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Abstract:
Lago Tupé is a black water "ria lake". It is connected with the Rio Negro throughout the year and shows similarly large fluctuations in water level, amounting to nearly 8 m in 1971. The exceptionally high minimum water level of the one-year sampling period implied that there was thermal stratification
in the lake throughout the year; because of this the oxygen content of the bottom water layers did not exceed 0,6 mg/l, and H2S formed in the hypolimnion during the period of decreasing water level. These
unfavourable external conditions led to the development of a qualitatively and, to a lesser extent, quantitatively poor benthic fauna in the lake centre. Dominant were Chaoboridae larvae (192 individuals/m² over the year) and Ostracoda (457 individuals/m² over the year); the Acari were sub-dominant (70 individuals/m² over the year), and Nematoda were also present 110 individuals/m² over the year). The mean annual biomass was 0.136 g/m². The corresponding patterns of abundance and biomass were also determined for the year.
The benthic fauna of the littoral zone was investigated during the phases of low and rising water levels, also at a locally fixed sampling station. The characteristic littoral fauna appears only during the low water phase (November to January) when Chironomidae larvae are dominant with an abundance of 43-74 %. The rest of the fauna consists of many groups with low relative abundances. When the water level begins to rise the qualitative composition alters completely within a few weeks, Chaoboridae
larvae and Ostracoda becoming dominant. Apart from the absence of Acari the resulting zoocoenose is qualitatively identical with the profundal zoocoenose. It is conceivable that some of the true littoral species migrate at rising water to the neighbouring inundated forest (igapó), and move back again
when the water level falls. The rhythmic faunal movements between profundal, littoral and igapô zones, caused by the large water level fluctuations, are schematically represented. Biomass values were also determined in the littoral zone. The values for both this and abundance were much higher than in the lake centre at any given time.
The feeding habits of the individual taxa of profundal and littoral zoocoenoses were examined. The Ostracoda are the detritophages of the profundal zoocoenose; they are fed on by the zoophagous
Acari and to a certain extent by the chaoborid larvae. In the true littoral zone the phytophages feed on blue green algae and diatoms growing on firm substrates; these are absent from both igapó and profundal zone. The fungi, which are an important food source in the igapó, play a lesser role in the littoral.
The larvae of the Tanypodinae are the zoophages of the true Littoral zone.
Finally, comparable studies of the zoobenthos of other black water lakes are discussed; these lend support to the inferences from Lago Tupé i.e. the profundal zone of the black water lakes has qualitatively and quantitatively the poorest benthic fauna of all lacustrine biotopes in the Central Amazon.
