General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Key words : buoyancy, artificial mixing, aeration, Microcystis, cyanobacteria Abstract In a storage reservoir, which is artificially mixed in order to reduce algal and especially cyanobacterial growth, the cyanobacterium Microcystis is still present . The aim of the research was to investigate why Microcystis was able to grow in the artificially mixed reservoir . From the results it could be concluded that the large shallow area in the reservoir allows this growth . The loss of buoyancy during the day was much higher in this shallow part than in the deep part. Assuming that the loss of buoyancy was the result of a higher carbohydrate content, a higher growth rate in the shallow part may be expected . A higher received light dose by the phytoplankton in the shallow mixed part of the reservoir than in the deep mixed part explains the difference in buoyancy loss . A significant correlation between the received light dose (calculated for homogeneously mixed phytoplankton) and the buoyancy loss was found . Apparently, the Microcystis colonies were entrained in the turbulent flow in both the shallow and the deep part of the reservoir. With a little higher stability on one sampling day, due to the late start of the artificial mixing, the loss of buoyancy at the deep site was higher than on the other days and almost comparable to the loss at the shallow site . Although the vertical biomass distribution and the temperature profiles showed homogeneous mixing, the colonies in the upper layers apparently received a higher light dose than those deeper in the water column . Determination of the buoyancy state of cyanobacteria appeared to be a valuable method to investigate the light history and hence their entrainment in the turbulent flow in the water column .

The Water Storage Corporation Brabantse Biesbosch manages three interconnected deep reservoirs in the south western part of The Netherlands. These reservoirs, with a total retention period of five to six months, are employed for storage and upgrading of polluted and eutrophic water from the River Meuse. In the first two reservoirs, with a total retention period of 4-5 months, probably due to adsorption/settling and biodegradation there is a reduction of geosmin and 2-methylisoborneol (2-MIB) down to 1-2 ng/l, i.e. 60-70% of the concentration in the river water. In the last reservoir Petrusplaat a large benthic cyanobacterial population consisting of various Oscillatoriaceae produces geosmin from February till June. In 1984 this production resulted in an average concentration of 6 ng/l, i.e. 5-6 times the concentration of the inlet of this reservoir. Since 1985 cyanobacterial growth is controlled by rooting up the bottom with a harrow. This resulted in average concentrations of 1-3 ng/l, i.e. twice the average concentration of the inlet. No further reduction of 2-MIB has been observed in this reservoir. It is concluded that storage of eutrophic river water in deep reservoirs reduces the concentration of geosmin and 2-MIB. Autochthonous production however may interfere with this water quality improvement.

For three years chironomid larvae were collected bimonthly with a grab sampler in two deep storage reservoirs in The Nethedands. Chironomid pupal exuviae were collected monthly, from April to November, with a handnet. Net samples yielded more taxa than grab samples. The taxa found in net samples originated from more habitats, than taxa found in grab samples. The relative contribution of Orthocladiinae was much larger in net samples. Based on the results obtained in the two reservoirs, some advantages and disadvantages of both sampling methods are discussed.

Long-term changes in distribution and taxonomic composition of chironomid larvae and oligochaetes in two water storage reservoirs in The Netherlands were studied. A succession among the chironomid spe-9 was observed. In the last 10-15 years chironomid densities varied. Compared with other lakes chironomid densities were high. Densities of chironomids were higher in the littoral zone than in the profundal zone. The opposite was found for oligochaetes. Densities and composition of the chironomid fauna in the two reservoirs were similar. Proc/adius, Tanytarsus and Harnischia dominated at all depths. However, pupal exuviae samples showed some differences in chironomid taxonomic composition between the two reservoirs. Orthocladiinae, rarely found in bottom samples, abounded in pupal exuviae samples.

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