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  • 1
    Digitale Medien
    Digitale Medien
    Top-down control in freshwater lakes: the role of nutrient state, submerged macrophytes and water depth (1997)
    Springer
    Hydrobiologia 342-343 (1997), S. 151-164 
    Details
    ISSN: 1573-5117
    Schlagwort(e): top-down control ; shallow lakes ; trophic structure ; trophic cascade ; macrophytes ; zooplankton ; fish ; biomanipulation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Biologie
    Notizen: Abstract 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1017046130329
    Standort Signatur Erwartet Verfügbarkeit
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  • 2
    Digitale Medien
    Digitale Medien
    Phosphorus release from resuspended sediment in the shallow and wind-exposed Lake Arresø, Denmark (1992)
    Springer
    Hydrobiologia 228 (1992), S. 91-99 
    Details
    ISSN: 1573-5117
    Schlagwort(e): shallow lakes ; resuspension ; internal P-loading ; equilibrium concentration
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Biologie
    Notizen: Abstract Wind-induced sediment resuspension occurs frequently in the shallow and eutrophic Lake Arresø, Denmark. The impact of resuspension on internal phosphorus loading was investigated by laboratory experiments studying P-release from the undisturbed sediment surface and by experiments simulating resuspension events. Phosphorus release from undisturbed sediment sampled in May and August was 12 mg and 4 mg m−2 d−1, respectively. During experimental simulation of resuspension, soluble reactive phosphate (SRP) increased by 20–80 µg l−1, which indicates that a typical resuspension event in the lake would be accompanied by the release of 150 mg SRP m−2. The internal P loading induced by resuspension is estimated to be 60–70 mg m−2 d−1, or 20–30 times greater than the release from undisturbed sediment. SRP release during simulation of resuspension was mainly dependent on the equilibrium conditions in the water column and was basically independent of the increase in suspended solids and the duration of resuspension. A second simulation of resuspension conducted 26 hours later, did not result in any further release of SRP from sediment sampled in May. In contrast, there was an additional SRP release from sediment sampled in August, indicating that an exchangable P pool, capable of altering equilibrium conditions, is built up between resuspension events. It is concluded that resuspension, by increasing the P flux between sediment and water, plays a major role in the maintenance of the high nutrient level in Lake Arresø. A relatively high release rate is maintained during resuspension because of the low Fe:P ratio and the high concentration of NH4Cl-extractable P in the sediment.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00006480
    Standort Signatur Erwartet Verfügbarkeit
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  • 3
    Digitale Medien
    Digitale Medien
    Internal phosphorus loading in shallow Danish lakes (1999)
    Springer
    Hydrobiologia 408-409 (1999), S. 145-152 
    Details
    ISSN: 1573-5117
    Schlagwort(e): shallow lakes ; phosphorus retention ; internal loading ; sediment
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Biologie
    Notizen: Abstract High phosphorus concentrations due to internal loading from the sediment with a strongly negative impact on lake water quality, is often seen in shallow lakes after a reduction of external loading. To analyse the nature of internal loading we studied 1. the seasonal phosphorus concentrations of 265 Danish shallow, mainly eutrophic lakes; 2. seasonal phosphorus mass balances and retention for eight years in 16 eutrophic lakes, and 3. phosphorus mass balances and changing sediment phosphorus profiles for 15 years in one hypertrophic lake. Lake water, inlets and outlets were routinely sampled 10–26 times annually. Total phosphorus (TP) concentrations during summer were two–four times higher than winter values in lakes with a mean summer total phosphorus concentration (TPsum) above 0.2 mg P l-1. Annual phosphorus retention decreased with increasing TPsumand was lower than predicted from the Vollenweider model, particularly in lakes with TPsumabove 0.2 mg P l-1. The seasonal phosphorus retention in lakes with TPsumbelow 0.1 mg P l-1was positive during the whole season, except July and August when mean retention ranged from −10 to −30% of inlet loading. In lakes with TPsumabove 0.1 mg P l-1, the retention was positive during winter, but negative from April to September. The negative retention was most pronounced in lakes with the highest TPsum, particularly in May and July when mean retention ranged from −50 to −68% in lakes with TPsumabove 0.2 mg P l-1. The retention was generally less negative in June, when a clearwater phase typically occurs and close to 0 also in lakes with a high TPsum. Mass balances from the hypertrophic lake have now shown a 15-yr net annual negative retention following reduced external loading. Sediment profiles suggest phosphorus release from depths down to 25 cm and that net internal phosphorus loading may persist for another 15 yrs. It is concluded that internal loading of shallow eutrophic lakes may have a considerable and persistent impact on summer TP after reduced external loading.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1017063431437
    Standort Signatur Erwartet Verfügbarkeit
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  • 4
    Digitale Medien
    Digitale Medien
    Trophic dynamics in turbid and clearwater lakes with special emphasis on the role of zooplankton for water clarity (1999)
    Springer
    Hydrobiologia 408-409 (1999), S. 217-231 
    Details
    ISSN: 1573-5117
    Schlagwort(e): alternative stable states ; turbid ; clearwater ; macrophytes ; fish ; trophic structure ; bottom-up ; top-down ; shallow lakes
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Biologie
    Notizen: Abstract Within a certain nutrient level shallow lakes may alternate between two states, a clearwater and a turbid state. To obtain more information on the characteristics of these two states, we compared seasonal variations in trophic structure and physico-chemical variables of two clear and two turbid lakes studied during seven or eight years. The clearwater lakes were characterised by a high abundance of submerged macrophytes, high piscivorous:planktivorous fish ratios, high zooplankton:phytoplankton ratios and low chlorophyll aduring summer. Submerged macrophytes were almost absent from the turbid lakes, planktivorous fish dominated, the zooplankton:phytoplankton ratio was low and summer chlorophyll awas high. While total phosphorus (TP) was almost constant throughout the season in the clearwater lakes, TP was substantially higher during summer in the turbid lakes reflecting high internal loading. In the clearwater lakes, mean summer chlorophyll awas only 45–51% of winter values, while summer chlorophyll awas 118–259% of winter values in the turbid lakes. Our data suggests that zooplankton, by grazing on phytoplankton, play a major role in maintaining clearwater conditions in eutrophic macrophyte-rich lakes, in particular during summer. In addition, results from a multiple regression on data from 37 lakes and the analyses of the seasonal dynamics in suspended solids provide some evidence that zooplankton grazing diminishes concentrations of detritus and inorganic suspended solids either directly by grazing or more indirectly. Using information also from literature, we argue that the role of zooplankton grazing for water clarity in macrophyte-rich lakes may increase from mesotrophic to eutrophic lakes.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1017071600486
    Standort Signatur Erwartet Verfügbarkeit
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