摘要:

SUMMARY.The distribution and quantitative development of aquatic macrophytes have been studied in oligotrophic Lake Kalgaard, Denmark. The vegetation is dominated by isoetid species, which are widely distributed (about 40% of the lake bottom) compared to emergent and floating‐leaved macrophytes (about 4%). Littorella uniflora dominates at depths of 0–2 m and Isoetes lacustris from 2.0 to 4.5m. Within the colonization area the mean midsummer biomass of Littorella is 112g organic dry weight m−2and that of Isoetes, 66 gm−2. The total biomass of these two species constitutes 99% of the biomass of submerged macrophytes.The perennial Littorella shows only small seasonal biomass variations. The vegetational biomass, the above‐ground fraction of the biomass, and the weight of individual plants all increased with the organic content of the sediment at water depths from 0 to 0.75 m. At the same time the interstitial concentrations of carbon dioxide, extractable inorganic nitrogen, and exchangeable inorganic phosphorus increased, thus supporting the hypothesis that an increasing organic content of the sediments at this low level creates a physiologically richer medium for the plants.The isoetid growth form is discussed in relation to the chemical environment of oligotrophic, softwa

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1979.tb01481.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

SUMMARY.Decomposition ofPotamogeton pectinatusin Swartvlei, a southern African coastal lake, followed an exponential pattern of decay. The rale constant was 0.0205 day−1and decay was virtually complete after 158 days. The original stock of ash, phosphorus and potassium was lost more rapidly than dry matter in the initial stages of decomposition due to leaching. Almost the entire stock of potassium and 60% of the phosphorus was lost in the first 7 days. The proportions of N and P in the decomposing detritus increase significantly in the final stages of decomposition, providing a rich food source for the abundant filter‐feeding animals of the littoral zone. The decomposition rate of P.pectinatusis compared with that of other aquatic macrophytes and the significance of the relatively rapid decay rate of this species in the ecology of Swartvlei lake is discus

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1979.tb01482.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

SUMMARY.Between 1967 and 1977, catches ofTilapia galilaeain Lake Kinneret fell from an annual average of about 200 tonnes to a low of 70 tonnes in 1974, and then returned to their former level. Total yearly mortality (Z) was estimated at 1.84 between 1970 and 1976. Assuming a natural mortality (M) of 0.94, total stock size averaged 300–700 tonnes in that period, while recruitment averaged<0.7–3 million per year. Three broods hatched in years of relatively high water levels (1970, 1971 and 1972) yielded especially low recruitment.The Von Bertalanffy growth coefficient (K) was estimated at 0.67, and final length (L∞) at 25–36cm total length. First year's growth, 23–152mm, was closely related to final length, which appeared to be affected by genetic variation in the population. Yearly fluctuations in size distribution were attributed to changes in the proportions of fast‐ and slow‐growers, as well as in the abundance of year‐classes. Recruitment to the fishery, at 17–20cm, occurred between ages l+and3+.The highest growth rates for age‐1T, galilaea, 1–2% mean weight per day, occurred in April and May, thePeridiniumseason. However, only a small proportion of age‐1 fish reached marketable size by July, when their food supply began to diminish. During the autumn months, there was hardly any length increment, and allT. galilaeabecame lean, especially the age‐0 group. It is surmised that recruitment of stocked fish could be increased by introduction in December and by use of fingerlings which are potentially able to grow to marketable size

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1979.tb01483.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

SUMMARY.In laboratory experiments, prey selection of zooplankters by starved perch fry (Perca fluviatilis) from Windermere, English Lake District, is temporally variable and there is usually, in the short‐term, a persistent succession from one actively selected prey species to another. Provided that the prey is visible, the patterns of prey selection are maintained irrespective of the accessibility of the prey. The fish do not optimize their energy intake per effort. The method of observing predation activity by fish shows qualitative and quantitative prey selection as time progresses in great detai

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1979.tb01484.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

SUMMARY.A 2‐year sampling programme of three benthic sites in a small eutrophic loch revealed large communities of ciliated protozoa consisting of a wide diversity of species (91 spp. over the 2 years). Species‐groups described with the aid of association analysis (normal and inverse) indicated the existence of a ‘ubiquitous’ species‐group consisting of at leastCyclidium glaucomaandAspidisca costata. Another species‐group containing large ciliates (Loxodes striatus, Frontonia leucas, Spirostomumspp. andParameciumspp.) was characteristic of the summer months at all sites. It is advocated that association analysis might increase the value of protozoa as pollution

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1979.tb01485.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

SUMMARY.Three experiments are described in which the rates of loss from suspension ofLycopodiumspores introduced at the water surface in a large butyl rubber enclosure (‘Lund Tube’) were monitored. Rates of arrival at the mud surface and in sediment traps placed in the water column were also measured. Results obtained by each method are compared with the predetermined intrinsic sinking behaviour of the individual particles. The experiments, which were carried out under differing conditions of water‐column stability (isothermal mixing, thermal microstratification, and autumnal destratification), demonstrate the importance of turbulent mixing in determining the effective settling rate of seston. A simple predictive model is developed to relate sinkinglosses to the vertical extent of broadly defined water layers. Recoveries in sediment traps can give an accurate synopsis of net sedimentation when they are operated in non‐turbulent water layers, but trap performance in turbulent flow is significantly modified by hydrodynamic perturbation generated by the traps the

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1979.tb01486.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1979.tb01487.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1979.tb01488.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

Hammond C. O. (1977)The Dragon flies of Great Britain and Ireland.Golterman H. L.(Ed.)(1977)Interactions Between Sediments and Freshwater.Schwoerbel J. (1977)Einführung in die Limnologie

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1979.tb01489.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

ISSN:0046-5070    

DOI:10.1111/j.1365-2427.1979.tb01490.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY