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Abstract
The occurrence of macrophyte- and phytoplankton-dominated states has been studied in shallow Lake Muggelsee, which, due to its rather high average depth and naturally eutrophic condition, differs from most typical shallow lakes. At the beginning of the 20(th) century, a diverse vegetation covered one third of the lake with a maximum colonisation depth of 4 m. Secchi depths were high although cyanobacteria blooms, dominated by Aphanizomenon flos-aquae and Microcystis spec., occurred in late summer. Increasing eutrophication finally led to disappearance of submerged plant stands around 1970 and a switch to phytoplankton dominance. Cyanobacteria were now mainly represented by Planktothrix agardhii and Limnothrix redekei, preventing a clear-water phase in spring. Turbidity increased significantly after disappearance of submerged plants. Since 1990, external nutrient loads have been reduced. Cyanobacteria structure reversed to dominance of Aphanizomenon and Microcystis resulting in a clear-water phase in spring and re-appearance of submerged macrophytes. Nevertheless, submerged plant stands still covered only 3 % of the total lake area in 1999. Average colonisation depth, biomass and colonisation densities of the dominant species Potamogeton pectinatus are low. Hitherto, re-colonisation by other species formerly present in the lake has not taken place. Exclosure experiments revealed that grazing by herbivorous birds or fish hamper re-colonisation.
... Recurrent and massive cyanobacterial blooms are reported worldwide as a consequence of amalgamation of eutrophication and global climate changes conferring elevation in water temperature and atmospheric CO 2 levels (L€ urling et al., 2017;Huisman et al., 2018;Burberg et al., 2020). Anthropic impact promotes eutrophication that leads to severe impacts such as disappearance of submerged macrophytes with subsequent shifting to a luxuriant growth dominated by cyanobacteria (Korner, 2001). Cyanobacteria are ancient group of ubiquitous, oxygenic photosynthetic microbes exhibiting high ecological adaptivity. ...
The responses of antioxidants and photosynthetic pigments to temporal variations in the bloom-forming cyanobacterium Microcystis aeruginosa dominating the eutrophic pond were investigated. Native-PAGE of superoxide dismutase (SOD) enzyme profile elucidated that while isoform number 1 identified as Fe-SOD showed its recurrent presence in all the three seasons, isoform 2 identified as Fe-SOD and isoform 3 as Mn-SOD were observed only in rainy season. However, densitometry analyses illustrated specific seasonal variations in the intensity of Fe-SOD isoform 1, with its intensity being the highest in winter season followed by rainy and summer seasons respectively. Interestingly, native-PAGE enzyme profile depicted the presence of three isoforms of catalase enzyme during rainy season while it was absent during winter and summer seasons. Correlogram elucidated that antioxidative enzymes SOD and catalase were positively correlated to phy-cocyanin while they remained negatively correlated to other polar and non-polar solvent soluble photosynthetic pigments. Good quality 3D models of Fe-SOD and Mn-SOD in Microcystis aeruginosa were generated by homology modeling. All metal binding positions were highly conserved and protein-protein interactions were also observed. The phylogenetic tree revealed that Microcystis sp. T1-4 is closest evolutionary homolog of Fe-SOD and Mn-SOD in Microcystis aeruginosa. Principal component analyses illustrated that enzymatic and non-enzymatic antioxidants contribute jointly and specifically to combat oxidative stress, may serve as biomarker of environmental cues and may be exploited for further biotechnological applications .
... This is essentially due to the imbalance of input and output of nutrients (Aguiar et al., 2011). As a result, species distribution in aquatic ecosystem is unbalanced, submerged plants disappear, phytoplankton overgrows gradually (algal blooms and red tides) and water becomes hypoxic or anoxic (less than 2 mg O 2 l −1 ) (Korner, 2001;Hu et al., 2014;Liu et al., 2015;Zhang et al., 2017). These cyanobacterial blooms will eventually destroy the flow of material and energy in the system, leading to sublethal effects and mortality in aquatic wildlife, the gradual extinction of the entire aquatic ecosystem (Dodds et al., 2008;Le et al., 2010) as well as poisoning in seafood consumers (Mat et al., 2018). ...
Nowadays, eutrophication is a very popular environmental problem in numerous waters around the world. The main reason of eutrophication is the enrichment of the nutrient, which results in the excessive growth of phytoplankton and some of them are toxic and harmful. Fortunately, some studies have shown that some bivalves can filter the overgrown phytoplankton in water, which may alleviate water eutrophication. However, the physiological effects of toxic cyanobacteria on filter feeding animal have not been clarified very well. In this experiment, digestive enzyme activities in Hyriopsis cumingii exposed to different concentrations of the toxic Microcystis aeruginosa (0, 5 * 105 and 5 *106 cell ml-1) at two dissolved oxygen (DO) levels (6 and 2 mg l-1) for 14 days were investigated. Toxic M. aeruginosa significantly affected all digestive enzyme activities throughout the experiment. At high toxic M. aeruginosa concentration, the activities of cellulase, amylase and lipase in digestive gland and stomach were significantly increased (P<0.05). However, hypoxia reduced the activities of cellulase, amylase and lipase in digestive gland and stomach. Conflicting effects were observed between toxic M. aeruginosa and DO in most digestive enzyme activities during the exposure period. Therefore, it is not conducive for the digestion and absorption of M. aeruginosa in H. cumingii under hypoxic conditions. H. cumingii is tolerant to toxic M. aeruginosa and may remove toxic cyanobacteria from waters under normal DO conditions.
