SEASONAL, DISCHARGE-DRIVEN PATTERNS OF MAYFLY ASSEMBLAGES IN AN INTERMITTENT NEOTROPICAL STREAM

1. Streams in the seasonal wet and dry tropics have highly dynamic dis charge regimes. Our study stream, located in mid-western Brazil, drain s into the Pantanal, South America's largest wetlands, and is characte rized by frequent spates in the rainy season and several weeks of inte rrupted flow in the dry season. 2. In order to understand how these se asonal flow patterns influence the aquatic fauna, floating litter was studied because: (i) this habitat is exposed to the current, and thus is likely to be most affected abiotically and biotically by changes in flow; and (ii) this habitat is abundant in unaltered tropical streams . Studies were conducted in a third- and a fourth-order reach. Mayflie s were chosen as study organisms because they were frequent and specie s-rich on floating litter, and because they should to be responsive to changes in current velocity. 3. In the course of 15 months, covering one rainy and two dry seasons, mayflies showed pronounced seasonal pat terns at family and genus level, which were evidently driven by discha rge. Two periods of high mayfly densities were observed in the course of one year. One maximum coincided with the peak of the rainy season, the other with the peak of the dry season, and both were distinct in f aunistic composition. At high current velocities the leptophlebiid Far rodes sp. was dominant and Leptohyphes sp., Acerpenna sp. and Paracloe odes sp, were frequent. In the dry season, when the river was reduced to isolated standing waters, 86-93% of all mayflies were Caenis sp. 4. Altogether sixteen mayfly genera from the families Baetidae, Caenidae , Leptohyphidae, Leptophlebiidae and Oligoneuriidae were recorded. Two genera are new to science, the baetid Aturbina was recently described (among others, from material from our study river), Acerpenna and Par acloeodes are new records for South America, and Miroculis and Terpide s are recorded for the first time south of the Amazon.