A CONCEPTUAL MODEL OF A SILTATION SYSTEM IN SHALLOW LAKES WITH LITTORAL VEGETATION

SUMMARY

In shallow lakes a siltation system develops when wind driven water turbulence erodes the lake floor, silt is resuspended and transported by means of waterdrift into the littoral vegetation. Here, due to the resistance of the vegetation, the turbulence is reduced and the silt deposited. Through this mechanism, the prevailing winds have a major rôle in the erosion and siltation pattern, lake morphology, vegetation pattern and in the mixing of water and nutrients from the open water and the reedswamp and vice versa. The model explains the uneven distribution of vegetation at the lakes Neusiedlersee (Austria) and Chilwa (Malawi) satisfactorily, if one assumes a positive correlation between siltation and reedswamp growth and spread.     

Allelopathy among Cyanoprokaryota and Other Algae Originating from Lake Neusiedlersee (Austria)

During the late eighties, the eutrophic shallow lake Neusiedlersee located at the austrian – hungarian border showed a massive development of cyanobacteria, which coincided with a strong decline of chlorococcales. Besides a change in nutrient conditions, allelopathy may have contributed to the dominance of cyanobacteria, too. To get a first insight into potential allelopathic effects, 13 cyanobacteria strains were tested originating for the most part from lake Neusiedlersee. Plate diffusion assays showed allelopathic activity of 6 strains indicating chemical interactions between cyanobacteria and other algae.     

Palaeolimnology of Neusiedlersee,Austria

Eighteen sediment cores from the Neusiedlersee reed belt were analysed for organic and inorganic carbon, nitrogen, organic and inorganic phosphorus and trace metal content. The depth variation of the distribution patterns of C, P and N in the cores are influenced by: 1. temporal variability of organic matter deposition (e.g. burial of Phragmites litter) 2. the impact of a number of astatic phases in the history of the lake, and 3. early palaeo-ecological changes associated with the alteration of marine to lacustrine environment. At some sites, due to mineralisation, the organic phosphorus compartment in the Pannonian sediments is completely exhausted; accordingly there is a parallel decrease in carbon and nitrogen content in the profiles. These changes in the pattern of the distribution of the nutrients (especially organic phosphorus) can be used to differentiate between the recent and the upper boundary layer of the tertiary sediments. There is no clear trace metal distribution pattern observed in the cores but the Pannonian sediments recorded the lowest levels. In contrast, at some locations surface sediments register a two to three fold increase. The highest C, P, N, values and trace metal concentrations are recorded in the surface sediments deposited after the drying up of the lake a hundred and twenty-two years ago.     

Assessment of Components Controlling Phytoplankton Photosynthesis and Bacterioplankton Production in a Shallow,Alkaline, Turbid Lake (Neusiedlersee,Austria)

Various aspects of phytoplankton photosynthesis were investigated between 1968 and 1980 in Neusiedlersee, Austria, a shallow (z̄ = 1.3 m), well mixed lake with much turbidity resulting from frequent resuspension of inorganic sediment particles. The influences of the great environmental variability on biomass, photosynthesis and phytoplankton production were determined and discussed with respect to the increasing nutrient concentrations in the lake. Investigations of bacterioplankton biomass and production revealed their importance for nutrient regeneration and as food for large zooplankton populations. The results are compared with those from several other shallow lakes, and an attempt is made to identify characteristics to distinguish the different types.     

Paleolimnology of Neusiedlersee,Austria

Neusiedlersee is a shallow, alkaline lake (Table 1), which came into existance by tectonic subsidence about 12000–14000 yr ago. At present, half of it is covered by Phragmites australis which developed after the lake fell dry for the last time in 1868.Due to its astatic character, with about 100–200 dry periods since the lake came into existence, most of the sediment of the open lake overlying a thick Tertiary layer presents a mixture of terrestrial and lacustrine components. Because of disturbance by wind, this lacks stratification. Preliminary investigations, however, demonstrated that within the fringing Phragmites belt — especially landward — such a stratification does still exist. In order to learn more about the history of the lake 18 deep freeze cores of between 135 and 190 cms in depth, were collected from the Phragmites belt and investigated for chemical data (see Gunatilaka, this volume) and the succession of ostracods. Most of the cores contain Tertiary material (foraminifera and ostracods) at their base, followed by a cold water fauna as indicated by the presence of Cytherissa lacustris. Indicators of high alkalinity (i.e. the remains of male Limnocythere inopinata and high salinity (Heterocypris salina) were found in only a few cores.