... In groundwater-fed lakes where CO 2 rich water enters the littoral zone, IBF would interrupt the added CO 2 contribution and thereby decrease the possibility for CO 2 -dependent macrophytes to survive. This process has been suggested to be involved in the complete disappearance of F. antipyretica from Lake Müggelsee (Figure 4) during the last century [190]. An overall lower availability of DIC in the littoral zone due to IBF preventing groundwater exfiltration into the lake may eventually lead to an overall loss of macrophytes. ...
Studies on induced bank filtration (IBF), a cost-effective and reliable drinking water production method, usually focus on processes affecting the target drinking water quality. We aim to expand this view by assessing potential impacts of IBF on surface water quality. We suggest that IBF can directly and indirectly affect several physical, chemical and biological processes in both the sediment and open water column, eventually leading to positive or negative changes in source water quality. Direct effects of IBF comprise water level fluctuations, changes in water level and retention time, and in organic content and redox conditions in littoral sediments. Indirect effects are mainly triggered by interrupting groundwater discharge into the surface water body. The latter may result in increased seasonal temperature variations in sediment and water and reduced discharge of solutes transported by groundwater such as nutrients and carbon dioxide. These changes can have cascading effects on various water quality, e.g., by facilitating toxic phytoplankton blooms. We propose investigating these potential effects of IBF in future field and laboratory studies to allow for more detailed insights into these yet unknown effects and their magnitude in order to assure a sustainable application of this valuable technique in the future.
... Eutrophication following enhancement of human activity has increased in frequency, intensity and geographical distribution of toxic blooms during the last several decades (Derwent et al., 1998;Ibelings et al., 2007). One of the more serious impacts of eutrophication on aquatic ecosystems is the disappearance of submerged macrophytes and the shift to a phytoplankton-dominated state (Korner, 2001). Over the last several decades, many regions throughout the world have experienced harmful algal blooms, which were caused by a variety of microalgal species (Karim et al., 2002;Aguiar et al., 2011). ...
Bloom forming algae and hypoxia are considered to be two main co-occurred stressors associated with eutrophication. The aim of this study was to evaluate the interactive effects of harmful algae Microcystis aeruginosa and hypoxia on an ecologically important mussel species inhabiting lakes and reservoirs, the triangle sail mussel Hyriopsis cumingii, which is generally considered as a bio-management tool for eutrophication. A set of antioxidant enzymes involved in immune defence mechanisms and detoxification processes, i.e. glutathione-S-transferases (GST), glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), lysozyme (LZM) in mussel haemolymph were analyzed during 14days exposure along with 7days depuration duration period. GST, GSH, SOD, GPX and LZM were elevated by toxic M. aeruginosa exposure, while CAT activities were inhibited by such exposure. Hypoxia influenced the immune mechanisms through the activation of GSH and GPX, and the inhibition of SOD, CAT, and LZM activities. Meanwhile, some interactive effects of M. aeruginosa, hypoxia and time were observed. Independently of the presence or absence of hypoxia, toxic algal exposure generally increased the five tested enzyme activities of haemolymph, except CAT. Although half of microcystin could be eliminated after 7days depuration, toxic M. aeruginosa or hypoxia exposure history showed some latent effects on most parameters. These results revealed that toxic algae play an important role on haemolymph parameters alterations and its toxic effects could be affected by hypoxia. Although the microcystin depuration rate of H. cumingii is quick, toxic M. aeruginosa and/or hypoxia exposure history influenced its immunological mechanism recovery.
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... Many studies have documented the existence of these alternative stable states in shallow lakes (e.g. Moss et al. 1996, Korner 2001, Dent et al. 2002, Bayley & Prather 2003, Dokulil & Teubner 2003, Jackson 2003. The clear water state is maintained by the action of macrophytes, which outcompete phytoplankton for nutrients, reduce sediment resuspension, and provide a refuge for zooplankton against predation by fish (Schriver et al. 1995, Scheffer et al. 1997, Van den Berg et al. 1998, Blindow et al. 2002. ...
Year One Annual Report Introduction The Vancouver Lake Watershed Partnership is interested in understanding the processes that lead to the formation, persistence and decline of cyanobacteria and other harmful algal blooms in Vancouver Lake and similar bodies of water. Vancouver Lake has experienced numerous nuisance blooms of cyanobacteria in recent years (e.g. Wierenga 2005, 2006), often necessitating closure of the Lake to recreational use. Indeed, there is growing evidence that the incidence of noxious cyanobacterial blooms in freshwater lakes and rivers is increasing worldwide, most often associated with increased nutrient inputs, i.e. eutrophication (e.g. Dokulil & Teubner 2000, Sellner et al. 2003, Yamamoto & Nakahara 2005). Excessive abundance or "blooms" of cyanobacteria may have detrimental effects on lake ecosystems and water quality, including development of surface scums, depleted oxygen levels, and (in some cases) production of toxins that can negatively affect aquatic life and humans (Carmichael 1992, Codd 1995, Sellner et al. 2003). This phenomenon is of great concern to water resource managers, particularly with respect to human health, as well as to the public whose use and enjoyment of these environments may be prohibited as a result. With support from the Partnership, Washington State University Vancouver initiated an assessment of the planktonic assemblage and environmental conditions in Vancouver Lake as a first step in identifying and quantifying the factors that influence cyanobacterial blooms. This report summarizes the results from the first year of the assessment (March 2007 to February 2008), which was focused on three specific objectives: 1) to determine the abundance, distribution and taxonomic composition of cyanobacteria, algae and zooplankton in Vancouver Lake over a full annual cycle, 2) to initiate some preliminary investigations of the biotic (e.g., grazers) and abiotic (e.g., temperature, nutrients) factors influencing these blooms, and 3) to analyze the extant data on cyanobacteria blooms in Vancouver Lake for spatial and temporal patterns and trends in abundance, as well as provide a literature review.
... The ongoing eutrophication of freshwater bodies (Carpenter et al., 1998;Tilman et al., 2001) has induced a decline or disappearance of macrophytes from many shallow water ecosystems Brouwer & Roelofs, 2001;Gulati & van Donk, 2002;Lamers et al., 2002). This has been observed in many shallow lakes in densely populated areas, for instance in the Loosdrecht lakes (Best et al., 1984;Gulati & van Donk, 2002; Van de Haterd & Ter Heerdt, 2007) and Lake Veluwemeer (Van den Berg et al., 1999;Ibelings et al., 2007) in The Netherlands, Lake Fure (SandJensen et al., 2008) and Lake Arresø ( Jeppesen et al., 2007) in Denmark and the Müggelsee in Germany (Korner, 2001). Increased nutrient availability can initially stimulate macrophyte growth as long as the water remains clear (Lombardo & Cooke, 2003;Nagasaka, 2004;Feuchtmayr et al., 2009). ...
Although many lake restoration projects have led to decreased nutrient loads and increased water transparency, the establishment or expansion of macrophytes does not immediately follow the improved abiotic conditions and it is often unclear whether vegetation with high macrophyte diversity will return. We provide an overview of the potential bottlenecks for restoration of submerged macrophyte vegetation with a high biodiversity and focus on the biotic factors, including the availability of propagules, herbivory, plant competition and the role of remnant populations. We found that the potential for restoration in many lakes is large when clear water conditions are met, even though the macrophyte community composition of the early 1900s, the start of human-induced large-scale eutrophication in Northwestern Europe, could not be restored. However, emerging charophytes and species rich vegetation are often lost due to competition with eutrophic species. Disturbances such as herbivory can limit dominance by eutrophic species and improve macrophyte diversity. We conclude that it is imperative to study the role of propagule availability more closely as well as the biotic interactions including herbivory and plant competition. After abiotic conditions are met, these will further determine macrophyte diversity and define what exactly can be restored and what not.
... The ongoing eutrophication of freshwater bodies (Carpenter et al., 1998; Tilman et al., 2001) has induced a decline or disappearance of macrophytes from many shallow water ecosystems (Sand-Jensen et al., 2000; Brouwer & Roelofs, 2001; Gulati & van Donk, 2002; Lamers et al., 2002). This has been observed in many shallow lakes in densely populated areas, for instance in the Loosdrecht lakes (Best et al., 1984; Gulati & van Donk, 2002; Van de Haterd & Ter Heerdt, 2007) and Lake Veluwemeer (Van den Berg et al., 1999; Ibelings et al., 2007) in The Netherlands, Lake Fure (SandJensen et al., 2008) and Lake Arresø (Jeppesen et al., 2007) in Denmark and the Müggelsee in Germany (Korner, 2001). Increased nutrient availability can initially stimulate macrophyte growth as long as the water remains clear (Lombardo & Cooke, 2003; Nagasaka, 2004; Feuchtmayr et al., 2009). ...
Although many lake restoration projects have led to decreased nutrient loads and increased water transparency, the establishment or expansion of macrophytes does not immediately follow the improved abiotic conditions and it is often unclear whether vegetation with high macrophyte diversity will return. We provide an overview of the potential bottlenecks for restoration of submerged macrophyte vegetation with a high biodiversity and focus on the biotic factors, including the availability of propagules, herbivory, plant competition and the role of remnant populations. We found that the potential for restora-tion in many lakes is large when clear water conditions are met, even though the macrophyte community composition of the early 1900s, the start of human-induced large-scale eutrophication in Northwestern Europe, could not be restored. However, emerging charophytes and species rich vegetation are often lost due to competition with eutrophic species. Distur-bances such as herbivory can limit dominance by eutrophic species and improve macrophyte diversity. We conclude that it is imperative to study the role of propagule availability more closely as well as the biotic interactions including herbivory and plant competition. After abiotic conditions are met, these will further determine macrophyte diversity and define what exactly can be restored and what not.
... In nutrient rich situations, competition for light between phytoplankton and macrophytes leads to macrophyte limitation and even disappearance (Scheffer et al. 1993; Declerck et al. 2005). Epiphytes may also contribute to drastically limiting aquatic plants, even in lakes with sufficient light availability (Weisner et al. 1997; Korner 2001; Jones et al. 2002), by shortening the clear-water phase with optimum growth conditions for submerged plants (Roberts et al. 2003). ...
This review aims to determine how environ- mental characteristics of aquatic habitats rule species occurrence, life-history traits and community dynamics among aquatic plants, and if these particular adaptations and responses fit in with general predictions relating to abiotic factors and plant communities. The way key abiotic factors in aquatic habitats affect (1) plant life (recruitment, growth, and reproduction) and dispersal, and (2) the dynamics of plant communities is discussed. Many factors related to plant nutrition are rather similar in both aquatic and terrestrial habitats (e.g. light, temperature, substrate nutrient content, CO2 availability) or differ markedly in intensity (e.g. light), variations (e.g. temperature) or in their effective importance for plant growth (e.g. nutrient content in substrate and water). Water movements (water- table fluctuations or flow velocity) have particularly drastic consequences on plants because of the density of water leading to strong mechanical strains on plant tissues, and because dewatering leads to catastrophic habitat modifi- cations for aquatic plants devoid of cuticle and support tissues. Several abiotic factors that affect aquatic plants, such as substrate anoxia, inorganic carbon availability or temperature, may be modified by global change. This in turn may amplify competitive processes, and lead ulti- mately to the dominance of phytoplankton and floating species. Conserving the diversity of aquatic plants will rely on their ability to adapt to new ecological conditions or escape through migration.
