The density and biomass of mesozooplankton and ichthyoplankton in the Negro and the Amazon Rivers during the rainy season: the ecological importance of the confluence boundary.

The boundary zone between two different hydrological regimes is often a biologically enriched environment with distinct planktonic communities. In the center of the Amazon River basin, muddy white water of the Amazon River meets with black water of the Negro River, creating a conspicuous visible boundary spanning over 10 km along the Amazon River. Here, we tested the hypothesis that the confluence boundary between the white and black water rivers concentrates prey and is used as a feeding habitat for consumers by investigating the density, biomass and distribution of mesozooplankton and ichthyoplankton communities across the two rivers during the rainy season. Our results show that mean mesozooplankton density (2,730 inds. m-3) and biomass (4.8 mg m-3) were higher in the black-water river compared to the white-water river (959 inds. m-3; 2.4 mg m-3); however an exceptionally high mesozooplankton density was not observed in the confluence boundary. Nonetheless we found the highest density of ichthyoplankton in the confluence boundary (9.7 inds. m-3), being up to 9-fold higher than in adjacent rivers. The confluence between white and black waters is sandwiched by both environments with low (white water) and high (black water) zooplankton concentrations and by both environments with low (white water) and high (black water) predation pressures for fish larvae, and may function as a boundary layer that offers benefits of both high prey concentrations and low predation risk. This forms a plausible explanation for the high density of ichthyoplankton in the confluence zone of black and white water rivers.

Goliath catfish spawning in the far western Amazon confirmed by the distribution of mature adults, drifting larvae and migrating juveniles.

We mapped the inferred long-distance migrations of four species of Amazonian goliath catfishes (Brachyplatystoma rousseauxii, B. platynemum, B. juruense and B. vaillantii) based on the presence of individuals with mature gonads and conducted statistical analysis of the expected long-distance downstream migrations of their larvae and juveniles. By linking the distribution of larval, juvenile and mature adult size classes across the Amazon, the results showed: (i) that the main spawning regions of these goliath catfish species are in the western Amazon; (ii) at least three species-B. rousseauxii, B. platynemum, and B. juruense-spawn partially or mainly as far upstream as the Andes; (iii) the main spawning area of B. rousseauxii is in or near the Andes; and (iv) the life history migration distances of B. rousseauxii are the longest strictly freshwater fish migrations in the world. These results provide an empirical baseline for tagging experiments, life histories extrapolated from otolith microchemistry interpretations and other methods to establish goliath catfish migratory routes, their seasonal timing and possible return (homing) to western headwater tributaries where they were born.

Papel trófico dll microbial loop en un lago de inundación en la amazonía central / The Trophic Role of Microbial Loop in an Amazonia Central Floodplain Lake

Con el propósito de evaluar la participación de bacterias heterotróficas en el flujo de carbono en la cadena trófica de un lago de inundación amazónico, recolectas mensuales de estos microorganismos fueron realizadas durante el año hidrológico de diciembre de 2007 hasta noviembre de 2008, en las regiones litoral, pelágica y de macrófitas acuáticas del lago Catalão, en la Amazonia central brasilera. Las bacterias fueron multiplicadas in vitro, usando como sustrato carbono orgánico disuelto (COD) del lago en cada una de las regiones estudiadas, para posterior análisis de la abundancia de isotopos estables de carbono y nitrógeno. Estos datos fueron confrontados con los valores de COD de los cuatro periodos limnológicos del lago (seca, inundación, llena y bajada de aguas). En general se encontró que la fuente principal de carbono de las bacterias heterotróficas fue aquella de origen C4, que presentó un aporte mínimo de 75%, en la medida en que el d13C de las bacterias presentó valor medio de -17,72‰ ± 2,25. Confrontando ese valor, con el d13C del zooplancton en el mismo período (- 33,04‰ ± 3,81) se concluye que el aporte de las bacterias heterotróficas al flujo de carbono hacia niveles tróficos superiores en el lago estudiado es mínimo.

In order to evaluate the role of heterotrophic bacteria on carbon flow in food chains of an Amazonian floodplain lake, monthly collections of these organisms were made during the hydrological year from December 2007 to November 2008. Littoral, pelagic, and aquatic macrophyte regions of the Catalão Lake in Central Amazonia were sampled and bacteria were multiplied in vitro, using dissolved organic carbon (COD) of each one of the regions studied as a substrate. The bacterial biomass obtained was used for stable isotope analyses of carbon and nitrogen. These data were confronted with COD values of the four hydrological periods of the lake (dry, rising, flood and fall). In general, it was found that the main source of carbon for heterotrophic bacteria was that of C4 origin, which presented a minimum contribution of 75% of bacterial biomass, to the extent that the bacteria d13C average value was -17.72‰ ± 2.25. Comparing this value with the d13C of zooplankton in the same period (-33.04‰ ± 3.81) permit concludes that the contribution of heterotrophic bacteria in the carbon flow to higher trophic levels in the Catalão lake is minimal.