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In southern Brazil, cold (La Niña) and warm (El Niño) episodes of the El Niño Southern Oscillation (ENSO) phenomenon cause drought and high rainfall, respectively. The low precipitation and freshwater outflow associated with La Niña during 1995-1996 were associated with an increase in the abundance of marine species in the Patos Lagoon estuary. During the 1997-1998 El Niño, high precipitation and river discharge were associated with low abundance of marine species in the estuary. ANOVA results showed a higher abundance during La Niña than El Niño for estuarine resident (RES) and estuarine dependent (DEP) fishes. During La Niña catch per unit of effort (CPUE) of RES increased from the marine to estuarine area, but during El Niño CPUE increased at the marine area and diminished during summer and autumn in some estuarine sites. DEP fishes had an opposite abundance pattern. During La Niña, these fishes were abundant at the coastal marine area and along some estuarine sites, but during El Niño, CPUE remained almost the same at the marine area but dropped along some estuarine sites. These different abundance patterns for dominant fish groups yielded a positive interaction between stations and climatic events. With higher river discharge and the consequent decline of dominant euryhaline fishes, such as Mugil platanus and Atherinella brasiliensis, freshwater species increased in abundance and richness in the shallow waters of the estuary. The ENSO phenomenon influences precipitation and estuarine salinity in southern Brazil and thereby seems to have a strong influence on recruitment, immigration, and emigration dynamics of fish species living within and adjacent to estuarine habitats. 2001 The Fisheries Society of the British Isles

We used stable C and N isotope ratios of tissues from 29 fish species from a large subtropical lagoon in southern Brazil to examine spatial variability in isotopic composition and vertical trophic structure across freshwater and estuarine habitats. Nitrogen isotope ratios indicated a smooth gradation in trophic positions among species, with most fishes occupying the secondary and tertiary consumer level. Fish assemblages showed a significant shift in their carbon isotopic signatures between freshwater and estuarine sites. Depleted carbon signatures (from À24.7& to À17.8&) were found in freshwater, whereas more enriched signatures (from À19.1& to À12.3&) were obtained within the estuarine zone downstream. Based on our survey of the C 3 and C 4 plants and isotopic values for phytoplankton and benthic microalgae reported for ecosystems elsewhere, we hypothesized that the observed d 13 C differences in the fish assemblage between freshwater and estuarine sites is due to a shift from assimilating organic matter ultimately derived from C 3 freshwater marsh vegetation and phytoplankton at the freshwater site (d 13 C ranging from À25& to À19&), to C 4 salt-marsh (e.g. Spartina) and widgeon grass (Ruppia maritima), benthic microalgae and marine phytoplankton at the estuarine sites (from À18& to À12&). Our results suggested that fish assemblages are generally supported by autochthonous primary production. Freshwater fishes that likely were displaced downstream into the estuary during periods of high freshwater discharge had depleted d 13 C values that were characteristic of the upper lagoon. These results suggest that spatial foodweb subsidies can occur within the lagoon.

Determining how assembly rules (e.g. limiting similarity, environmental filtering and neutrality) shape community structure along environmental gradients and across spatial scales is still controversial. The study of functional relationships between coexisting species may help to disentangle among these assembly rules. Here, we compared pairwise functional dissimilarities between fish species to their corresponding pairwise co-occurrences. Fish assemblages (n  835) were sampled monthly in Patos-Mirim system (Brazil) using both bottom trawling and beach seining. Species occurrences were recorded and functional traits related to locomotion and food acquisition were measured on several individuals from each species. The region studied was divided in two areas corresponding to each side of a floodgate located in São Gonçalo Channel: a freshwater channel up the floodgate and, down the floodgate, the Patos Lagoon estuary. The relationship between functional dissimilarity and co-occurrence between species pairs was assessed using quantile regressions for each month and at different spatial scales. Overall, quantile regression coefficients between functional dissimilarities (either based on locomotion types or feeding habits) and co-occurrence values were negative, suggesting that co-occurrence increases with functional redundancy regardless of spatial scale. Our results support the assumption that environmental filters have more influence than biotic interactions on the structure of fish assemblages even locally.

