typically found in agricultural run-off (ARO) on the growth of macrophytes, periphyton, and phytoplankton in microcosms. We applied a one-level replicated design to test whether ARO induces a regime shift and a multifactorial dose–response design to model stressor thresholds and disentangle stressor interactions along a gradient. The individual stressors did not induce a regime shift, but the full ARO did. Nitrate and pesticides acted synergistically, inducing a shift with increasing phytoplankton biomass and decreasing macrophyte biomass. Warming amplified this effect and lowered critical thresholds for regime shifts. Shallow aquatic ecosystems in agricultural landscapes affected by global warming thus increasingly risk shifting to a turbid, phytoplankton-dominated state, and negatively impacting ecosystem service provisioning. Multiple stressor interactions must be considered when defining safe operating spaces for aquatic systems.

РАСТИТЕЛЬНЫЙ ПОКРОВ МЕЛКОВОДНОГО ГИПЕРТРОФНОГО ОЗЕРА В РАЗЛИЧНЫХ ГИДРОЛОГИЧЕСКИХ УСЛОВИЯХ

Comparison of the distribution of aquatic vegetation in shallow hypertrophic Lake Nero (Yaroslavl oblast, Russia) in different hydrological conditions – during low water phase in 2002–2004 and water level rise in 2017 is presented. The total area covered by aquatic vegetation increased from 14.8 km2 in 2000–2004 to 15.8 km 2 in 2017. Correspondingly, the lake area covered by macrophytes showed its increase from 27% to 29%. The observed increase in the total plant coverage was mainly driven by helophytes which increased in the coverage area from 8.4 km2 to 11.2 km2. The coverage area of submerged vegetation during this period decreased from 5.4 km2 to 3.6 km2. Potamogeton perfoliatus showed the highest reduction in the coverage area by factor of 2.6 in the central part of the lake. In contrast, the coverage area of Ceratophyllum demersum and Myriophyllum verticillatum did not change mostly in the shallow, southern part of the lake. The analysis of literature and our data revealed that increase in water level due to building a dam on the River Veksa in the 1980s and impact of climatic factors had a negative influence on submerged vegetation. Thus, the high water level is one of the mechanisms stabilising the ecosystem of the Lake Nero in the turbid, phytoplankton-dominated state.

Pelagic energy flow supports the food web of a shallow lake following a dramatic regime shift driven by water level changes

Across the globe, lake ecosystems are exposed to a variety of human disturbances. A notable example is shallow lakes where human-induced eutrophication or water level fluctuation may result in a switch from a clear-water, macrophyte-dominated state to a turbid, phytoplankton-dominated state. Yet, few investigations have described synchronous changes in biotic assemblage composition and food web framework under such a shift between alternative states. We used stable carbon and nitrogen isotopes to test the extent to which switching from macrophyte to phytoplankton dominance in Lake Gucheng, triggered by a water level increase, would alter ecosystem structure and change the basal resources supporting the food web. We found that invertebrates and fish compensated for a reduction of macrophyte and epiphyte resources by deriving more energy from the alternative pelagic energy channel, where benthic invertebrates act as crucial links between primary producers and higher consumers by transporting δ13C-depleted pelagic algae to the benthic zone. Although consumers can respond to large shifts in energy allocation and stabilize food web dynamics through their ability to feed across multiple energy pathways, our study suggest that energy subsidies may promote trophic cascades and enhance the stability of the turbid regime.

Sedimentary Diatom Records Reveal the Succession of Ecosystem in Lake Xihu, Dali over the Past 50 Years

In recent decades, intense human activities have caused a decline in many lake ecosystems in Yunnan Province, rendering the transformation of the lake from a clear macrophyte-dominated state to a turbid phytoplankton-dominated state. Improved understanding of the ecological changes in lake ecosystem has significant implications for management. In this study, a small lake in Dali Prefecture of Yunnan Province, i.e., Lake Xihu, was selected. Combined with diatom records and physicochemical proxies from the lake sediments, this paper focuses on the long-term ecological changes in Lake Xihu, Dali since the mid-1960s. The results show that the Lake Xihu, Dali has undergone a significant shift in stable states over the past 50 years. Prior to 2000, the benthic-epiphytic species (i.e., Cocconeis placentula, Staurosira construens, Gomphonema angustum, and Achnanthidium minutissimum) dominated in diatom assemblages, indicating oligotrophic conditions; since 2000, benthic-epiphytic species (i.e., Encyonopsis microcephala and Navicula cryptocephala) and planktonic species (i.e., Cyclotella atomus, Cyclotella meneghiniana, Stephanodiscus hantzschii, and Aulacoseira granulata) dominated successively, indicating mesotrophic to eutrophic conditions. Principal component analysis based on the diatom assemblages in temporal scale showed the response of diatoms succession to nutrients. Redundancy analysis also confirmed that nutrient enrichment was the main driving force for the succession of diatom assemblages in Lake Xihu, Dali. In the past 50 years, climate change and human activities (i.e., agricultural reclamation, fertilization, animal husbandry, and fishery) have enhanced the accumulation of nutrients in the lake. The continuous loading of nutrients promoted the propagation of planktonic algae, and also the productivity of the lake, rendering the transformation to a turbid phytoplankton-dominated state.