... Noordhuis et al., 2002;Irfanullah and Moss, 2004), or fish (e.g. Korner, 2001;Matsuzaki et al., 2007). While fish stocks including the turbidity-inducing carp have declined (Lammens and van den Berg, 2001), reported pressure from bird grazing is extreme. ...
The Ramsar site of Lake Uluabat, western Turkey, suffers from eutrophication, urban and industrial pollution and water abstraction, and its water levels are managed artificially. Here we combine monitoring and palaeolimnological. techniques to investigate spatial and temporal limnological variability and ecosystem impact, using an ostracod and mollusc survey to strengthen interpretation of the fossil record. A combination of low invertebrate Biological Monitoring Working Party scores (<10) and the dominance of eutrophic diatoms in the modern lake confirms its poor ecological status. Palaeolimnological analysis of recent (last >200 yr) changes in organic and carbonate content, diatoms, stable isotopes, ostracods and molluscs in a lake sediment core (UL20A) indicates a 20th century trend towards increased sediment accumulation rates and eutrophication which was probably initiated by deforestation and agriculture. The most marked ecological shift occurs in the early 1960s, however. A subtle rise in diatom-inferred total phosphorus and an inferred reduction in submerged aquatic macrophyte cover accompanies a major increase in sediment accumulation rate. An associated marked shift in ostracod stable isotope data indicative of reduced seasonality and a change in hydrological input suggests major impact from artificial water management practices, all of which appears to have culminated in the sustained loss of submerged macrophytes since 2000. The study indicates it is vital to take both land-use and water management practices into account in devising restoration strategies. in a wider context, the results have important implications for the conservation of shallow karstic lakes, the functioning of which is still poorly understood. (c) 2008 Elsevier Ltd. All rights reserved.
... Many lakes have experienced a transition from a clear state with abundant macrophyte growth to a turbid state characterized by the excess growth of microscopic algae. Submerged macrophytes can hardly survive under these conditions and the overall ecological quality of the lake ecosystem is poor (e.g., Korner 2001; Irfanullah and Moss 2004). A moderate reduction of nutrient loading often does not lead to a clear state due to internal loading (Sondergaard and others 2003) and resilience of the turbid state (Scheffer 2001). ...
Many lakes have experienced a transition from a clear into a turbid state without macrophyte growth due to eutrophication. There are several measures by which nitrogen (N) and phosphorus (P) concentrations in the surface water can be reduced. We used the shallow lake model PCLake to evaluate the effects of three measures (reducing external nutrient loading, increasing relative marsh area, and increasing exchange rate between open water and marsh) on water quality improvement. Furthermore, the contribution of different retention processes was calculated. Settling and burial contributed more to nutrient retention than denitrification. The model runs for a typical shallow lake in The Netherlands showed that after increasing relative marsh area to 50%, total phosphorous (TP) concentration in the surface water was lower than the Maximum Admissible Risk (MAR, a Dutch government water quality standard) level, in contrast to total nitrogen (TN) concentration. The MAR levels could also be achieved by reducing N and P load. However, reduction of nutrient concentrations to MAR levels did not result in a clear lake state with submerged vegetation. Only a combination of a more drastic reduction of the present nutrient loading, in combination with a relatively large marsh cover (approximately 50%) would lead to such a clear state. We therefore concluded that littoral marsh areas can make a small but significant contribution to lake recovery.
In this paper we analyze a long-term dataset on the recovery from eutrophication of Lake Veluwe (The Netherlands). Clear hysteresis
was observed in a number of ecosystem variables: the route to recovery differed significantly from the route that led to loss
of clear water. The macrophyte dominated state disappeared in the late 1960s at TP above 0.20mgl−1, whereas its return occurred at less than 0.10mgTPl−1. Several regime shifts resulting in the occurrence of three alternative stable states were observed over a period of 30years.
The turbid state showed resistance to change, despite a strong and prompt reduction in Chl-a following reduction of external P-loading. The most important component that determined hysteresis in the return to clear
water was not internal P-loading, but a high level of nonalgal light attenuation (through sediment resuspension) maintained
by the interaction between wind and benthivorous fish. Although Chara was able to re-colonize the most shallow parts of the lake, recovery stalled and for a number of years clear (above charophyte
beds) and turbid (deeper parts of the lake) water co-existed, as a separate alternative state on route to full recovery. Lake-wide
clear water was re-established after bream density had been reduced substantially. This allowed a return of zebra mussels
to the lake, whose high filtration capacity helped in maintaining clear water. In this study, we were able to identify the
main drivers of hysteresis and regime shifts, although formal demonstration of cause and effect was not possible on the basis
of field data alone. We argue that resilience of the present clear water state of Lake Veluwe very much depends on sizable
populations of a few keystone species, especially Chara (stoneworts) and Dreissena (zebra mussels), and that careful management of these species is equally important as control of nutrients. Lake management
should strive to maintain and strengthen resilience of the ecosystem, and this should offer protection against a renewed collapse
of the clear state.
The effects of nutrient additions on aquatic systems have been frequently studied. Typically, these studies report an increase
in algal biomass and a decrease in species diversity in response to nutrient increases. However, it is not clear why comparable
aquatic communities respond differently to nutrient additions of similar magnitudes. We tested the effects of the rate and
amount of nutrient load on community structure in 760l mesocosms; treatments manipulated the total amount of nutrients that
entered an aquatic system (small versus large load) and the temporal pattern in which these nutrients entered the system (annually,
monthly, or weekly). We found that the effects of the loading rate of nutrients were at least as important as the total amount
of the nutrients for several response variables. Although these effects were manifested in several ways, the response to the
different rates was most prominent within groups of the primary producers, which showed large shifts in composition and abundance.
