摘要:

AbstractAs a part of a whole‐lake, long‐term experiment in biomanipulation in. the hypertrophic Bautzen reservoir (G.D.R.), during three years (1984–1986) the dynamics of mouse‐related LD 50 ofMicrocystis aeruginosawas compared with the biomass development of this blue‐green and the grazing pressure exerted byDaphnia galeata.Since the three summer averages of the biomass ofD. galeatarevealed strong differences due to decreasing predation activity of fish from 1984 to 1986, the effects of different grazing pressure onMicrocystistoxicity could be investigated under field conditions.Microcystiswas nontoxic at the beginning of the growing season and developed high toxicity during its first strong biomass increase in summer in all three years. But this decrease of the LD 50 together with the first biomass increase of the season is found in quite different periods in different years (1984: August, 1985: July, 1986: June). It is obvious that the higher the mean effective filtration rate ofD. galeataduring summer is found the faster the toxicity ofMicrocystisis formed. If these observations are combined with findings of other authors, the conclusion can be drawn that the development of toxicMicrocystisblooms seems to be promoted by a combination of five conditions:(1) Presence of a mixture of toxic and nontoxicMicrocystisstrains at the beginning of the growing season even if the portion of toxic strains is very low, (2) physical and chemical growth conditions which favourMicrocystisover other phytoplankton, (3) high grazing pressure by zooplankton on edible food particles over a rather long period, (4) patchy distribution of the differentMicrocystisstrains if nonselective filtrators such as Daphnia dominate the zooplankton, and (5) absence of defense mechanisms ofMicrocystisagainst grazing which are not coupled with toxicity (e.g. large colony size). These conclusions contribute to a better understanding of the possibilities and limits of in‐lake eutrophication control by biomanipulation and emphasize the need to combine top‐down and bottom‐up control mechanisms in eutrophic and hype

ISSN:0020-9309    

DOI:10.1002/iroh.19890740302

出版商:Akademie Verlag, Berlin    

年代:1989

数据来源: WILEY

摘要:

AbstractLarge errors are made when laboratory measurements of the ingestion rate of herbivorous zoo‐plankton are used to estimate grazing in planktonic environments. The errors arise because the spatial heterogeneity in natural plankton distributions is greater than the heterogeneity present in laboratory experiments. Two probability models are developed for extrapolating laboratory measurements of ingestion to planktonic environments. Both models require sample statistics of the plankton distributions, and these are estimated from sampling studies of the small scale distribution of marine plankton. One model predicts laboratory measurements overestimate the ration obtained by an individual copepod by 30 %. A second model predicts errors exceeding ±50 % are possible when laboratory measurements are used to estimate the grazing by a population of cope‐

ISSN:0020-9309    

DOI:10.1002/iroh.19890740303

出版商:Akademie Verlag, Berlin    

年代:1989

数据来源: WILEY

摘要:

AbstractThough rotifers play an important role in many pelagic ecosystems, there is a lack for studies on diurnal variations in feeding behaviour. Diurnal feeding rhythms of estuarine populations ofBrachionus plicatilisandKeratella cruciformis f. eichwaldiand a pond population ofK. cochleariswere investigated using14C‐Chlorella and14C‐labelled natural bacteria populations, respectively, as a tracer food duringin situexperiments. A51Cr/14C double tracer technique was used to determine Assimilation efficiencies.All species had about two times higher feeding rates during day than during dark hours. There was a tendency for higher values during the afternoon. No trend was found regarding diurnal changes in assimilation efficiency.Diurnally segregated niches between microphagous daytime active rotifers and nighttime active crustacean populations in pelagic ecosystems are demonstrated and their ecological significance with special regard to changes in food quality and predation pressure is discus

ISSN:0020-9309    

DOI:10.1002/iroh.19890740304

出版商:Akademie Verlag, Berlin    

年代:1989

数据来源: WILEY

摘要:

AbstractThe life history and distribution ofSialis lutariaL. in the exposed littoral zone of Lake Esrom, Denmark were examined during 1979/80.Sialisshowed an annual life cycle with mean annual abundance of 20 and 40 ind m−2at 0.4 and 2.0 m depth, respectively. Growth peaked during July with rates of about 10 % d−1. Net annual production averaged 0.2 g DW at 0.4 m depth and 0.8 g DW at 2.0 m depth. The annual P/B‐ratios were estimated in the range of 4.

ISSN:0020-9309    

DOI:10.1002/iroh.19890740305

出版商:Akademie Verlag, Berlin    

年代:1989

数据来源: WILEY

摘要:

AbstractThe effect of pH on copper toxicity to two planktonic blue‐green algae,Aphanizomenon gracileandOscillatoria redekei, was investigated. Growth rates of the algae without copper treatment decrease with pH,Aphanizomenonis earlier and more affected thanOscillatoria.On the other hand, pH‐lowering leads sooner to a toxicity enhancement inOscillatoria.In the acid range, toxicity retardation occurs inAphanizomenon.At pH 5.1, shortening of the interval between copper toxicity and copper stimulus is characteristic for both spec

ISSN:0020-9309    

DOI:10.1002/iroh.19890740306

出版商:Akademie Verlag, Berlin    

年代:1989

数据来源: WILEY

摘要:

AbstractPhotosynthetically active radiation (PAR) was registered daily in 0.25 m and 11.25 m in 3 years. Upwards irradiance of green light was 1 % of downward irradiance. Incubator14C experiments showed that the phytoplankton enhanced its carbon uptake substantially with increasing concentrations of the carrier CO2in the ampoules. Severe carbon limitation of photosynthesis occurred in spring and summer 1961, where the real carbon fixation was only about 34 % of that calculated by the usual procedure. Utilization of light and DIC by the phytoplankton and its compensation depth was determined. Photoinhibition occurred down to 10–11 m, and net primary production was nearly always positive in any depth down to the lake botto

ISSN:0020-9309    

DOI:10.1002/iroh.19890740307

出版商:Akademie Verlag, Berlin    

年代:1989

数据来源: WILEY

摘要:

AbstractThe relationship between phosphorus concentration and estimated phosphorus load in Lake Dal—a marl lake—is described for the past decade. The concentration of this element in the lake system remained constant during this period although the P load increased by about 40 %. This phenomenon is discussed in connection with various P‐removal mechanisms operating in the lake s

ISSN:0020-9309    

DOI:10.1002/iroh.19890740308

出版商:Akademie Verlag, Berlin    

年代:1989

数据来源: WILEY

摘要:

AbstractSeasonal, local and monthly vertical variations in physical and chemical characteristics of Aswan High Dam Lake (AHDL) were followed during the period from March 1982 to February 1984. The results show that there were distinct seasonal thermal variations. Similarly, thermal stratification was evident from late spring through early autumn. Oxygen content appeared to vary with the changes in pH value. Water transparency in the southern part was greatly reduced by the incoming silt laden flood especially in summer months. Consequently, nitrate‐nitrogen and total residue increased considerably in this part during summer. The recorded minimum N‐concentration (2.35μg l−1) was certainly limiting the growth of phytoplankton. Suspended and organic matter exhibited somewhat irregular variations at least during the period of this

ISSN:0020-9309    

DOI:10.1002/iroh.19890740309

出版商:Akademie Verlag, Berlin    

年代:1989

数据来源: WILEY

ISSN:0020-9309    

DOI:10.1002/iroh.19890740311

出版商:Akademie Verlag, Berlin    

年代:1989

数据来源: WILEY

ISSN:0020-9309    

DOI:10.1002/iroh.19890740312

出版商:Akademie Verlag, Berlin    

年代:1989

数据来源: WILEY