Effects of Submerged Vegetation on Water Clarity Across Climates

A positive feedback between submerged vegetation and water clarity forms the backbone of the alternative state theory in shallow lakes. The water clearing effect of aquatic vegetation may be caused by different physical, chemical, and biological mechanisms and has been studied mainly in temperate lakes. Recent work suggests differences in biotic interactions between (sub)tropical and cooler lakes might result in a less pronounced clearing effect in the (sub)tropics. To assess whether the effect of submerged vegetation changes with climate, we sampled 83 lakes over a gradient ranging from the tundra to the tropics in South America. Judged from a comparison of water clarity inside and outside vegetation beds, the vegetation appeared to have a similar positive effect on the water clarity across all climatic regions studied. However, the local clearing effect of vegetation decreased steeply with the contribution of humic substances to the underwater light attenuation. Looking at turbidity on a whole-lake scale, results were more difficult to interpret. Although lakes with abundant vegetation (>30%) were generally clear, sparsely vegetated lakes differed widely in clarity. Overall, the effect of vegetation on water clarity in our lakes appears to be smaller than that found in various Northern hemisphere studies. This might be explained by differences in fish communities and their relation to vegetation. For instance, unlike in Northern hemisphere studies, we find no clear relation between vegetation coverage and fish abundance or their diet preference. High densities of omnivorous fish and coinciding low grazing pressures on phytoplankton in the (sub)tropics may, furthermore, weaken the effect of vegetation on water clarity.     

Long Transients Near the Ghost of a Stable State in Eutrophic Shallow Lakes with Fluctuating Water Levels

Alternative stable states in shallow lakes have received much attention over the past decades, but less is known about transient dynamics of such lakes in the face of stochastic perturbations such as incidental extremes in water levels driven by climatic variability. Here, we report on the ecosystem dynamics of 70 lakes in the floodplains of the Lower Rhine in The Netherlands from 1999 to 2004. In any particular year, most lakes were either in a macrophyte-dominated clear state or in a contrasting state with turbid water and sparse submerged macrophyte cover. Macrophyte dominance was positively related to the occurrence of drawdown, and negatively to lake surface area and mean depth. We did not find a relation with nutrient levels. Remarkably, shifts between the two contrasting states were common, and episodes of low water levels appear to be an important external driver. A dry period before our study and the exceptionally dry summer of 2003 caused widespread drawdown of floodplain lakes, resulting in establishment of submerged macrophytes in the next year upon refill. In the 4 years without drawdown, many lakes returned to a macrophyte-poor turbid state. Although some lakes turned turbid again quickly, others took several years to shift into the turbid state. A model analysis suggests that such prolonged transient vegetated states may be explained by the fact that the system dynamics slow down in the vicinity of the “almost stable” macrophyte-dominated state. Such a “ghost” of an equilibrium causes the system to stick around that state relatively long before slipping into the only true stable state. Our results support the idea that transient dynamics rather than equilibrium may be the key to understanding the overall state of some ecosystems. A practical implication of our findings is that artificial stabilization of the water level in shallow lakes may have been an important factor aggravating the permanent loss of submerged macrophytes due to cultural eutrophication.     

Scaling of Pelagic Metabolism to Size,Trophy and Forest Cover in Small Danish Lakes

ABSTRACT We tested whether pelagic light and nutrient availability, metabolism, organic pools and CO₂-supersaturation were related to lake size and surrounding forest cover in late summer–autumn measurements among 64 small (0.02–20 ha), shallow seepage lakes located in nutrient-rich, calcareous moraine soils in North Zealand, Denmark. We found a strong implicit scaling to lake size as light availability increased significantly with lake size while nutrient availability, phytoplankton biomass and dissolved organic matter declined. Forest lakes had significantly stronger net heterotrophic traits than open lakes as higher values were observed for light attenuation above and in the water, dissolved organic matter, pelagic community respiration (R) relative to maximum gross primary production (R/GPP) and CO₂-supersaturation. Total-phosphorus was the main predictor of phytoplankton biomass (Chl) despite a much weaker relationship than observed in previous studies of larger lakes. Maximum gross primary production increased with algal biomass and decreased with dissolved organic matter, whereas community respiration increased with dissolved organic matter and particularly with gross primary production. These results suggest that exogenous organic matter supplements primary production as an energy source to heterotrophs in these small lakes, and particularly so in forest lakes experiencing substantial shading from the forest and dissolved humic material. This suggestion is supported by 20–30-fold CO₂ supersaturation in the surface water of the smallest forest lakes and more than sixfold supersaturation in 75% of all measurements making these lakes among the most supersaturated temperate lakes examined so far.     