Exploring How Cyanobacterial Traits Affect Nutrient Loading Thresholds in Shallow Lakes: A Modelling Approach

Globally, many shallow lakes have shifted from a clear macrophyte-dominated state to a turbid phytoplankton-dominated state due to eutrophication. Such shifts are often accompanied by toxic cyanobacterial blooms, with specialized traits including buoyancy regulation and nitrogen fixation. Previous work has focused on how these traits contribute to cyanobacterial competitiveness. Yet, little is known on how these traits affect the value of nutrient loading thresholds of shallow lakes. These thresholds are defined as the nutrient loading at which lakes shift water quality state. Here, we used a modelling approach to estimate the effects of traits on nutrient loading thresholds. We incorporated cyanobacterial traits in the process-based ecosystem model PCLake+, known for its ability to determine nutrient loading thresholds. Four scenarios were simulated, including cyanobacteria without traits, with buoyancy regulation, with nitrogen fixation, and with both traits. Nutrient loading thresholds were obtained under N-limited, P-limited, and colimited conditions. Results show that cyanobacterial traits can impede lake restoration actions aimed at removing cyanobacterial blooms via nutrient loading reduction. However, these traits hardly affect the nutrient loading thresholds for clear lakes experiencing eutrophication. Our results provide references for nutrient loading thresholds and draw attention to cyanobacterial traits during the remediation of eutrophic water bodies.

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

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

Exploring the influence of hydrology on the threshold phosphorus-loading rate in shallow lakes

Abstract The Alternative Stable States Model describes the two contrasting conditions of shallow lakes that occur in response to external phosphorus (P) loading: 1) a clear-water, macrophyte-dominated state during times of lesser external P loading, and 2) a turbid, phytoplankton-dominated state during periods of greater P loading. Determination of the P loading rate that distinguishes the two stable states, i.e. the threshold phosphorus loading rate (TPL), is fundamental to shallow-lake science and management. We used data from nine shallow subtropical lakes in Florida (USA) to generate a parsimonious model that predicts the TPL from the hydraulic detention time (HDT). The model reveals that the TPL is more sensitive to changes in HDT in lakes with short ( 10 years) HDTs. We compared our results with published data from 54 geographically distributed shallow aquatic systems and found that the Florida-based HDT-TPL model has general applicability. Given many lakes worldwide are shallow, our findings have the potential to improve understanding and management of numerous aquatic ecosystems around the globe.

Stocking of herbivorous fish in eutrophic shallow clear-water lakes to reduce standing height of submerged macrophytes while maintaining their biomass

To balance the conservation value versus recreational use of shallow lakes, moderate herbivory may be needed in eutrophic lakes to avoid near surface growth while maintaining high vegetation biomass close to the sediment. However, over-grazing or even complete elimination of macrophytes by grass carp (Ctenopharyngodon idella) commonly used for control purposes has often been observed, leading to a shift from a clear to a turbid phytoplankton-dominated state. We hypothesized that slow-growing and smaller-sized herbivorous fish species might be more suitable than grass carp to obtain the desired moderate control because they consume the top part of the vegetation without severely affecting the lower plant parts. To test the hypothesis, the effects of Wuchang bream (Megalobrama amblycephala), an endemic medium-sized herbivorous cyprinid, and grass carp on the biomass, density and trait of the macrophyte Vallisneria denseserrulata were compared in an enclosure experiment. We found that V. denseserrulata grew less tall but did not lose biomass under moderate herbivory by Wuchang bream due to increased plant density and leaf weight per length, whereas excessive herbivory by grass carp had strong negative effects on the plant biomass. Moreover, the plant had more and thicker leaves in the fish treatments than in the fishless controls. The growth of grass carp was much faster than that of Wuchang bream. Our findings suggest that stocking of Wuchang bream in proper densities may be more useful than grass carp for the management of V. denseserrulata and likely also other macrophyte species. More tests, especially at different fish densities are, however, needed to draw any firm conclusions regarding this hypothesis.

