In order to detect the cause of recurrent fish mortality in the ponds of the "Maria-Hendrika"-park of Ostend (Belgium), hydrobiological samplings have been made during 1968, twice a month, at two different places. A 24-hours-cycle, with samplings each second hour, has been undertaken in September. Next to the detailed physico-chemical analysis of the water, qualitative and quantitative plankton studies as well as analyses of chlorophyl and organoseston were each time performed. The results show that the ponds are severely eutrophicated (large amount of nitrates and phosphates, dominance of blue-green algae and development of important amounts of plankton). As concerns their degree of pollution, the ponds are alpha-mesosaprobic. The continuous presence of a certain amount of nitrites betray an important ammoniac concentration in the lower layers of the water; moreover the occurrence of the blue-green algae Oscillatoria redekei, typical for the presence of H2S, is indicative of the formation of important quantities of hydrogen sulfide in the sediments of these shallow ponds. Fish thus suffer chronical poisoning by NH3 and H2S, poisoning which becomes acute, especially for weakened specimens, when an important oxygen deficit occurs, as for instance happens at the moment of bacterial decomposition of water blooms.
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