Shallow lake restoration by nutrient loading reduction—some recent findings and challenges ahead

Shallow lakes respond to nutrient loading reductions. Major findings in a recent multi-lake comparison of data from lakes with long time series revealed: that a new state of equilibrium was typically reached for phosphorus (P) after 10–15 years and for nitrogen (N) after <5–10 years; that the in-lake Total N:Total P and inorganic N:P ratios increased; that the phytoplankton and fish biomass often decreased; that the percentage of piscivores often increased as did the zooplankton:phytoplankton biomass ratio, the contribution of Daphnia to zooplankton biomass, and cladoceran size. This indicates that enhanced resource and predator control often interact during recovery from eutrophication. So far, focus has been directed at reducing external loading of P. However, one experimental study and cross-system analyses of data from many lakes in north temperate lakes indicate that nitrogen may play a more significant role for abundance and species richness of submerged plants than usually anticipated when total phosphorus is moderate high. According to the alternative states hypothesis we should expect ecological resistance to nutrient loading reduction and P hysteresis. We present results suggesting that the two alternative states are less stable than originally anticipated. How global warming affects the water clarity of shallow lakes is debatable. We suggest that water clarity often will decrease due to either enhanced growth of phytoplankton or, if submerged macrophytes are stimulated, by reduced capacity of these plants to maintain clear-water conditions. The latter is supported by a cross-system comparison of lakes in Florida and Denmark. The proportion of small fish might increase and we might see higher aggregation of fish within the vegetation (leading to loss of zooplankton refuges), more annual fish cohorts, more omnivorous feeding by fish and less specialist piscivory. Moreover, lakes may have prolonged growth seasons with a higher risk of long-lasting algal blooms and at places dense floating plant communities. The effects of global warming need to be taken into consideration by lake managers when setting future targets for critical loading, as these may well have to be adjusted in the future. Finally, we highlight some of the future challenges we see in lake restoration research.     

Impact of fish predation on cladoceran body weight distribution and zooplankton grazing in lakes during winter

1. It is well accepted that fish, if abundant, can have a major impact on the zooplankton community structure during summer, which, particularly in eutrophic lakes, may cascade to phytoplankton and ultimately influence water clarity. Fish predation affects mean size of cladocerans and the zooplankton grazing pressure on phytoplankton. Little is, however, known about the role of fish during winter. 2. We analysed data from 34 lakes studied for 8–9 years divided into three seasons: summer, autumn/spring and winter, and four lake classes: all lakes, shallow lakes without submerged plants, shallow lakes with submerged plants and deep lakes. We recorded how body weight of Daphnia and then cladocerans varied among the three seasons. For all lake types there was a significant positive correlation in the mean body weight of Daphnia and all cladocerans between the different seasons, and only in lakes with macrophytes did the slope differ significantly from one (winter versus summer for Daphnia). 3. These results suggest that the fish predation pressure during autumn/spring and winter is as high as during summer, and maybe even higher during winter in macrophyte‐rich lakes. It could be argued that the winter zooplankton community structure resembles that of the summer community because of low specimen turnover during winter mediated by low fecundity, which, in turn, reflects food shortage, low temperatures and low winter hatching from resting eggs. However, we found frequent major changes in mean body weight of Daphnia and cladocerans in three fish‐biomanipulated lakes during the winter season. 4. The seasonal pattern of zooplankton : phytoplankton biomass ratio showed no correlation between summer and winter for shallow lakes with abundant vegetation or for deep lakes. For the shallow lakes, the ratio was substantially higher during summer than in winter and autumn/spring, suggesting a higher zooplankton grazing potential during summer, while the ratio was often higher in winter in deep lakes. Direct and indirect effects of macrophytes, and internal P loading and mixing, all varying over the season, might weaken the fish signal on this ratio. 5. Overall, our data indicate that release of fish predation may have strong cascading effects on zooplankton grazing on phytoplankton and water clarity in temperate, coastal situated eutrophic lakes, not only during summer but also during winter.     

