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  • 1
    Language: English
    In: Oikos, May, 2011, Vol.120(5), p.766(10)
    Description: To authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1600-0706.2010.18553.x Byline: Sabine Hilt, Jan Kohler, Hans-Peter Kozerski, Egbert H. van Nes, Marten Scheffer Abstract: Regime shifts between clear and turbid water states are commonly found in shallow lakes. These shifts are attributed to a positive feedback between water clarity and submerged macrophytes (underwater plants). Altering the retention time of the water may influence these interactions and thus potentially reduce the probability of alternative stable states. Here we assessed the effect of water retention time on the occurrence of alternative states in water quality of flushed lakes, chains of lakes and rivers using a spatially explicit simple model. Our results indicate that increased flushing of lakes rapidly decreases the range of parameters with alternative stable states up to their total disappearance at a flushing rate of about 50% the algal growth rate. Similarly, in a chain of lakes or in rivers with low flowing velocity, our model predicts that alternative stable states can only occur for systems with a high retention time. Despite the lack of hysteresis at lower water retention times, we predict that abrupt changes between clear and turbid states are still possible both in time and in space. Over a wide range of parameters, the equilibrium state of the chain of lakes shows a steep gradient of vegetation cover. Further, the transient dynamics of the model often include rapid shifts in time. For example, a local regime shift that occurs upstream may propagate through the whole lake chain or river due to a domino effect. All results of the simple model could qualitatively be reproduced with a more mechanistic model. The abrupt rather than gradual response of submerged macrophytes to reduced turbidity levels still makes river systems rather resilient to management measure. The importance of the initial turbidity and the observed domino effect suggest that restoration measures should start upstream and that these measures should eventually trigger regime shifts downstream. Author Affiliation: (1)Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Muggelseedamm 310, DE-12587 Berlin, Germany (2)Dept of Aquatic Ecology and Water Quality Management, Wageningen Univ., PO Box 47, NL-6700 AA Wageningen, the Netherlands Article History: Paper manuscript accepted 14 September 2010 Article note: S. Hilt, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Muggelseedamm 310, DE-12587 Berlin, Germany. E-mail: hilt@igb-berlin.de
    Keywords: Rivers -- Analysis
    ISSN: 0030-1299
    E-ISSN: 16000706
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  • 2
    In: Freshwater Biology, October 2013, Vol.58(10), pp.2027-2036
    Description: During eutrophication, submerged macrophytes in temperate European shallow lakes are thought to undergo a sequence from seasonally ‘stable’ conditions characterised by high water clarity in spring and summer, through ‘crashing’ conditions where the water is clear in spring but dominated by phytoplankton in late summer, to ‘turbid’ conditions with year‐round phytoplankton dominance. However, it is not known whether this sequence is reversed during re‐oligotrophication and whether this contributes to the often observed delay in macrophyte recovery during lake restoration. We analysed long‐term (100 years) data on macrophyte species presence, maximum colonisation depth, Secchi depth and seston concentration in shallow Lake Müggelsee during eutrophication from around 1900 and during re‐oligotrophication that started in 1990. The current clonal diversity of the dominant species (Potamogeton pectinatus) was investigated to determine whether vegetative dispersal was predominant during its re‐establishment. During eutrophication, Lake Müggelsee went through a crashing phase for c. 70 years with a gradual decline in macrophyte species diversity from c. 24 to 5 species. From around 1970, the lake became turbid and was dominated by phytoplankton for the next 20 years. Following a reduction in external nutrient loading by 50% from 1990, spring clear‐water conditions immediately re‐appeared, and P. pectinatus started to re‐establish from a few stands that had survived in very shallow areas. By 2011, species diversity had increased to 25 species and maximum colonisation depth had reached 3.2 m. Despite a continuing dominance of P. pectinatus, seasonally persistent (Ceratophyllum demersum) and late‐season associated (Najas marina) species re‐appeared suggesting potential for seasonally stable macrophyte conditions in future. Based on microsatellite analyses, more recently established P. pectinatus stands had lower genotype diversity and were comprised of only a small subset of genotypes from shallower areas, suggesting that vegetative dispersal was more important than seed dispersal for plant re‐establishment. We argue that this prevailing reproduction by tubers in combination with negative effects of herbivory and periphyton shading, shown for P. pectinatus in earlier studies in this lake, contributed to the long duration of macrophyte re‐establishment.