In order to measure the attenuation of light by periphyton a method has been developed which assures that natural periphyton architecture and therefore its optical characteristics remain preserved. This method has been applied to analyze the transmittance of periphyton of four lakes of different tropic states situated in the Osterseen Lake District (Upper Bavaria, Germany). The seasonal variation of the periphyton's transmittance has been studied using standard microscope glass slides exposed 1 m beneath the water surface within macrophyte beds. The study ran from February to November 1997 in two eutrophic lakes (Lake Waschsee, Lake Sengsee) and from May to November 1997 in the meso-oligotrophic Lake Eishaussee and in the oligotrophic Lake Herrensee.Generally the exposure-period, the seasonality, and the tropic state of the habitat affected the transmittance spectra of the periphyton. In all lakes the attenuation by periphyton increased with longer colonization times but at a different time scale. The periphyton of the nutrient-rich and the nutrient-poor lakes differed clearly in composition and architecture. The eutrophic lakes were characterized by a relatively thick but loosely attached, unstable periphyton community, which was translucent to a certain degree even at the end of the growth period. The transmittance of this periphyton fluctuated considerably and high percentages of filamentous green algae in this periphyton contributed to attenuation maxima within the range 400–500 nm and 650–700 nm due to photosynthetic pigments. By contrast, the periphyton of the meso-oligotrophic and of the oligotrophic lake was quite compact and nearly impervious to light.
Cyanotoxins are a group of compounds produced by cyanobacteria that can have severe physiological effects on other organisms, including humans. The potential allelopathic effects of Microcystis aeruginosa, a toxic cyanobacterium, on the duckweed plant, Lemna gibba L., were examined using three experimental methods: (1) a series of toxicity bioassays, (2) evaluation of toxin production by M. aeruginosa in the direct and indirect presence of L. gibba, and (3) inhibition of oxygen evolution in photosynthesis. The results showed that, first, there were no clear dose-dependent effects of the microcystin-LR standard or the toxic M. aeruginosa culture filtrate on any of the end points measured in the toxicity bioassays (plant and frond number, dry weight, growth rate, chlorophyll content; one-way ANOVA, p > 0.05). In those cases in which an EC(50) value could be obtained, chlorophyll a was the most sensitive end point, as it had the lowest EC(50) value (14.47 microg/L microcystin-LR) of all the end points. Second, the presence of L. gibba did not result in higher microcystin-LR production in the toxic M. aeruginosa culture. And, last, oxygen evolution was not affected in isolated chloroplasts exposed directly to microcystin-LR. Therefore, microcystins from the toxic cyanobacterium Microcystis aeruginosa do not appear to have an allelopathic effect on the common aquatic macrophyte Lemna gibba.
The environmental conditions for submerged macrophytes in sewage channels are different to those of natural waters. Mainly 6 eutraphent species occur in two communities on the flowing stretch of the Berlin treated sewage.channel Wuhle. The survival time of the above ground biomass of the annual species is relatively long due to the higher water temperatures in winter. The average biomass of the different parts of the flowing stretch is between 50 and 830 g DM/m2 in summer. With decreasing concentrations of ammonia and BOD an increase of the biomass and the colonization density could be noticed over the last few years.
Cladoceran composition and diel horizontal migration were studied in 2, 10 and 25 m diameter macrophyte exclosures established in the littoral zone of shallow Lake Stigsholm, Denmark. The exclosures were protected from waterfowl grazing, but open to fish. The macrophyte community comprised Potamogeton pectinatus, Potamogeton pusillus and Callitriche hemzaphroditica. Cladocerans were sampled randomly every third hour inside and outside the macrophyte exclosures during a 24 h period. In the 2 m exclosure, the pelagic species Ceriodaphnia spp. and Bosmina spp. dominated during the day, mean density being as high as 3430 indiv. l(-1). During the night, density decreased to 10-20% of the daytime density, thus indicating diel horizontal migration. In the 10 and 25 m exclosures, the daytime mean density of Ceriodaphnia spp. was 865 and 202 indiv. l(-1), respectively, and did not decrease at night. In contrast to the pelagic species, the density of macrophyte-associated species tended to be higher in the 10 and 25 m exclosure than in the 2 m exclosure. In the daytime, Eurycercus lamellatus density in the 2, 10 and 25 m macrophyte exclosures was 7, 28 and 16 indiv. l(-1), respectively, while that of Simocephalus vetulus was 11, 171 and 92 indiv. l(-1), respectively. There was no day-night difference in the density of macrophyte-associated species We conclude that cladoceran community composition varies with macrophyte bed size, and that the edge zone between the bed and open water is an important daytime refuge for potentially migrating pelagic cladocerans.
The impact of sediment composition and waterfowl grazing on colonization of submerged macrophytes in shallow biomanipulated Lake Væng was studied by means of growth experiments: curly leaved pondweed (Potamogeton crispus L.) was transplanted to separate pots containing each type of sediment found in Lake Væng, and the pots were planted at two locations, one sheltered and one exposed. Half of macrophyte pots were behind a fence to protect them against grazing by water- fowl (mainly coots, Fulica atra L.). Sediment composition had some impact on macrophyte growth, being highest on mud from the centre of the lake. The fenced-in macrophytes became up to 6.5 times longer than the unfenced macrophytes. It was concluded that macrophytes were able to grow on all substrates present in Lake Væng, but waterfowl grazing probably controlled and delayed macrophyte colonization.