Natural and anthropogenic forcing of Holocene lake ecosystem development at Lake Uddelermeer (The Netherlands)

Lake Uddelermeer (The Netherlands) is characterized by turbid conditions and annual blooms of toxic cyanobacteria, which are supposed to be the result of increased agricultural activity in the twentieth century AD. We applied a combination of classic palaeoecological proxies and novel geochemical proxies to the Holocene sediment record of Lake Uddelermeer (The Netherlands) in order to reconstruct the natural variability of the lake ecosystem and to identify the drivers of the change to the turbid conditions that currently characterize this lake. We show that the lake ecosystem was characterized by a mix of aquatic macrophytes and abundant phytoplankton between 11,500 and 6000 cal year BP. A transition to a lake ecosystem with clear-water conditions and relatively high abundances of ‘isoetids’ coincides with the first signs of human impact on the landscape around Lake Uddelermeer during the Early Neolithic (ca. 6000 cal year BP). An abrupt and dramatic ecosystem shift can be seen at ca. 1030 cal year BP when increases in the abundance of algal microfossils and concentrations of sedimentary pigments indicate a transition to a turbid phytoplankton-dominated state. Finally, a strong increase in concentrations of plant and faecal biomarkers is observed around 1950 AD. Canonical Correspondence Analysis suggests that reconstructed lake ecosystem changes are best explained by environmental drivers that show long-term gradual changes (sediment age, water depth). These combined results document the long-term anthropogenic impact on the ecosystem of Lake Uddelermeer and provide evidence for pre-Industrial Era signs of eutrophication.

Twenty-three-year timeline of ecological stable states and regime shifts in upper Amazon oxbow lakes

Regime shifts in shallow lakes are often associated with anthropogenic impacts, such as land-use change, non-point source nutrient loading, and overfishing. These shifts have mostly been examined in lakes in temperate and boreal regions and within anthropogenically disturbed basins. Here, it is demonstrated that tropical floodplain lakes in a region of virtually no human disturbance naturally undergo frequent regime shifts. We demonstrate this using satellite imagery to provide a 23-year time series of 22-oxbow lakes or “cochas” along 300 km of the Manu River in SE Peru. In any year, a majority of these lakes is in a macrophyte-free, phytoplankton-dominated state. However, over the 23 years covered by images, roughly a third of the lakes experienced abrupt shifts to a floating macrophyte state. Macrophyte cover persisted for ≤ 3 year. Analysis of water level fluctuations sampled on a subset of the lakes for 1 year suggests that lake isolation from streams and the main river facilitates regime shifts. Multiple forcing factors, both internal and external to the lakes themselves, could drive the observed regime shifts, but insufficient data exist from this remote region to identify the key processes.

Herbivory of Omnivorous Fish Shapes the Food Web Structure of a Chinese Tropical Eutrophic Lake: Evidence from Stable Isotope and Fish Gut Content Analyses

Studies suggest that, unlike the situation in temperate lakes, high biomasses of omnivorous fish are maintained in subtropical and tropical lakes when they shift from a turbid phytoplankton-dominated state to a clear water macrophyte-dominated state, and the predation pressure on large-bodied zooplankton therefore remains high. Whether this reflects a higher degree of herbivory in warm lakes than in temperate lakes is debatable. We combined food web studies using stable isotopes with gut content analyses of the most dominant fish species to elucidate similarities and differences in food web structure between a clear water macrophyte-dominated basin (MDB) and a turbid phytoplankton-dominated basin (PDB) of Huizhou West Lake, a shallow tropical Chinese lake. The δ13C–δ15N biplot of fish and invertebrates revealed community-wide differences in isotope-based metrics of the food webs between MDB and PDB. The range of consumer δ15N (NR) was lower in MDB than in PDB, indicating shorter food web length in MDB. The mean nearest neighbor distance (MNND) and standard deviation around MNND (SDNND) were higher in MDB than in PDB, showing a markedly low fish trophic overlap and a more uneven packing of species in niches in MDB than in PDB. The range of fish δ13C (CR) of consumers was more extensive in MDB than in PDB, indicating a wider feeding range for fish in MDB. Mixing model results showed that macrophytes and associated periphyton constituted a large fraction of basal production sources for the fish in MDB, while particulate organic matter (POM) contributed a large fraction in PDB. In MDB, the diet of the dominant fish species, crucian carp (Carassius carassius), consisted mainly of vegetal matter (macrophytes and periphyton) and zooplankton, while detritus was the most important food item in PDB. Our results suggest that carbon from macrophytes with associated periphyton may constitute an important food resource for omnivorous fish, and this may strongly affect the feeding niche and the strength of the top-down trophic cascade between fish and zooplankton in the restored, macrophyte-dominated basin of the lake. This dual effect (consumption of macrophytes and zooplankton) may reduce the chances of maintaining the clear water state at the prevailing nutrient levels in the lake, and regular removal of large crucian carp may therefore be needed to maintain a healthy ecosystem state.