Bimodal transparency as an indicator for alternative states in South American lakes

1. The alternative state theory claims that shallow lakes may have either clear water, and be dominated by submerged macrophytes, or turbid water and be dominated by phytoplankton. Most evidence for this theory comes from studies in temperate or boreal regions of Europe. Because of differences in the strength of trophic interactions, such as in the pressure of zooplankton grazing on phytoplankton, this influential theory might not apply elsewhere. 2. Here, we test the theory for South American lakes, combining field data and Landsat satellite data. We studied the frequency distribution of primary producers and water transparency, looking for potential bimodality separating clear and turbid lakes. A bimodal distribution might be observed if there are indeed alternative states, although would not itself be sufficient evidence for the theory. Possible shifts between alternative states were analysed by comparing satellite data from 1987 to 2005. 3. In our field data, there was a bimodal pattern in phytoplankton abundance and possibly in the abundance of submerged macrophytes, but not in water transparency. Analyses of the larger satellite data set revealed bimodality in lake transparency in 2005, but less so in 1987. In 1987, the lakes were generally clearer, and the transition to higher turbidity was more gradual than in 2005. The stronger bimodality in the more recent data, and the overall lower transparency, could have been caused by an increase in fertiliser use and subsequent eutrophication but also by differences in hydrology. Further, 1987 was much wetter than 2005, which could have caused dilution of suspended particles, leading to clearer water. 4. While a bimodal distribution in the abundance of primary producers and water clarity is not decisive evidence for or against the theory of alternative states, our data clearly fail to refute it.     

Temporal Changes between Ecological Regimes in a Range of Primary and Secondary Salinised Wetlands

Many rivers and wetlands in south-western Australia are threatened by salinisation due to rising saline watertables, which have resulted from land clearing and the replacement of deep-rooted perennial species with shallow-rooted annual species. A four to six weekly sampling program of water quality, submerged macrophytes and macroinvertebrates was undertaken at six wetlands, from September 2002 to February 2004, to investigate seasonal variation in a range of primary and secondary saline systems. The wetlands dried and filled at different times in response to local rainfall patterns, and salinities varied accordingly with evapoconcentration and dilution. Two types of clear-water wetlands were recognised; those dominated by submerged aquatic macrophytes (Ruppia, Lepilaena and Lamprothamnium) and those dominated by benthic microbial communities. Two types of turbid wetlands were also recognised; those with high concentrations of phytoplankton and those with high concentrations of suspended sediments. A primary saline lake and two lakes that have only recently been affected by secondary salinisation persisted in a clear, macrophyte-dominated regime throughout most of the study period, except during drying and filling. Two lakes with a long history of secondary salinisation (70 years) moved between regimes over the study period. A clear, benthic microbial community – dominated regime only persisted at the wetland which contained permanent water throughout the study period. The turbid regimes were only present during drying and refilling phases. A richer and more abundant macroinvertebrate fauna was associated with the clear, macrophyte- dominated wetlands. Our results suggest that the development of management guidelines that recognise the presence of different ecological regimes and that consider the interactions between water regime, salinity, and primary and secondary production will be more useful in protecting biodiversity and ecological function in these systems than managing salinity as a single factor.     