The impact of light climate during the growing season and light climate history on the growth and survival of a Potamogeton pectinatus L. population was studied in a eutrophic shallow lake, Lake Veluwe (Netherlands), during the period 1986-1988. Four different light conditions were created in an experimental setup by manipulating the photon flux density using artificial shading (three levels of artificial shading and one control situation without artificial shading), during the growing season of 1986. During the growing season of 1987, part of the P. pectinatus vegetation in the experimental setup was artificially shaded in the same way as in 1986, while another part was not shaded. No artificial shading was applied at all during the growing season of 1988. Growth conditions for P. pectinatus were less favourable in 1987 than in 1986. On the whole, above- and below-ground biomass values were lower, the growing season was shorter, maximum above-ground biomass was lower and was reached earlier in the season, and net tuber production was lower in 1987 than in 1986 in the control situations. In both years, the above- and below-ground biomass values were lower, the growing season was shorter and maximum above-ground biomass was lower with higher levels of shading. The impact of shading on relative above-ground biomass development was similar in 1986 and 1987, whereas the impact of shading on relative tuber bank size at maximum above-ground biomass was more pronounced in 1987 than in 1986 in the experimental areas which were shaded both seasons. Mean individual tuber ash-free dry weight was lowest at the highest level of shading in both years. It is concluded that light climate is a dominant factor in controlling the biomass and tuber bank dynamics of a P. pectinatus population in Lake Veluwe. Additionally, water quality and meteorological characteristics are involved. At the highest level of shading, the photosynthetic tissue was able to sustain tuber growth and maintenance of the vegetation during a relatively short period only. As a consequence, the vegetation sloughed early in the growing season and the tuber bank size was smallest. The maximum above-ground biomass was positively correlated with the size of the hibernated tuber bank. Therefore, the biomass development in one season is at least partly determined by the light conditions of a previous growing season(s) through the size of the hibernated tuber bank. The Lake Veluwe vegetation has the potential to recuperate from negative shading effects on above-ground biomass and tuber bank size under field conditions. This may require more than one growing season. depending on actual growth conditions and size of the hibernated tuber bank.
One aspect of the present work on Lake Müggelsee has been a quantitative assessment of phytoplankton changes over the annual cycle and from year to year as a basis for primary production measurements. The investigations of plankton analysis were carried out in Lake Müggelsee (Großer Müggelsee, Berlin), a eutrophic, polymictic shallow lake with an area of 7.2 km ² , a mean depth of 4.9 m. The lake was described as a species rich water with partially considerable developments of diatoms and blue‐green algae as was shown by plankton analyses from the beginning of this century. Since the end of the 60s, when macrophytes disappeared, Lake Müggelsee has been recognized as a lake with extensive phytoplankton development characterized by dense and irregular development of blue green algae and diatoms. Especially solitary centric diatoms increased in number and in biovolume from 1979 to 1990 and occurred with greater diameters in spring during the seasonal course.
The specific content of chlorophyll a was estimated according to the algal composition. It is suggested that the observed decrease in Chl a per unit biovolume is associated with species size rather than the time of development depending on the unknown portion of inactive algal cells resuspended from the sediment surface.
Biomass and community structure of phytoplankton have been analysed in the eutrophic, shallow, polymictic Müggelsee and in the inflowing Spree since 1980. In both lake and river, mean biomass and vitality of phytoplankton were similar. A regular annual succession of planktonic algae was found in both the river and the lake. The same species were found to be dominant, although the actual biomass of single phytoplankton populations differed normally from inflow to lake. Lake phytoplankton was characterized by higher proportions of Cyanobacteria (45.9 ± 11.3% resp. 34.5 ± 8.2%) and lower proportions of centric diatoms (35.1 ± 9.5% resp. 41.9 ± 10.3%, average April-October, 1980–90). The less abundant Pennales, Chlorophytes, and unidentified planktonic algae < 10 μm had attained a relatively higher importance in the river than in the lake. In the last decade, a pronounced shift from Cyanobacteria to diatoms (mainly Centrales) has been observed in the river.
Seasonal and long-term variability of the nutrients phosphorus, nitrogen, and silicon were investigated in the eutrophic Lake Müggelsee and its inflow from 1979 to 1990. The lake concentrations were internally controlled during the summer season and by the inflowing river in winter, respectively. The lake acts as a sink of nutrients. 18% of N and 3.5% of P were retained in the 12-years-period. Phosphorus retention differed from year to year. The average retention rate of the investigated period was only 4 kgP/d, but in some years it was negative due to strong P-release from sediment. The P-retention rate was also influenced by bank filtration (withdrawal for a waterworks).
Lake Müggelsee, a polymictic eutrophic lake at the eastern border of the city of Berlin (area: 7.3 km2, mean depth: 4.9m) is mainly influenced by the River Spree (flowing through the lake) and subject to various usages, especially to municipal water supply and recreation. Geomorphologic development, morphometric features, climatic conditions, hydrological regime, and position in the hydrographic net are described. Short information is given on fauna and flora of the lake, and on characteristics of the catchment area. Water quality parameters are summarized as 12 year average (1979–1990) and the eutrophic state is compared to other lakes. Finally, a short report on scientific research activities is given.
Monitoring data obtained from 231 freshwater lakes and ponds in the Netherlands, covering the period 1980–1996, were used to analyse the relationships between (a) transparency and chlorophyll-a, and the effect of system characteristics on this relationship, (b) chlorophyll-aand nutrient concentrations, and the effect of biological variables and (c) nutrient concentrations and nutrient loading. (a) Chlorophyll-aimposes a maximum on water transparency, but deviations from this maximum can be large. Reducing chlorophyll-a, therefore, does not guarantee a sufficient improvement of transparency. Soil type and the average depth of a lake were shown to influence the relationship between chlorophyll-aand transparency. (b) The maximum ratios of both chlorophyll-a: total-P and chlorophyll-a: total-N were higher in systems dominated by filamentous cyanobacteria than in systems dominated by other algae, indicating the efficiency of the former group with respect to nutrients. In systems with an areal coverage with submersed macrophytes above 5%, concentrations of chlorophyll-aand nutrients were lower than in systems with lower coverages. The ratios between chlorophyll-aand nutrients were lower at coverages larger than 10%. This indicates both bottom-up and top-down control of algae by macrophytes. Grazing pressure by zooplankton was also found to lower the chlorophyll-a: nutrient ratios. (c) System specific linear relationships were found between the average concentrations of total-P and total-N in the incoming water and the summer mean concentration in the lake. This allows the assessment of admissible loads for individual lakes, with narrower confidence limits compared to traditional relationships based on combined data from many lakes. From the analysis, it is concluded that the chain of relationships from nutrient loading to transparency is complex, and depends on biological variables as well as system characteristics.
