Map of St. Lucia Lake indicating the different regions referred to in the present paper. The insert is a larger scale diagram of the lowest regions of the system. WATER CONDITIONS Salinity Samples of water were brought back to Cape Town and the salinity detennined by the silver nitrate titration method. The titrations were performed by Mr. D. Sacks of the Oceanography Department and are given to the first decimal point in Table I. During the winter expedition only a limited number of samples was taken as it was understood that regular determinations were being made by the lake ranger at Charter's Creek. It was later felt, however, that the ranger's method of determination was not sufficiently accurate and in January the number of samples was increased. To make good the deficiencies in the winter period the determinations of Joubert and Davies of the Oceanographic Research Institute, Durban, for their June expedition of 1964 have been included, and to give a better idea of the change from winter to summer the determinations of Davies and Joubert for the spring of 1964 have also been listed. In the table only the average salinity for the area is given and the number of determinations upon which each is based is shown between brackets. These figures show clearly that salinities were rising throughout the entire system. In July, 1964, low salinity conditions were present in the Lakes and Northern Narrows, with higher values in the Southern Narrows and Estuary Basin, which are subject to tidal influence. The values given for the latter areas are averages, but the salinity will, of course, fluctuate with the tides and was found in fact to vary between 9•6 and 34•8%0 (sea water= 35•0%0) at this time. This is a normal salinity gradient as might be expected in any estuary receiving freshwater increments at its source and subject to tidal influence at the mouth. In spring the normal salinity gradient was still apparent, though all values except that for False Bay were slightly higher (the value for the Estuary Basin was taken on a falling tide). In summer not only were all averages quite considerably higher, but that in the Southern
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