Dynamics of phytoplankton pigment characteristics in the Biya head water

Concentrations of phytoplankton photosynthetic pigments were measured at the Biya head water (in the immediate vicinity of Lake Teletskoe) every 10 days from July 1998 through December 2001. Comparison of phytoplankton pigment characteristics at the river’s head waters and at the surface of the lake’s pelagic zone for the ice-free period shows that the monitoring data give a clear picture of the formation and functioning of the lake phytoplankton. Analysis of seasonal fluctuations of pigment characteristics and their proportions revealed characteristic stages in the development of lake algal cenoses. We have demonstrated similarities and differences in the seasonal dynamics of the phytoplankton between Lake Teletskoe, a deep oligotrophic lake in the south of West Siberia, as compared with the general pattern observed in temperate lakes. According to the concentration of chlorophyll a, the trophic status of the lake’s pelagic zone and the Biya head water is ultraoligotrophic-oligotrophic. Yellow:green pigment ratio ranks Lake Teletskoe among “carotenoid lakes“. Low coefficients of linear correlation between chlorophyll a, water level and biogenic matter concentration shows that the relationships analyzed are far more complex than has been inferred from the combined effect of abiotic and biotic factors.     

Maximum growing depth of macrophytes in Loch Leven,Scotland, United Kingdom,in relation to historical changes in estimated phosphorus loading

Eutrophication is common in shallow lakes in lowland areas. In their natural state, most shallow lakes would have clear water and a thriving aquatic plant community. However, eutrophication often causes turbid water, high algal productivity, and low species diversity and abundance of submerged macrophytes. A key indicator of the ecological state of lake ecosystems is the maximum growing depth (MGD) of aquatic plants. However, few studies have yet quantified the relationship between changes in external phosphorus (P) input to a lake and associated variation in MGD. This study examines the relationship between these variables in Loch Leven, a shallow, eutrophic loch in Scotland, UK. A baseline MGD value from 1905 and a series of more recent MGD values collected between 1972 and 2006 are compared with estimated P loads over a period of eutrophication and recovery. The results suggest a close relationship between changes in MGD of macrophytes and changes in the external P load to the loch. Variation in MGD reflected the ‘light history’ that submerged macrophytes had been exposed to over the 5-year period prior to sampling, rather than responding to short term, within year, variations in water clarity. This suggests that changes in macrophyte MGD may be a good indicator of overall, long term, changes in water quality that occur during the eutrophication and restoration of shallow lakes.     

A Theory for Cyclic Shifts between Alternative States in Shallow Lakes

Some shallow lakes switch repeatedly back and forth between a vegetation dominated clear-water state and a contrasting turbid state. Usually such alternations occur quite irregularly, but in some cases the switches between states are remarkably regular. Here we use data from a well-studied Dutch lake and a set of simple models to explore possible explanations for such cyclic behavior. We first demonstrate from a graphical model that cycles may in theory occur if submerged macrophytes promote water clarity in the short run, but simultaneously cause an increased nutrient retention, implying an accumulation of nutrients in the long run. Thus, although submerged plants create a positive feedback on their own growth by clearing the water, they may in the long run undermine their position by creating a slow “internal eutrophication”. We explore the potential role of two different mechanisms that may play a role in this internal eutrophication process using simulation models: (1) reduction of the P concentration in the water column by macrophytes, leading to less outflow of P, and hence to a higher phosphorus accumulation in the lake sediments and (2) a build-up of organic matter over time resulting in an increased sediment oxygen demand causing anaerobic conditions that boost P release from the sediment. Although the models showed that both mechanisms can produce cyclic behavior, the period of the cycles caused by the build-up of organic material seemed more realistic compared to data of the Dutch Lake Botshol in which regular cycles with a period of approximately 7 years have been observed over the past 17 years.     

Temporal variability in algal biomass and primary productivity in Florida lakes relative to latitudinal gradients,organic color and trophic state

Seasonal patterns in primary productivity and algal biomass in subtropical Florida lakes along increasing gradients of both dissolved organic color and phytoplankton biomass are presented. Chlorophyll a concentrations and gross primary productivity generally reached maxima during the summer and were most depressed in winter months, regardless of color or trophic classification. Primary productivity was more strongly correlated with chlorophyll a, nutrient concentrations and water clarity in clearwater (< 75 Pt units) than in colored (> 75 Pt units) systems. Amplitudes in algal biomass were considerably smaller than temperate lakes. Variability in primary production in Florida lakes was intermediate to patterns in the temperate zone and tropics, but was more closely aligned to northern latitudes. Within the Florida peninsula, variability of primary productivity decreased from north to south and corresponded to latitudinal gradients in climatic regimes.     