Studies of the plant contribution to the food of fish in eutrophic lakes in Poland show that roach (Rutilus rutilus) and rudd (Scardinius erythrophthalmus), which frequently dominate in biomass and numbers, are the main plant consumers. Submerged macrophytes and filamentous algae are the most important plant food. The fish examined feed selectively on macrophytes and show a preference for species with low cost/ benefit ratio. The variability of plant ingestion according to temperature and the size of fish is discussed, and estimates of the annual consumption of algae and macrophytes by fish in the lakes examined are given. The possibility and effectiveness of mechanical and enzymatic processing of the ingested plants are presented. Qualitative and quantitative descriptions of faeces as well as their possible impact on the lake system are given. The herbivorous fish studied were found to be relatively efficient grazers but inefficient assimilators, thus they play a significant role in initiating the internal, biologically mobilized, nutrient sources.
The shallow, hypertrophic L. Groer Mggelsee, Germany,needs to be converted into a meso- to eutrophic level by means ofrestoration of the catchment area. Despite efforts inphosphorus(P) loading reduction in the inflow, this polymictic lakestilldemonstrates an year-to-year fluctuating and strong internal Ploading with increasing pore-water phosphate concentrationsduringsummer due to formation of anoxic microlayers at the sedimentsurface. This P release is indirectly governed by externalfactorsincluding the supply of dissolved oxygen and nitrate viainflow(high runoff), changes in land use (e.g., decreasing nitrogenfertilization) and climatic factors, and is facilitated bywind-induced rapid changes of stratification and mixingevents.Important management implications are: first, declines innitrateconcentrations in the inflow may result in depletion ofelectronacceptors with respect to phosphate desorption and increasethe Prelease. Second, the P release will continue for a long periodandthus delay the recovery of the lake. In dredging the uppermost1 msediment layer, the P content of the sediment would decreasefrom3.5 to between 0.5 and 1.0 mg P g dry weight–1and result inreduced capability of internal P loading. Because of the highcosts, a decision should be made about dredging or whether thelimnological improvement of the lake can be achieved viadephosphorization between 11 and 16 years.
Eighteen shallow lakes in The Netherlands were subjected to biomanipulation, i.e. drastic reduction of the fish stock, for the purpose of lake restoration. The morphology and the nutrient level of the lakes differed, as did the measures applied. In some lakes biomanipulation was accompanied by reduction of the phosphorus loading. In all but two lakes, the Secchi disk transparency increased after the fish removal. Eight lakes (no phosphorus loading reduction, except for one lake) showed a strong and quick response to the measures: the bottom of the lake became visible (`lake bottom view'') and there was a massive development of submerged macrophytes. In eight other lakes the water transparency increased, but lake bottom view was not obtained. In the biomanipulated lakes the decrease in total phosphorus and chlorophyll aand the increase in Secchi disk transparency were significantly stronger than the general trend occurring in Dutch lakes where no measures had been taken. The improvement in the Secchi depth and chlorophyll awas also stronger than in lakes where only the phosphorus load was reduced. The critical factor for obtaining clear water was the extent of the fish reduction in winter. Significant effects were observed only after >75% fish reduction. Success seems to require substantial fish manipulation. In such strongly biomanipulated lakes, wind resuspension of the sediment never prevented the water from becoming clear. No conclusion can be drawn with respect to the possible negative impact of cyanobacteria or Neomysison grazing by Daphniaand consequently on water clarity. In all lakes where there had been a high density of cyanobacteria or years with a high density of Neomysisother factors such as insufficient fishery may explain why lake bottom view was not obtained. In all lakes with additional phosphorus loading reduction the fish stock has been reduced less drastically (15–60%). In these lakes the effects on transparency were less pronounced than in the lakes with > 75% fish removal. Daphniagrazing seems responsible for spring clearing in all clear lakes but one. In three lakes the reduction of benthivorous fish also increased the transparency. The factors that determine water clarity in summer are less obvious. In most clear lakes a low algal biomass coincided with a macrophyte coverage of more than 25% of the lake surface area. However, it was not clear what mechanism caused the low algal biomass in summer, although inorganic nitrogen concentrations were regularly found to be very low. Daphniagrazing in open water seemed to be of little importance for suppressing the algal biomass in summer. Although in most lakes the total phosphorus concentration decreased after the biomanipulation, the dissolved phosphorus concentration remained too high to cause phosphorus limitation of the algal growth. In four out of six clear lakes for which there are long-term data the transparency decreased again after 4 years. In one lake with lower nutrient levels the Secchi disk transparency increased over the years. However, the number of lakes with low nutrient levels is too small for conclusions to be drawn regarding the impact of nutrient levels on the stability of the clear water state.