were observed in locations dtroughout the world. In southern Brazil, E1 Nino events are associated with increased rainfall and higher freshwater discharge into Patos Lagoon, a large coastal lagoon that empties into the Atlantic Ocean. Based on interdecadal meteorological and biological data sets encompassing the two strongest E1 Nino events of the la.~t 50 yr, we evaluated the hypothesis that E1 Nino-induced hydrological changes are a major driving force controlling the interannual variation in the structure and dynamics of fishes in the Patos Lagoon estuary. High rainfall in the drainage basin of the lagoon coincided with low salinity in the estuarine area during both E1 Nino episodes. Total rainfall in the drainage basin was higher (767 versus 711 mm) and near-zero salinity conditions in the estuarine area lasted about 3 mo longer during the 1997-1998 E1 Nino event compared with the 1982-1983 event. Hydrological changes triggered by both E1 Nino events had similar relationships to fish species composition and diversity patterns, but the 1997-1998 event appeared to have stronger effects on the species a.~emblage. Although shifts in species composition were qualitatively similar during the two E1 Nino events. distance between E1 Nino and non-EI Nino assemblage multivariate centroids was greater during the 1996-2000 sampling period compared with the 1979-1983 period. We provide a conceptual model of the principal mechanisms and processes connecting the atmospheric-oceanographic interactions triggered by the E1 Nino phenomena and their effect on the estuarine fish assemblage. stood, Timmermann et al. (1999) showed that, under a scenario of increasing greenhouse-gas concentrations, EI Nino-like conditions would become more frequent. The frequency and intensity of EI Nino events and their effects may also be influenced by the phases of interdecadal modes of sea surface temperature variation (Mestas-Nunez and Enfield 1999, 2001). If El Nino events increase in severity and duration in response to global change, it will become critically important to understand how meteorological and hydrological patterns associated with EI Nino events affect ecosystems. Patos Lagoon (3~, 490W) represents an appropriate ecosystem to evaluate how disturbances induced by EI Nino can affect the dynamics and structure of fish estuarine assemblages. EI Nino episodes have affected the western coasts of the Americas frequently over the last 5,000 yr (Rollins et al. 1986), which coincides with the geomorphologic age (5,000-6,000 yr) estimated for the Patos Lagoon system on the Atlantic coast of southern Brazil (Knoppers and Kjerfve 1999). High rainfall in southern Brazil during EI Nino events (Grimm et al. 1998, 2000) increases freshwater discharged .

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Taim Hydrological System is a unique subtropical wetland in southern Brazil harboring an exceptional biological diversity. In 1978, an ecological reserve was established to protect part of this area from increasing anthropogenic impacts in its surroundings. Fishes have been poorly investigated in this system. Based on a multi-gear sampling study encompassing several years (2001)(2002)(2003)(2004)(2005), and on comparisons with previous fish inventories, we provide an up-to-date species list of fishes occurring in the Taim Wetland. In addition, we made the first preliminary description of fish dominance patterns found in the main lakes of the system (Flores, Nicola, Jacaré and Mangueira). Checklist comparison resulted in 62 fish species distributed in 24 families, with Characidae (19 species) and Cichlidae (7) showing the highest species richness. Six species are cited for the first time in the reserve: Trachelyopterus lucenai (Auchenipteridae), Hoplosternum littorale (Callichthyidae), Rineloricaria cadeae (Loricariidae), Eigenmannia trilineata (Sternopygidae), Odontesthes mirinensis and O. perugiae (Atherinopsidae). Apparently, the black catfish T. lucenai invaded the system in the last decade and became one of the dominant species in the pelagic waters. Although differences in gears hindered direct comparisons, differences in species composition and dominance patterns between shallow margins and pelagic waters of lakes seem to occur in the lakes. A more diverse assemblage dominated by small fishes (< 5cm) occurred in the protected, vegetated lakes margins. In contrast, an assemblage with lower species richness comprised of larger fishes (> 15 cm) seemed to dominate in pelagic waters.O Sistema Hidrológico do Taim é um banhando subtropical localizado no sul do Brasil que abriga uma diversidade biológica excepcional. Em 1978, uma reserva ecológica foi estabelecida para proteger parte dessa área do crescente impacto de ações antrópicas nos seus arredores. Os peixes tem sido muito pouco investigados nessa região. Baseado em coletas com vários amostradores durante vários anos (2001)(2002)(2003)(2004)(2005), e em comparações com estudos prévios, esse trabalho fornece uma lista de espécies atualizada dos peixes ocorrendo no Taim. Também é apresentada a primeira descrição preliminar dos padrões de dominância nos principais lagos que compõem o sistema (Flores, Nicola, Jacaré e Mangueira). A análise comparada indicou a presença de 62 espécies de peixes distribuídas em 24 famílias, sendo Characidae (19 espécies) e Cichlidae (7) as famílias com o maior número de espécies. Seis espécies são registradas pela primeira vez na reserva: Trachelyopterus lucenai (Auchenipteridae), Hoplosternum littorale (Callichthyidae), Rineloricaria cadeae (Loricariidae), Eigenmannia trilineata (Sternopygidae), Odontesthes mirinensis e O. perugiae (Atherinopsidae). Aparentemente, o penharol T. lucenai invadiu o sistema na última década e tornou-se uma das espécies dominantes nas zonas pelágicas das lagoas. Embora diferenças nos amostradores não p...