    Keywords: Biodiversity ; Eutrophication ; Restoration ; Shallow Lakes ; Submerged Macrophytes
    ISSN: 0046-5070
    E-ISSN: 1365-2427
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  • 3
    Language: English
    In: Hydrobiologia, 2016, Vol.778(1), pp.91-103
    Description: Groundwater influx can significantly contribute to nutrient and carbon budgets of lakes, and its influence is the strongest in littoral areas dominated by macrophytes and periphyton. We have reviewed the effects of groundwater-borne nitrogen and phosphorus and dissolved inorganic and organic carbon (DIC, DOC) on these benthic primary producers in lakes. We develop a hypothesis for groundwater effects including the less studied impacts of periphyton shading on macrophytes. Groundwater-borne nutrients and DIC promote both macrophytes and periphyton. Direct studies on groundwater-borne DOC effects are lacking, but coloured DOC contributes to light attenuation and thus can restrict the growth of benthic primary producers. We predict that above certain threshold levels of nutrient influx by groundwater, periphyton and macrophyte biomass should decline owing to shading by phytoplankton and periphyton, respectively. However, because of their higher light requirements, those thresholds should be lower for macrophytes. For macrophytes, a threshold level is also predicted for a shift from DIC limitation to light limitation. Differences in light requirements are expected to result in lower thresholds of DOC loading for declines of macrophytes than periphyton.
    Keywords: Dissolved inorganic carbon ; Dissolved organic carbon ; Light ; Macrophytes ; Nutrients ; Periphyton
    ISSN: 0018-8158
    E-ISSN: 1573-5117
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  • 4
    Language: English
    In: Ecology, December 2013, Vol.94(12), pp.2754-66
    Description: In aquatic systems, terrestrial dissolved organic matter (t-DOM) is known to stimulate bacterial activities in the water column, but simultaneous effects of autumnal leaf input on water column and sediment microbial dynamics in littoral zones of lakes remain largely unknown. The study's objective was to determine the effects of leaf litter on bacterial metabolism in the littoral water and sediment, and subsequently, the consequences for carbon cycling and food web dynamics. Therefore, in late fall, we simultaneously measured water and sediment bacterial metabolism in the littoral zone of a temperate shallow lake after adding terrestrial particulate organic matter (t-POM), namely, maize leaves. To better evaluate bacterial production (BP) and community respiration (CR) in sediments, we incubated sediment cores with maize leaves of different quality (nonleached and leached) under controlled laboratory conditions. Additionally, to quantify the incorporated leaf carbon into microbial biomass, we determined carbon isotopic ratios of fatty acids from sediment and leaf-associated microbes from a laboratory experiment using 13C-enriched beech leaves. The concentrations of dissolved organic carbon (DOC) increased significantly in the lake after the addition of maize leaves, accompanied by a significant increase in water BP. In contrast, sediment BP declined after an initial peak, showing no positive response to t-POM addition. Sediment BP and CR were also not stimulated by t-POM in the laboratory experiment, either in short-term or in long-term incubations, except for a short increase in CR after 18 hours. However, this increase might have reflected the metabolism of leaf-associated microorganisms. We conclude that the leached t-DOM is actively incorporated into microbial biomass in the water column but that the settling leached t-POM (t-POML) does not enter the food web via sediment bacteria. Consequently, t-POML is either buried in the sediment or introduced into the aquatic food web via microorganisms (bacteria and fungi) directly associated with t-POM(L) and via benthic macroinvertebrates by shredding of t-POM(L). The latter pathway represents a "benthic shortcut" which efficiently transfers t-POM(L) to higher trophic levels.
