DIURNAL BUOYANCY CHANGES OF MICROCYSTIS IN AN ARTIFICIALLY MIXED STORAGE RESERVOIR

In a storage reservoir, which is artificially mixed in order to reduce algal and especially cyanobacterial growth, the cyanobacterium Microc ystis is still present. The aim of the research was to investigate why Microcystis was able to grow in the artificially mixed reservoir. Fro m 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 re ceived light dose by the phytoplankton in the shallow mixed part of th e reservoir than in the deep mixed part explains the difference in buo yancy loss. A significant correlation between the received light dose (calculated for homogeneously mixed phytoplankton) and the buoyancy lo ss was found. Apparently, the Microcystis colonies were entrained in t he turbulent flow in both the shallow and the deep part of the reservo ir. With a little higher stability on one sampling day, due to the lat e start of the artificial mixing, the loss of buoyancy at the deep sit e was higher than on the other days and almost comparable to the loss at the shallow site. Although the vertical biomass distribution and th e 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 cyanobact eria appeared to be a valuable method to investigate the light history and hence their entrainment in the turbulent flow in the water column .