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Abstract:
The tropical wet savanna of Roraima possesses both seasonally flooded and permanently flooded,
or perennial, lagoons. The seasonally flooded lagoons show no noteworthy aquatic flora and fauna. Two
types of perennial lagoons can be distinguished, depression lagoons and ox-bow lakes. The origin and hydrography of these are discussed.
The very shallow, basin-shaped depression lagoons are chiefly fed by rainwater. Their waters have
almost the lowest electrolyte concentration known. Their aquatic fauna and flora, however, are surprisingly well developed, and the possible causes for this are shown. Chironomid larvae are dominant in the macrobenthos. The vegetation-free sand flats in the centre of these lagoons are exclusively colonised by larvae of
two Tanytarsus species with a density of 2 400 individuals/m².
The ox-bow lakes have a box-shaped cross-section with steep sloping banks. These lakes are connected with the rivers in the rainy season. Their water is slightly richer in electrolytes. There is free-floating as
well as submerged vegetation here. The benthic fauna of the shore regions is similar to that in the depression
lagoons. An investigation of the fauna in the muddy deep sediments has however still to be done.