    Keywords: Ecosystem ; Bacteria -- Metabolism ; Carbon -- Chemistry
    ISSN: 0012-9658
    E-ISSN: 19399170
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  • 5
    In: Freshwater Biology, November 2013, Vol.58(11), pp.2264-2276
    Description: Regime shifts are commonly associated with the loss of submerged macrophytes in shallow lakes; yet, the effects of this on whole‐lake primary productivity remain poorly understood. This study compares the annual gross primary production (GPP) of two shallow, eutrophic lakes with different plant community structures but similar nutrient concentrations. Daily GPP rates were substantially higher in the lake containing submerged macrophytes (586 ± 23 g C m−2 year−1) than in the lake featuring only phytoplankton and periphyton (408 ± 23 g C m−2 year−1; P 〈 0.0001). Comparing lake‐centre diel oxygen curves to compartmental estimates of GPP confirmed that single‐site oxygen curves may provide unreliable estimates of whole‐lake GPP. The discrepancy between approaches was greatest in the macrophyte‐dominated lake during the summer, with a high proportion of GPP occurring in the littoral zone. Our empirical results were used to construct a simple conceptual model relating GPP to nutrient availability for these alternative ecological regimes. This model predicted that lakes featuring submerged macrophytes may commonly support higher rates of GPP than phytoplankton‐dominated lakes, but only within a moderate range of nutrient availability (total phosphorus ranging from 30 to 100 μg L−1) and with mean lake depths shallower than 3 or 4 m. We conclude that shallow lakes with a submerged macrophyte–epiphyton complex may frequently support a higher annual primary production than comparable lakes that contain only phytoplankton and periphyton. We thus suggest that a regime shift involving the loss of submerged macrophytes may decrease the primary productivity of many lakes, with potential consequences for the entire food webs of these ecosystems.
    Keywords: Macrophytes ; Oxygen Curves ; Periphyton ; Regime Shift ; Trophic Status
    ISSN: 0046-5070
    E-ISSN: 1365-2427
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  • 6
    Language: English
    In: Hydrobiologia, 2014, Vol.737(1), pp.57-66
    Description: We investigated the effects of Eurasian watermilfoil ( Myriophyllum spicatum ) on a natural phytoplankton community in 85 l mesocosms. Changes in phytoplankton community composition, biomass, primary productivity, nitrogenase activity and the abundance of toxic and non-toxic Microcystis aeruginosa subpopulations were investigated during 13 days of exposure. We aimed to test whether the known allelopathic activity of M. spicatum towards certain algae and cyanobacteria could be proven under field-like conditions with potential interference of resource competition and zooplankton grazing. The presence of M. spicatum had only short-term inhibitory patterns on total phytoplankton and green algae, whereas consistent negative effects were observed on cyanobacteria biomass. Quantitative PCR analyses revealed that toxic and non-toxic M. aeruginosa were equally inhibited. Total phytoplankton gross primary production was significantly suppressed by the presence of M. spicatum. Despite the partial interference by nutrient limitation, there are indications for a significant impact of macrophyte-excreted allelochemicals on phytoplankton. We argue that more mesocosm experiments with complex natural phytoplankton communities are needed to unravel the ecological relevance of macrophyte allelopathy.
    Keywords: Allelopathy ; Mesocosm experiment ; Microcystis aeruginosa ; Myriophyllum spicatum ; Submerged macrophytes
    ISSN: 0018-8158
    E-ISSN: 1573-5117
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  • 7
    Language: English
    In: Hydrobiologia, 2014, Vol.737(1), pp.321-331
    Description: Alien aquatic plant species can strongly affect all types of freshwater ecosystems. Their number has more than doubled between 1980 and 2009 in Germany, and currently 27 are known and their number is still increasing. Eleven have been classified as invasive, but only four are managed yet, mainly by weed cutting. Most of the alien aquatic plant species were probably introduced as aquarium and pond waste. Despite this fact, 18 of the 27 known alien species are traded as ornamentals for aquaria or garden ponds in German shops. Alien species can most successfully be controlled when their management starts as soon as possible after their introduction. In Germany, the delay between first records and start of management actions seems too long for successful control. The public awareness of alien aquatic plants and problems they can cause in Germany is still limited despite a number of recent projects. At present, Black lists are developed that help nature conservationists, stakeholders and politicians to select those alien species for which prevention measures should be implemented. These, however, are not legally binding and laws regulating trade in Black listed plant species are strongly needed to reduce their impact on the environment and economy.
