ABSTRACT. Composition and microhabitat preferences of Odonata (Insecta) immatures in a lowland section of the Rio Ubatiba, Maricá-RJ, Brazil. Monthly sampling was held from May, 1999 to May, 2000 in the Rio Ubatiba, a lowland river situated at Maricá district, Rio de Janeiro State, Brazil. Seven different substrates were collected along fifty meters. A total of 1,279 larvae of Odonata were collected and identified in 27 species, belonging to five families. The most numerous species were Acanthagrion lancea (Selys, 1876), Hetaerina auripennis (Burmeister, 1839), Micrathyria hesperis Ris, 1911 and Telebasis filiola (Perty, 1834). The greatest number of individuals were found in "riparian plants in depositional areas". In general, the species recorded showed habitat preferences: Dythemis multipunctata Kirby, 1894, Erythrodiplax sp., M. hesperis, T. filiola, A. lancea, Erythemis sp., Coryphaeschna adnexa (Hagen, 1861) and H. auripennis seem to prefer organic substrates and Brechmorhoga sp., B. praecox (Hagen, 1969) and Progomphus complicatus Selys, 1854, inorganic substrates.

Indirect foodweb interactions often determine the dynamics of aquatic ecosystems. Predators may reduce the activity of herbivores, which, in turn, may cause basal resources to increase. We studied the patterns of distribution of periphyton (organic and inorganic mass and chlorophyll) on rocks in pools in two Atlantic rainforest coastal streams that varied with respect to the presence or absence of fish. A steep waterfall apparently prevented most fish species from colonising the upper parts of one stream; the other stream was apparently naturally without most species of fish. When fish were present, atyid shrimps and baetid mayflies were less abundant compared with parts of the streams without fish. Concomitant with this, the quantity of periphyton organic and inorganic masses was much greater in the presence of fish. Previous experiments showed that atyid shrimps (Potimirim glabra) and baetid mayflies could reduce the quantity of periphyton by grazing and bioturbation. We deduce that fish inhibit the grazing and bioturbing activities of Potimirim and baetid mayflies, which resulted in larger quantities of inorganic and organic mass of periphyton in parts of streams with fish. Cascading interactions may be common in Atlantic coastal forest streams.