FINE-LAYER ZONATION AND SHORT-TERM CHANGES OF MICROBIAL COMMUNITITES IN 2 COASTAL MEROMICTIC LAKES (MADANG PROVINCE, PAPUA-NEW-GUINEA)

Early in December 1990, an incursion of seawater caused destruction of the meromictic stratification in Lake Nagada and Lake Siar. The re-es tablishment of the meromictic features, the progressive deeping of the halocline and its separation from the oxycline resulted in a strong v ertical niche segregation. This strongly affected the distribution of microorganisms in the mixo- and monimolimnia and their microzonation i n relation to the physicochemical gradients. The phytoplankton communi ty was relatively simple in both lakes, in contrast to the diverse pop ulations of bacterioplankton and heterotrophic organisms. The microbia l populations in the lower mixolimnion and across the oxic-anoxic boun dary are thought to be major centres of production and decomposition o f organic matter in the lakes. Over a period of about two months, as p hysicochemical conditions changed, very rapid changes took place in th e composition of both the mixo- and monimolimnetic populations of micr oorganisms. The striking differences between the populations of photot rophic bacteria in the two lakes, and their waxing and waning, can onl y partly be explained.