Relative importance of periphyton and phytoplankton in turbid and clear vegetated shallow lakes from the Pampa Plain (Argentina): a comparative experimental study

We analyzed experimentally the relative contribution of phytoplankton and periphyton in two shallow lakes from the Pampa Plain (Argentina) that represent opposite scenarios according to the alternative states hypothesis for shallow lakes: a clear lake with submerged macrophytes, and a turbid lake with high phytoplankton biomass. To study the temporal changes of both microalgal communities under such contrasting conditions, we placed enclosures in the littoral zone of each lake, including natural phytoplankton and artificial substrata, half previously colonized by periphyton until a mature stage and half clean to analyze periphyton colonization. In the clear vegetated shallow lake, periphyton chlorophyll a concentrations were 3–6 times higher than those of the phytoplankton community. In contrast, phytoplankton chlorophyll a concentrations were 76–1,325 times higher than those of periphyton in the turbid lake. Here, under light limitation conditions, the colonization of the periphyton was significantly lower than in the clear lake. Our results indicate that in turbid shallow lakes, the light limitation caused by phytoplankton determines a low periphyton biomass dominated by heterotrophic components. In clear vegetated shallow lakes, where nitrogen limitation probably occurs, periphyton may develop higher biomass, most likely due to their higher efficiency in nutrient recycling.     

Can allelopathically active submerged macrophytes stabilise clear-water states in shallow lakes?

Inhibition of phytoplankton by allelochemicals released by submerged macrophytes is supposed to be one of the mechanisms that contribute to the stabilisation of clear-water states in shallow lakes. The relevance of this process at ecosystem level, however, is debated because in situ evidence is difficult to achieve. Our literature review indicates that allelopathically active species such as Myriophyllum, Ceratophyllum, Elodea and Najas or certain charophytes are among the most frequent submerged macrophytes in temperate shallow lakes. The most common experimental approach for allelopathic interference between macrophytes and phytoplankton has been the use of plant extracts or purified plant compounds. Final evidence, however, requires combination with more realistic in situ experiments. Such investigations have successfully been performed with selected species. In situ allelopathic activity is also influenced by the fact that phytoplankton species exhibit differential sensitivity against allelochemicals both between and within major taxonomic groups such as diatoms, cyanobacteria and chlorophytes. In general, epiphytic species apparently are less sensitive towards allelochemicals than phytoplankton despite living closely attached to the plants and being of key importance for macrophyte growth due to their shading. Light and nutrient availability potentially influence the sensitivity of target algae and cyanobacteria. Whether or not additional stressors such as nutrient limitation enhance or dampen allelopathic interactions still has to be clarified. We strongly propose allelopathy as an important mechanism in the interaction between submerged macrophytes and phytoplankton in shallow lakes based on the frequent occurrence of active species and the knowledge of potential target species. The role of allelopathy interfering with epiphyton development is less well understood. Including further levels of complexity, such as nutrient interference, grazing and climate, will extend this ecosystem-based view of in situ allelopathy.     

Changes in functional composition and diversity of waterbirds: The roles of water level and submerged macrophytes

