Apobaetis insolitus n. sp., Apobaetis kallawaya n. sp. and Apobaetis niger n. sp. (Ephemeroptera: Baetidae) are described from nymphs collected in Argentina and Bolivia. Apobaetis insolitus can be distinguished by the labrum with anterior margin with a U-shaped emargination, segment II of labial palpi with triangular apically rounded distomedial projection. Apobaetis kallawaya can be distinguished by segment II of labial palpi with triangular, slender and apically pointed distomedial projection, the posterior margins of the abdominal terga with blunt spines and paraprocts with blunt spines apically. Apobaetis niger can be distinguished by the anterior margin of labrum with medial lobe and segment II of labial palpi with triangular apically rounded distomedial projection. The three new species possess tarsal claws with two rows of denticles. A key to all species known from nymphs is presented.

Nitrogen dynamics were investigated in the top centimeters of bottom sediment in three different geomorphologic units (riffles, sand bars, pools) and along two contrasted reaches (nitrogen-rich, nitrogen-poor) of a rural stream. Some predictions of links between geomorphology and processes were verified: (i) hydrological exchanges and nitrification were lower in pools than in riffles and bars, whereas (ii) ammonification and denitrification were higher in pools. Studies of denitrification potential in six other reaches of three N-rich streams support these conclusions and demonstrate that pools are spots of high microbial activity in streams.

Three new species of the genus Harpagopalpus are described from Madagascar, allowing a redefinition of diagnostic character states of the monotypic family Harpagopalpidae, previously known only from Western Africa (two species) and India (one species. The separation of the subgenera Harpagopalpus and Harpagopalpellus is supported by new characters. Harpagopalpus octoporus K.Viets, 1924 and H. tetraporus K.Viets, 1924 are redescribed and a lectotype is designated of H. octoporus. A fourth new species is described from Cameroon, based on a male specimen erroneously considered to be the female of H. octoporus by K. Viets.

Tropocyclops bopingi n.sp. is a 0.4 mm cyclopoid found in Liuxi He Reservoir, Guangdong, South China. It is probably widespread in South China, and appears in the Fauna Sinica under the name T. parvus. True T. parvus is a Central American species that is here redescribed from its type locality, Lake Petén Itzá in Guatemala. T. bopingi is related to T. tenellus from the limnetic zone of Lake Tanganyika and other lakes in Central Africa, and all of them live under climatic conditions that are transitory between subtropical and tropical. T. tenellus is even smaller and more buoyant than T. bopingi and, at least in Lake Tanganyika, it tends to occur closer to the water surface than other zooplankton during the daytime. It does not carry egg balls, but extrudes eggs one at a time. T. tenellus and T. bopingi appear morphologically closer to each other than to T. parvus, but all three are ecologically similar: extremely small, pelagic species that presumably feed on small particles in the water. I hypothesize that environmental conditions in subtropical-tropical transition zones combine with predation and competitive pressure to offer a selective advantage to such omnivorous species in the open water of large lakes or reservoirs.

The prediction of the composition and structure of fish communities in reservoirs is essential in the management of the fisheries, an activity fundamental in emerging countries where human riverine populations depend on freshwater fish for their animal protein supply. The transformation of the river in a reservoir constitutes an environmental filter for the riverine fish species which, before anything else, must find food resources in the reservoir for their individual maintenance. After the closure of Petit- Saut Dam in 1994, fish were sampled by gillnets in the Sinnamary River in the period 1994-2000 and in the Petit-Saut Reservoir between 1998 and 2000. The stomach contents of the main species were analysed for 6 main food items. Analyses of similarity were conducted in order to compare river and reservoir diets of individual species on one hand and the trophic structure of fish assemblages on the other. Twelve species out of 23 analysed showed significant modifications in their feeding habits. The changes affected species from every feeding guild as established from the river stomach contents data. As a consequence the trophic structure was deeply modified in the reservoir. The abundance of piscivores and herbivores decreased while the relative importance of omnivores and invertivores increased. There was a general tendency for species to increase their food niche breath. The ability of species to modify their feeding habits is shown to be of fundamental importance for the trophic organization of the fish community in the Petit-Saut Reservoir.