    Keywords: Ornamental trade ; Control ; Eradication ; Public awareness ; Alien aquatic plants ; Prevention
    ISSN: 0018-8158
    E-ISSN: 1573-5117
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  • 8
    Language: English
    In: Hydrobiologia, 2014, Vol.737(1), pp.99-110
    Description: A number of mechanisms result in a feedback between water clarity and macrophytes and, consequently, the occurrence of alternative stable states in shallow lakes. We hypothesize that bottom-up mechanisms and interactions within the benthic food web are more important in a charophyte-dominated clear-water state, while top-down mechanism and interactions in the planktonic food web prevail at angiosperm dominance. Charophytes, which dominate at lower nutrient concentrations and develop higher densities than most angiosperms, can have a higher influence on sedimentation, resuspension, and water column nutrients. During dominance of dense submerged vegetation like charophytes, zooplankton can be hampered by low food quality and quantity and by high predation pressure from juvenile fish, which in turn are favoured by the high refuge potential of this vegetation. Grazing pressure from zooplankton on phytoplankton can therefore be low in charophytes, but the main feedback in angiosperm-dominated ecosystems. Charophytes offer a higher surface than most angiosperms to periphyton, which favors benthic invertebrates. These support macrophytes by grazing periphyton and constitute a central link in a trophic cascade from fish to periphyton and macrophytes. To test these hypotheses, more experiments and field measurements comparing the effect of charophytes and angiosperms on water clarity are needed.
    Keywords: Alternative stable state ; Angiosperms ; Characeae ; Macroinvertebrates ; Shallow lakes ; Zooplankton
    ISSN: 0018-8158
    E-ISSN: 1573-5117
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  • 9
    Language: English
    In: PLOS ONE, 2013
    Description: Interspecific differences in the response of microalgae to stress have numerous ecological implications. However, little is known of intraspecific sensitivities and the potential role of local genetic adaptation of populations. We compared the allelochemical sensitivity of 23 Pediastrum duplex Meyen strains, a common component of the freshwater phytoplankton. In order to test for local genetic adaptation, strains were isolated from water bodies with and without the allelopathically-active submerged macrophyte Myriophyllum. Strains were assigned to P. duplex on the basis of cell shape and colony morphology and only P. duplex strains that belonged to the same lineage in an ITS rDNA phylogeny were used. Inhibition of strain growth rates and maximum quantum yields of photosystem II were measured after exposure to tannic acid (TA) and co-culture with Myriophyllum spicatum. Growth rate inhibition varied over one order of magnitude between the P. duplex strains. There was no correlation between the presence of Myriophyllum in the source location and the sensitivity of the strains to TA or the presence of Myriophyllum, suggesting that at least strong unidirectional local adaptation to Myriophyllum had not taken place in the studied water bodies. The maximum quantum yield of photosystem II of TA exposed algae decreased, whereas the yield of algae exposed to M. spicatum was slightly higher than that of the controls. The ranking of P. duplex strain sensitivities differed between the types of exposure (single additions of TA versus co-existence with M. spicatum) and the parameter measured (growth rate versus maximum quantum yield), emphasizing the importance of measuring multiple traits when analysing strain-specific sensitivities towards allelochemicals. The observation that sensitivities to allelochemicals vary widely among strains of a single freshwater algal species should be taken into account if evaluating ecological consequences of allelopathic interactions.
    Keywords: Biology And Life Sciences ; Marine-Phytoplankton ; Chlorophyll Fluorescence ; Submerged Macrophytes ; Polyphenolic Allelochemicals ; Myriophyllum-Spicatum ; Genetic Adaptation ; Water ; Inhibition ; Growth ; Sphaeropleales
    ISSN: 1932-6203
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  • 10
    Language: English
    In: Hydrobiologia, 2006, Vol.570(1), pp.95-99
    Description: In shallow lakes, submerged macrophytes contribute to the stabilization of the clear water state. If lost, a number of mechanisms prevent re-colonization. Lake Müggelsee (730 ha) lost its submerged vegetation due to increasing eutrophication and switched to phytoplankton dominance in 1970. After the reduction of nutrient loading in 1990, Potamogeton pectinatus L. started re-colonizing the lake. During the following years, it spread at a mean rate of 2.5 ha per year to all available areas 〈80 cm depth. Between 1993 and 1999, decreasing maximum biomass indicated hampered growth. Exclosure experiments revealed that herbivory reduced the aboveground biomass by more than 90%. Both waterfowl and fish were found to contribute to the grazing pressure despite a low abundance of the known herbivorous fish species and waterfowl in spring and summer. Protection of stands against grazing resulted in higher biomass of shoots, whereas shoot and tuber density did not change. Both shading by phytoplankton and periphyton, as well as grazing pressure, prevented the submerged vegetation of Lake Müggelsee from developing back to a dense zone that contributed to the reduction of turbidity.
    Keywords: submerged macrophytes ; restoration ; exclosures ; fish herbivory
    ISSN: 0018-8158
    E-ISSN: 1573-5117
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