1. Water level and submerged macrophytes are critical players for the functioning of shallow lake ecosystems; understanding how waterbird communities respond to changes in both can have important implications for conservation and management. Here, we evaluated the effects of changes in water level and submerged macrophyte status on wintering waterbird community size, functional group abundances, functional diversity (FD), and community assembly by using a dataset compiled over 50 years.
2. We built generalised linear models to evaluate the effects of water level and submerged macrophyte status on the above-listed attributes of the waterbird communities by using mid-winter waterbird censuses, water level measurements, and submerged macrophyte surveys, along with submerged macrophyte macrofossil records from two shallow lakes in Turkey. Using a relevant set of functional traits, we defined functional groups, calculated four FD measures, and simulated null distributions of the FD measures for assessing assembly rules.
3. We found that macrophyte-dominated years had significantly higher abundances of waterbirds in one of the study lakes, and had more diving herbivores and omnivores in both lakes, while diving/scooping fish-eating waterbird abundance was lower in macrophyte-dominated years. Community size in Lake Beyşehir exhibited a negative association with water level; surprisingly, however, none of the functional group abundances and FD indices were significantly related to water level.
4. In our study communities, standardised effect sizes of functional richness and functional dispersion—two indices that are particularly sensitive to community assembly processes—were mostly lower than those of randomly assembled communities, which implies functional clustering. Shifts to a scarce-macrophyte state were associated with increases in these two indices, possibly due to either changes in the relative strength of environmental filtering and limiting similarity in community assembly or sampling of transitional communities. Further studies covering a wider range of the trophic/macrophyte status spectrum are needed to be certain.
5. The results of this study indicate that shifts between abundant and scarce-macrophyte states can have significant effects on wintering waterbird abundances, FD and community assembly. The results also suggest that shallow lakes in macrophyte-dominated states can support more wintering waterbirds, especially diving omnivores, some of which are globally threatened.

     

Phytoplankton–bacterioplankton interactions in a neotropical floodplain lake (Laguna Bufeos,Bolivia)

Laguna Bufeos is a floodplain lake of the river Ichilo, a tributary of the Amazon basin situated in Bolivia. Nutrient addition assays involving whole water (<200 μm) as well as fractionated water (<0.8 μm) treatments were carried out in incubation tubes to test whether bacterial growth is limited by the availability of inorganic nutrients and to test whether bacteria are able to utilize inorganic nutrients directly or are stimulated by inorganic nutrients through increased production of phytoplankton. The assays were carried out during two extreme hydrological conditions, the high-water and the low-water period. During the high-water period experiment, neither N or P limited bacterial growth rates. During the low-water period, bacterial growth was P limited. Bacterial growth was stimulated in the fractionated as well as in the whole water treatments, indicating that bacterial growth was directly stimulated by P. Bacterial growth corrected for grazing losses (determined by means of dilution experiments) was significantly higher in the fractionated water containing only bacteria when compared to the whole water containing also grazers and phytoplankton. This suggests that bacterial growth was suppressed by competition with phytoplankton rather than stimulated through the production of dissolved organic matter by phytoplankton.     

Alteration of Ecosystem Function by Zebra Mussels in Oneida Lake: Impacts on Submerged Macrophytes

Dreissenid mussels (the zebra mussel Dreissena polymorpha and the quagga mussel D. bugensis) are ecosystem engineers that modify the physical environment by increasing light penetration. Such a change is likely to affect the distribution and diversity of submerged macrophytes. Filter-feeding by these mussels has been associated with increased water clarity in many North American and European lakes. In this study, we report the increase in water clarity of Oneida Lake, New York, USA, for 1975–2002 and argue that the increase was caused by zebra mussel invasion rather than declines in nutrients. Over the study period, although mean total phosphorus decreased significantly, the main increase in water clarity occurred after the zebra mussel invasion in 1991. The average depth receiving 1% surface light increased from 6.7 m to 7.8 m after the invasion of zebra mussels, representing a 23% areal expansion. The maximum depth of macrophyte colonization, as measured by diver and hydroacoustic surveys, increased from 3.0 m before the invasion of zebra mussels to 5.1 m after their establishment. In addition, macrophyte species richness increased, the frequency of occurrence increased for most species, and the composition of the macrophyte community changed from low-light–tolerant species to those tolerating a wide range of light conditions. Comparisons with observations reported in the literature indicate that increased light penetration alone could explain these changes in macrophyte distribution and diversity. Such changes will increase the importance of benthic primary production over pelagic production in the food web, thereby representing an overall alteration of ecosystem function, a process we refer to as “benthification”.     