Changes in the abundance, species richness and species diversity of zooplankton were studied in the Paraguay River. Fifty-nine sites were studied at two hydrological phases between Porto Cáceres (16º03’S-26º23’W) and the confluence with the Paraná (Argentine, 26º53’S-58º23’W), representing a distance of 2270 km. Zooplankton densities varied between 1 and 60 ind.l-1 at high water and between 11 and 100 ind.l-1 at low water. Multiple regression analysis revealed that the hydrological phase explained 64% of the variability in zooplankton density. Surveys found 196 taxa in the Paraguay River (including both the main course and its floodplain). The greater species richness was registered in the upper section. There were significant differences in the species richness and species diversity of Rotifera (Monogononta) between hydrological phases. In the main channel, the hydrological phase explained 54% of variability in species richness, whereas water temperature and electric conductivity explained less than 22% of the variability in species diversity. Rotifera was the most abundant group in both study periods. Despite the total number of species registered in the main channel, only six planktonic genera dominated the zooplankton assemblage (Polyarthra, Synchaeta, Filinia, Keratella and Lecane). The most abundant cladocerans belonged to Bosminidae (Bosminopsis sp.) and the dominant copepods were in the nauplii and juveniles stages. A longitudinal pattern in dominant taxa was not defined at high water. At low water, three species-site groups were separated in relation to environmental variables. The pulsing of the river determines the degree of connectivity with the floodplain and local features had a greater effect on zooplankton assemblages than large-scale landscape patterns.

The epikarstic waters of a restricted sector of the Ojo Guareña cave (north Iberian Peninsula) were investigated to characterize the physico-chemical variation in an annual cycle, to improve the scarce knowledge of the aquatic cave fauna on the Iberian Peninsula, to look for distribution patterns of species per habitat along the annual cycle, and to search for the environmental basis (either physico- or hydro-geochemical) that could explain species distribution in the epikarst. The habitats studied included all puddles and gours present in the sector selected making a total of 51 chemical and 42 biological samples for the entire cycle. The waters show no appreciable contaminationand exhibit small chemical variations throughout the year which are patently affected by external weather conditions. The 53 taxa found belong to ten higher taxonomic groups (Oligochaeta, Turbellaria, Mollusca, Copepoda, Ostracoda, Isopoda, Bathynellacea, Tardigrada, Acarina and Cnidaria), and consist mainly of crustacea with a total of 27 species. Fourteen species were stygobionts (belonging to ten genera), nine of which are new to science and ten of which are endemic. Cave pools that appeared to be more stable in terms of water volume and mineralization, had a lower pCO2 and were carbonate oversaturated, harboured the greatest number of taxa. It is these pools that can maintain strictly cave dwelling species. Pools with lower levels of mineralization and greater water volume fluctuations had a lower diversity of fauna and in general lacked stygobiotic species.

The morphology of the larval stages of Arrenurus nodosus, A. knauthei and A. stecki is described. Particular attention is paid to the different characters separating the three species. The larvae of the three species differ in the size and shape of the dorsal plate. Distinct differences appear between the proportions in the length of the lateral margins of the epimeres, which are similar in A. knauthei and A. stecki, but different in A. nodosus. There are differences in the shape of the excretory pore plates. In A. stecki the shape of the anal plate is slightly different owing to the truncated proleg. The structure of the pedipalps are very similar. The PIII1 setae are always bipectinate, the PIV1 is pectinate in A. knauthei and smooth in the other species, the PV6 is short and thick in A. nodosus, and long and thin in the other two species. Distinct differences also occur in the feathering of legs. The ITi8 seta is short in A. knauthei and long in the other two, the LITi10 seta is long in A. nodosus and short in the other two, and IIITi10 is thick and pectinate in A. nodosus, and smooth and fairly thin in the other two.

The diversity of chironomid larvae in relation to flooding and drought phases in a marginal fluvial wetland of the Middle Paraná River floodplain was analyzed. Four sampling stations in the right bank of the Tiradero Viejo River towards the interior of the marginal wetland were established: one at the riverside, and three more at 10, 30 and 40 m from the river. Bottom sediment samples were extracted during the hydrosedimentological cycle of 2001. A total of 25 taxa of Chironomidae were recorded. The values of taxonomic richness, density and diversity were higher in the marginal fluvial wetland than in the riverside, increasing when the wetland is disconnecting from the river. Polypedilum (Tripodura) spp., Polypedilum (Polypedilum) spp., Ablabesmyia (Karelia) spp., Coelotanypus sp., Monopelopia cf. boliekae, Goeldichironomus holoprasinus and Chironomus gr. decorus spp. were dominant in marginal wetlands. The results indicate that the variations in the hydrosedimentological regime, the degree of disconnection with the river and the progressive desiccation at each one of the stations of the marginal fluvial wetland are limitant factors in the distribution and abundance of aquatic chironomids. The different assemblages of Chironomidae were strongly conditioned by temporal changes related to the gradient of inundation-drought and to the hydroperiod dynamics.