Fish assemblage in a dammed tropical river: an analysis along the longitudinal and temporal gradients from river to reservoir

We analysed changes in the fish assemblage structure along a longitudinal gradient of the Paraíba do Sul River and Funil Reservoir. We tested the hypothesis that shifts from lotic to lentic environment affect the richness and structure of the assemblage which are modulated by seasonal rainfall changes. Standardised monthly samplings were carried out from October 2006 to September 2007 in four zones: 1) river upstream from the reservoir; 2) upper part of the reservoir; 3) lower part of the reservoir, and 4) river downstream from the reservoir. Fishes were caught using gillnets deployed for 15 hours. We collected a total of 4550 specimens, representing 35 species and 5 orders. The highest richness and diversity were recorded in zone 2, the transitional zone between river and reservoir. In this ecotone, lotic and lentic species overlap. Greater abundance and biomass was recorded in the river upstream from the reservoir (zone 1); however, there are no differences between the zones in the structure of assemblages during the wet season. During the dry season, the assemblage structure is more differentiated between zones, although no differences in abundance and biomass occur. The seasonal flow of the river is the major driving factor to influence the fish assemblage structure along the longitudinal gradient from the river to the reservoir.

Mudanças na estrutura da assembleia de peixes foram analisadas ao longo de um gradiente longitudinal do rio Paraíba do Sul e Reservatório do Funil. A hipótese testada foi a de que mudanças do ambiente lótico para o lêntico afetam a riqueza e a estrutura da assembleia de peixes que são moduladas pelas variações sazonais da pluviosidade. Coletas mensais padronizadas foram realizadas entre outubro de 2006 e setembro de 2007 em quatro zonas: 1) rio a montante do reservatório; 2) parte alta do reservatório; 3) parte baixa do reservatório, e 4) rio a jusante do reservatório. Os peixes foram capturados com redes de espera que ficaram em operação por 15 horas. Um total de 4.550 espécimes foi coletado, representando 35 espécies e 5 ordens. A maior riqueza e diversidade foram registradas na zona 2, a zona de transição entre rio e reservatório. Neste ecótono, espécies lóticas e lênticas se sobrepõem. Maior abundância e biomassa foram observadas no rio a montante do reservatório (zona 1), no entanto, não houve diferenças na estrutura da assembleia de peixes entre as zonas durante a estação chuvosa. Durante a estação seca, a estrutura da assembleia de peixes foi mais diferenciada entre as zonas, porém sem diferenças na abundância e biomassa. O fluxo sazonal do rio é o principal fator a influenciar a estrutura da assembleia de peixes ao longo do gradiente longitudinal rio-reservatório.

Seasonal changes in the rotifer (Rotifera) diversity from a tropical high altitude reservoir (Valle de Bravo, Mexico).

We studied the zooplankton community composition at different depths (2, 4, 8, 12 and 20 m) of Valle de Bravo, a drinking water reservoir in central Mexico during December 2005 to November 2006.Temperature, dissolved oxygen, pH and chlorophyll a were measured simultaneously. While physicochemical values were similar to those found in previous studies, total zooplankton abundance was higher (mean 847 ind. l-1), doubling the mean abundance found in the previous year. Nevertheless, Zooplankton remained dominated by the rotifer genera Keratella, Polyarthra and Trichocerca, which constituted nearly 80% of the total numerical abundances. We encountered 23 rotifer species of which 5 of them (Lepadella rhomboides, Cephalodella catellina, Trichocerca elongata, T. porcellus and Dicranophorus forcipatus) were recorded for the first time from this reservoir. Shannon Wiener diversity index showed that the annual mean species diversity index was similar at depths of 2, 4 and 8 m, but were reduced at 12 and 20 m. Regardless of depth, the highest diversity value of 1.82 was observed during January, while the lowest (0.07) during March. Principal component analysis showed that temperature, dissolved oxygen, water column depth and chlorophyll a have combined effects on the abundance of dominant rotifer species. The highest rotifer density was observed in April (>1600 individuals l-1), while the lowest was recorded during January (<50 ind. l-1). During the study period, the most abundant rotifer species were Keratella chochlearis, Polyarthra vulgaris, Trichocecra elongata and Anuraeopsis fissa.