Grazing by herbivorous birds is often cited as an important factorin suppressing macrophyte development in shallow lakes undergoingrestoration, thus delaying the attainment of the stable clear waterstate. Development and succession of macrophyte communities andsize, diet and grazing pressure of coot (Fulica atra)populations upon macrophytes, were monitored over the seasonalcycle at ten shallow lakes of varying nutrient status, in theNorfolk Broads in eastern England. In spring, territorial breedingbirds were at relatively low density and included only a smallproportion of macrophytes in their diet, resulting in low grazingpressure on macrophytes. In summer, there was a significantrelationship between macrophyte cover and bird density,illustrating the importance of macrophytes in the dispersion phasefor birds following breeding. Macrophytes comprised the bulk ofbird diet where they were available and the consumption ofmacrophytes was up to 76 fold higher than in spring. However,losses to grazing in both periods were negligible when compared topotential growth rates documented in the literature. Grazingexperiments at two biomanipulated lakes confirmed that birds werenot responsible for limiting macrophytes during the springcolonisation phase or in the summer growth period. During theperiod of autumnal senescence and over the winter months where somemacrophyte species remain available, e.g. as developed individualsor dormant buds, grazing by birds may conceivably have an impact onthe development and structure of macrophyte populations insubsequent growing seasons.The relative importance of bird grazing compared to other factorslimiting the development of macrophytes in shallow lakes isdiscussed in the light of other experimental studies.
Based on data from 233 Danish lakes, enclosure experiments, full-scaleexperiments and published empirical models we present evidence that top-downcontrol is more important in shallow lakes than in deep lakes, excepting lakeswith a high abundance of submerged macrophytes. The evidence in support is: (1)That at a given epilimnion total phosphorus concentration (TP) the biomass offish per m2 is independent of depth, which means that biomassper m3is markedly higher in shallow lakes. (2) That the biomass of benthic invertebratesis higher in shallow lakes, which means that the benthi-planktivorous fish areless dependent on zooplankton prey than in deep lakes. By their ability to shiftto zooplankton predation their density can remain high even in periods whenzooplankton is scarce and they can thereby maintain a potentially high predationpressure on zooplankton. (3) That the possibilities of cladocerans to escapepredation by vertical migration are less. (4) That the zooplankton:phytoplanktonmass ratio per m2 is lower and presumably then also thegrazing pressure onphytoplankton. (5) That nutrient constraints appear to be weaker, as evidenced bythe fact that at a given annual mean TP, summer TP is considerably higher inshallow lakes, especially in eutrophic lakes lacking submerged macrophytes. (6)That negative feedback on cladocerans by cyanobacteria is lower as cyanobacterialdominance is less frequent in shallow lakes and more easily broken (at least inNorthern temperate lakes), and (7) That top-down control by benthi-planktivorousfish is markedly reduced in lakes rich in submerged macrophytes because theplants serve as a refuge for pelagic cladocerans and encouragepredatory fish at the expense of prey fish. We conclude that manipulation of fishand submerged macrophytes may have substantial impact on lake ecosystems, inparticular in shallow eutrophic lakes. On the contrary, if the conditions formore permanent changes in plant abundance or fish community structure are lackingthe feed-back mechanisms that endeavour a return to the original turbid state willbe particularly strong in shallow lakes.
The impact of grazing waterfowl on submerged macrophytes was studied in shallow (mean depth 0.8 m), eutrophic Lake Stigsholm, Denmark (area 21 ha). Two types of experiment were conducted, small-scale exclosure experiments (1 m2 plots) and large-scale enclosure experiments (100 m2 plots). In both experiments, shoots of Potamogeton crispus L. were planted in densities ranging from 1 to 8 m−2. Potamogeton pectinatus L. and Callitriche hermaphroditica L. were also present. The herbivorous waterfowl community foraging in the lake comprised mainly coot (Fulica atra L.), densities ranging from 0 to 9 individuals ha−1, and a few mute swans (Cygnus olor Gmel.), 0.2 individuals ha−1. During the growing season, macrophyte biomass, shoot length, shoot number per square metre and shoot height became significantly greater in exclosures/enclosures protected against grazing by waterfowl than in unprotected exclosures/enclosures. The largest differences recorded were in total shoot length and number of shoots per square metre, the maximum values found in the protected exclosures being 18.0 m and 156 m−2, respectively, compared with 5.5 m and 53 m−2 in the unprotected exclosures. In addition, the percentage of stubble (number of shoots lacking an apex relative to the total number of shoots) was greatest in the unprotected exclosures, with a maximum summer percentage of 25% compared with only 12% in the protected exclosures. The study provides further evidence that waterfowl may suppress macrophyte biomass in lakes with a low abundance of submerged macrophytes. We predict that by grazing on submerged macrophytes, waterfowl may have a negative impact on lake water quality, particularly in lakes that are in the recovery phase and in which macrophytes are about to re-colonise following a reduction in external nutrient loading.
Six years after application of biomanipulation in 1987, Lake Zwemlust (The Netherlands) returned during the summer from a clear water state dominated by aquatic vegetation to a turbid state characterized by high algal biomass. Herbivory and growth of epiphytes on macrophytes were the main factors triggering the switch. Selective herbivory by coots (Fulica atra) and rudd (Scardinius erythrophthalmus) caused a change in macrophyte species composition from a dominance of Elodea nuttallii in 1988/1989 to Ceratophyllum demersum in 1990/1991, and finally to Potamogeton berchtholdii in 1992/1993/1994. Observations revealed a general lack of epiphytes associated with Elodea and Ceratophyllum, while Potamogeton showed a progressive coverage with epiphytes, causing Potamogeton to decline markedly during late summer. Phytoplankton blooms, dominated by cyanobacteria, appeared again during three consecutive autumns, 1992, 1993 and 1994, with chlorophyll-a concentrations reaching 60-240 mu g l(-1). [KEYWORDS: biomanipulation; coots; epiphytes; herbivory; Lake Zwemlust; macrophytes Submerged plants; aquatic plant; stable states; shallow lakes; water; macrophytes; eutrophication; phytoplankton; ecosystem;community]