LAKE NAGADA AND LAKE SIAR, 2 COASTAL MEROMICTIC LAKES IN MADANG PROVINCE, PAPUA-NEW-GUINEA

Two coastal lakes near Madang, Papua New Guinea, exhibit the strong bi otic and physicochemical stratification characteristic of meromictic l akes. With a frequency not known, but only under the influence of exce ptional tides, seawater breaks into the lakes, destroying the pre-exis ting stratification. After break-ins in 1990 a new stratification was rapidly established in both lakes. In Lake Nagada, only two weeks afte r the break-in, there was a strong chemocline at a depth of only 20 cm . Many characteristic features of meromixis were displayed, including an intense oxycline, high concentrations of sulphides below the chemoc line and a plate of photosynthetic bacteria in the plane of the interf ace. Over the next two months the chemocline was driven deeper, to 1.7 5 m, and a strongly mesothermic temperature pattern developed. Events in Lake Siar were similar exept that the chemocline formed at a deeper level, 2.65 m. Although the frequency of episodic salt-water incursio n is not known we use the terminology of meromixis for these lakes.