Repeated Fish Removal to Restore Lakes: Case Study of Lake Væng, Denmark—Two Biomanipulations during 30 Years of Monitoring

Biomanipulation by fish removal has been used in many shallow lakes as a method to improve lake water quality. Here, we present and analyse 30 years of chemical and biological data from the shallow and 16 ha large Lake Væng, Denmark, which has been biomanipulated twice with a 20-year interval by removing roach (Rutilus rutilus) and bream (Abramis brama). After both biomanipulations, Lake Væng shifted from a turbid, phytoplankton-dominated state to a clear, water macrophyte-dominated state. Chlorophyll a was reduced from 60–80 μg·L−1 to 10–30 μg·L−1 and the coverage of submerged macrophytes, dominated by Elodea canadensis, increased from <0.1% to 70%–80%. Mean summer total phosphorus was reduced from about 0.12 to 0.07 mg·L−1 and total nitrogen decreased from 1.0 to 0.4 mg·L−1. On a seasonal scale, phosphorus and chlorophyll concentrations changed from a summer maximum during turbid conditions to a winter maximum under clear conditions. The future of Lake Væng is uncertain and a relatively high phosphorus loading via the groundwater, and the accumulation of a mobile P pool in the sediment make it likely that the lake eventually will return to turbid conditions. Repeated fish removals might be a relevant management strategy to apply in shallow lakes with a relatively high external nutrient loading.

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.

Distribution of planktonic crustaceans and its diurnal changes in a hypertrophic shallow lake: does the switch from turbid-water state to clear-water state matters?

The paper presents results of post hoc analysis of diurnal changes in the crustacean distribution along the horizontal transect in the shallow hypertrophic lake, which undergone a switch from turbid-water, phytoplankton-dominated state (chl-a : 257.8 ± 100.2 µg dm−3 , Secchi depth: 0.17 ± 0.02 m) to clear-water plant-dominated state (chl-a : 26.8 ± 4.8 µg dm−3 , Secchi depth: 1.03 ± 0.29 m). Changes in crustacean biomass were observed in two consecutive years during two July days (every sixth hour, starting with the noon) in three sampling sites (emergent macrophytes, submerged macrophytes and center), situated along the horizontal transect between the lake shore and the center of the lake. Analysis revealed that: (a) in turbid-water conditions crustaceans aggregated at night-time near the water surface, both in the littoral zone and the lake center; (b) in clear-water state a nocturnal increase in crustacean biomass was noted only in submerged macrophyte site, but concerned only cladocerans. In conclusion, we hypothesized that switch from phytoplankton-dominated to plant-dominated state effects in change of predator-avoidance strategy showed by planktonic crustaceans.

The impact of bird herbivory on macrophytes and the resilience of the clear-water state in shallow lakes: a model study

Shallow lakes have the potential to switch between two alternative stable states: a clear macrophyte-dominated and a turbid phytoplankton-dominated state. Observational and experimental studies show that in some lakes herbivory by birds may severely decrease macrophyte biomass, while in other lakes, the removed biomass by herbivory is compensated by regrowth. These contradictory outcomes might arise because of interplay between top-down control by bird herbivory and bottom-up effects by nutrient loading on macrophytes. Here, we use the ecosystem model PCLake to study top-down and bottom-up control of macrophytes by coots and nutrient loading. Our model predicted that (1) herbivory by birds lowers the critical nutrient loading at which the regime shift occurs; (2) bird impact on macrophyte biomass through herbivory increases with nutrient loading; and (3) improved food quality enhances the impact of birds on macrophytes, thus decreasing the resilience of the clear-water state even further. The fact that bird herbivory can have a large impact on macrophyte biomass and can facilitate a regime shift implies that the presence of waterfowl should be taken into account in the estimation of critical nutrient loadings to be used in water quality management.

