THE LIMNOLOGY OF LAGOA POELELA

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Abstract
The limnology of Lagoa Poelela in southern Moçambique was investigated in July 1973. This lake is connected to the sea via a 75 km long channel, the area is 65 km2, it has a mean depth of 13,7m and a maximum depth of 24 m. The water was saline (8%0) and the major ions, with the exception of sodium, were in the proportions expected of diluted sea water. The concentration of sodium was twice as high as expected; this sodium enrichment has been found in other coastal lakes. Temperatures were relatively uniform from the surface to the bottom and the oxygen saturation at 20 m was 80% of the surface value. Light penetration was higher than for other southern African lakes and at 22 m was 3% of surface illumination. Aquatic macrophytes were absent from shallow water in the main lake, only being found deeper than 3 metres where Potamogeton pectinatus and Naias? marina were common. Naias was found to a depth of 10 m and Chara sp. to 12 m. Only 11 species of fish were found. The fish population was dominated by the cichlids Tilapia rendalli and T. mossambica although some estuarine species were also found. It is suggested that the domination of this brackish lake by cichlids reflects a general tendency for cichlids to increase in numbers in estuarine systems which become isolated from the sea.

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