Migratory benthic fishes may induce regime shifts in a tropical floodplain pond

1. Alternative states are a widely recorded phenomenon in shallow lakes, which may shift between turbid‐ and clear‐water conditions. Here, we investigate whether such shifts in a tropical floodplain pond may be related to the effect of the flood pulse regime on the community structures of fish and macrophytes. 2. Using a long‐term data set, we demonstrate how benthic fish migration together with colonisation by submerged plants affected the transition from a turbid to a macrophyte‐dominated state in a floodplain pond without top‐down control. 3. In our study, the turbid state occurred mostly during low water phases and was largely characterised by high values for the biomass of benthic fish, chlorophyll‐a and total phosphorous. 4. During the period of rising water levels, the migration of benthic fish out of the pond occurs simultaneously with the establishment of submerged plants, while water turbidity decreases along with phytoplankton and nutrient concentrations, inducing a clear‐water phase. However, when submerged plants are absent and fish migration is low, a transient state is generated. 5. We suggest that, in contrast to temperate ponds and shallow lakes, where the main driving mechanisms establishing alternative states are related to cascading effects via the food chain, in tropical ponds and shallow lakes it is resuspension of sediments by benthic fish that plays the most significant role in establishing alternative states. However, the effect of the flood pulse regime plays an important role in the temporal dynamics of fish community structure by controlling benthic fish migration.     

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.     

Contrasting impacts of invasive engineers on freshwater ecosystems: an experiment and meta-analysis

Invasion by common carp (Cyprinus carpio) and red swamp crayfish (Procambarus clarkii) in shallow lakes have been followed by stable-state changes from a macrophyte-dominated clear water state to a phytoplankton-dominated turbid water state. Both invasive carp and crayfish are, therefore, possible drivers for catastrophic regime shifts. Despite these two species having been introduced into ecosystems world-wide, their relative significance on regime shifts remains largely unexplored. We compared the ecological impacts of carp and crayfish on submerged macrophytes, water quality, phytoplankton, nutrient dynamics, zooplankton and benthic macroinvertebrates by combining an enclosure experiment and a meta-analysis. The experiment was designed to examine how water quality and biological variables responded to increasing carp or crayfish biomass. We found that even at a low biomass, carp had large and positive impacts on suspended solids, phytoplankton and nutrients and negative impacts on benthic macroinvertebrates. In contrast, crayfish had a strong negative impact on submerged macrophytes. The impacts of crayfish on macrophytes were significantly greater than those of carp. The meta-analysis showed that both carp and crayfish have significant effects on submerged macrophytes, phytoplankton, nutrient dynamics and benthic macroinvertebrates, while zooplankton are affected by carp but not crayfish. It also indicated that crayfish have significantly greater impacts on macrophytes relative to carp. Overall, the meta-analysis largely supported the results of the experiment. Taken as a whole, our results show that both carp and crayfish have profound effects on community composition and ecosystem processes through combined consequences of bioturbation, excretion, consumption and non-consumptive destruction. However, key variables (e.g. macrophytes) relating to stable-state changes responded differently to increasing carp or crayfish biomass, indicating that they have differential ecosystem impacts.     

Ceratophyllum demersum hampers phytoplankton development in some small Norwegian lakes over a wide range of phosphorus concentrations and geographical latitude

1. Data on submerged and floating-leafed macrophytes, phytoplankton, nutrients (N, P) and calcium were collected from twenty-four small lakes ( 1 km²) over a wide range of latitudes in Norway. The majority of the investigated lakes were mesotrophic or eutrophic, and most of the lakes were markedly affected by diffuse and point-source runoff from agriculture. According to their macrophyte species composition, the majority of the lakes can be classified as Potamogeton lakes or Chara lakes, or a combination of these.
2. This study is consistent with the 'two alternative stable states' hypothesis. We observed clearwater lakes with dense macrophyte cover over a wider range of total P concentration than has been reported previously: from 30 to more than 700 mg P m^–3. The clearwater state was only observed in lakes with mean depths of less than 1.9 m.
3. Most clear lakes with high cover of submerged vegetation showed indications of N limitation.
4. In this study nearly all the macrophyte-dominated lakes with P concentrations above 30 mg m^–3 had dense stands of Ceratophyllum demersum (L.). This indicates that Ceratophyllum may also play an important role in stabilizing and maintaining a clearwater state at high P concentrations.