Macrophyte species strongly affects changes in C, N, and P stocks in shallow lakes after a regime shift from macrophyte to phytoplankton dominance

Shallow lakes are important stocks of carbon (C), nitrogen (N), and phosphorus (P), yet little is known about the influence of alternative primary producer dominance on C stocks or the impact of different macrophyte species on the role of shallow lakes as elemental stocks. We used Yangtze shallow lakes dominated by a monsoon climate as a research site to test the hypothesis that changes in elemental stocks in the water column and sediment after a shift to a phytoplankton-dominated state depend on the macrophyte species originally present. We used a dual approach, combining multi-year monitoring and multi-lake comparisons of lakes that were, at least once, dominated either by fast-decomposing Potamogeton crispus or slow-decomposing P. maackianus. Elemental concentrations generally decreased in the water column and increased in sediment after a shift from P. maackianus presence to absence. Only a minor reallocation of elemental stocks was found in lakes where P. crispus disappeared. This difference is likely caused by a combination of the different biomass and decomposition rates between species, further illustrated by the amount of dead plant material in the sediment after loss of plants. After P. maackianus loss, plant material was found in the sediment in high amounts for up to 6 years, whereas after P. crispus loss the coarse material was absent in <1 year. Suspended and dissolved concentrations (i.e., the mobile pool) of C increased 1.5–1.9-fold and P increased 2.0–4.3-fold after the shift, whereas N tended to decrease or stay unchanged. Higher mobile pools of C and P after macrophytes loss implies a more vulnerable watershed, supporting higher phytoplankton biomass in the lakes and causing serious downstream eutrophication problems.

Geo-engineering experiments in two urban ponds to control eutrophication

Many urban ponds experience detrimental algal blooms as the result of eutrophication. During a two year field experiment, the efficacy of five in situ treatments to mitigate eutrophication effects in urban ponds was studied. The treatments targeted the sediment phosphorus release and were intended to switch the ponds from a turbid phytoplankton-dominated state to a clear-water state with a low phytoplankton biomass. Two eutrophic urban ponds were each divided into six compartments (300–400 m2; 210–700 m3). In each pond the following treatments were tested: dredging in combination with biomanipulation (involving fish biomass control and the introduction of macrophytes) with and without the addition of the flocculant polyaluminiumchloride, interception and reduction of sediment phosphorus release with lanthanum-modified bentonite (Phoslock®) in combination with biomanipulation with and without polyaluminiumchloride; biomanipulation alone; and a control. Trial results support the hypothesis that the combination of biomanipulation and measures targeting the sediment phosphorus release can be effective in reducing the phytoplankton biomass and establishing and maintaining a clear-water state, provided the external phosphorus loading is limited. During the experimental period dredging combined with biomanipulation showed mean chlorophyll-a concentrations of 5.3 and 6.2 μg L−1, compared to 268.9 and 52.4 μg L−1 in the control compartments. Lanthanum-modified bentonite can be an effective alternative to dredging and in combination with biomanipulation it showed mean chlorophyll-a concentrations of 5.9 and 7.6 μg L−1. Biomanipulation alone did not establish a clear-water state or only during a limited period. As the two experimental sites differed in their reaction to the treatments, it is important to choose the most promising treatment depending on site specific characteristics. In recovering the water quality status of urban ponds, continuing attention is required to the concurrent reduction of external phosphorus loading and to maintaining an appropriate fish community.

Ecological classification of lakes: Uncertainty and the influence of year-to-year variability

Regular monitoring of lakes is important to determine their ecological state and development and of key significance when deciding whether action should be taken to improve their quality, for instance by reducing the external loading of nutrients. Imprecise or inadequate knowledge of the ecological state increases the risk of misclassification and of wrong management decisions. Based on Danish lake data, we aimed to determine temporal variations, in particular natural year-to-year differences, and to describe the uncertainty in assessing the ecological state of lakes. We analysed environmental data from ca. 350 Danish lakes (1100 lake years), including three case studies, with long-term data series (up to 24 years), with no significant changes in external nutrient loading. We used summer means of selected water chemical variables, phytoplankton and submerged macrophytes as indicators of ecological state and found considerable variations in all indicators, which could not be ascribed alone to meteorological variation. In shallow lakes, chlorophyll a concentrations exhibited large year-to-year variations, especially at TP ranging between 0.05 and 0.15mgL−1 where the lakes may shift between a macrophyte- and a phytoplankton-dominated state. For example, chlorophyll a varied by a factor 5–10 between years and was particularly low when submerged macrophyte coverage exceeded 20% compared with lakes without macrophytes. Use of a multimetric index including four phytoplankton indicators reduced the coefficient of variation. Generally, the 95% confidence interval of ecological classification was approximately 50% lower when the assessment of ecological state was based on 4–5 years’ measurements than if based on only one year's measurements. Knowledge and awareness of the uncertainty of indicators used in ecological classification are highly relevant for lake managers and policy makers when defining efficient monitoring and restoration strategies.