Lake Mahega: a mesothermic, sulphato-chloride lake in western Uganda

In May, 1971, Lake Mahega had pronounced mesothermy (40.W C at one metre). Solar heating of a bloom of baeteria and the blue-green alga, Synechococcus bacillaris Butch., probably caused the high temperature. A total ionic concentration gradient increasing from 192,600 mg 1itre-1 at the surface to 415,200 mg•litre- 1 at three metres stabilized the thermally inverted water. Nearly equal amounts of chloride and sulphate accounted for about 90% of the anionic composition. Sodium was the major cation. Crystals or the triple salt, northupite (Na2 CO3. MgCO3. NaCl) and of thenardite (Na2SO4) were mixed with the surface sediment. We believe it is possible that primary northupite depnsition is occurring. Lake Mahega is also the first mesothermic, sulphato-chloride lake reported for East Africa.

Microstructures layering, mechanisms in the middle waters of the Persian Gulf

Layered structures, known as micro structures in marine environments are common features of which their formation mechanisms are first reviewed. Some methods of measuring such features based on the measurements and theories are presented for the Persian Gulf. This includes determination of layers with temperature inversion (TI) associated with double diffusive convection (DDC). The relevant associated parameters are estimated from ROPME CTD data for late winter and early summer of 1992. Only in certain parts temperature inversion and DDC are observed which seem to produce layered structures. Observations show that the places with TI and DDC are mainly confined to the frontal regions where the water entering the Persian Gulf and water exiting it meet, nearly along the axis of the Gulf. TI and DDC is mainly observer in the northern bound of the front. Typical density ratio for regions with TI and DDC is 0.7 to 0.2 and the mean depth is at about 37 ± 3 m for the Persian Gulf. TI and DDC are also found in the outflow from the Persian Gulf to the Oman Gulf which is found to be at a depth of about 250 m. Horizontal addiction and reduction of solar heating seem to be the main reasons in producing layers with TI and DDC. It is also found that the regime of DDC in the Persian Gulf is more diffusive and the flow associated with intrusion layers with TI is non-isopycnal (more unstable). However for the Oman sea both diffusive and finger regime are observed and the flow is inferred to be isopycnal (more stable statically). Typical heat and salt fluxes due to DDC are found to be 6 W/m2 and 0.36 W/m2 respectively. Effective salinity diffusivity, Ks and heat diffusivity, Kr have been estimated for the places with DDC in the Persian Gulf and Oman Gulf (Ks=1.1 *10-7 m2/s, KT= 1.88*10-6 m2/s). Their values are within the values obtained by others. The buoyancy frequency for the Persian Gulf with typical mean value of 0.05s-1 is much higher than these of the free Oceans. Such large values of N (typically 0.05 s-1) indicate that processes such as tide can produce strong internal waves which may be another factor in producing layered structures. This requires separate study.