RESUMO
Paleoecological and geomorphological studies indicate that, during the middle Holocene, there was a predominance of drier conditions with grassy savannahs replacing forests across the South American continent. Modern savannahs are composed mainly of C4 plants and soils developed under this type of vegetation show enrichment in 13C compared to soils under C3 vegetation cover. If soils contain stabilized organic matter formed in the middle Holocene, we hypothesize that former C4 vegetation would be evidenced by a large enrichment of 13C in soil organic matter (SOM). We investigate this possibility examining the depth variation of carbon isotopic composition in 21 soil profiles collected by different researchers at 14 different sites in Brazil. Of these, profiles from only three sites showed a marked increase of 13C with depth (9-10 enrichment in δ13C difference between the surface soil and deepest depth); two sites showed intermediate enrichment (4-5), and nine sites showed a small enrichment of approximatelly 2.5. The majority of sites showing all-C3 derived SOM were in the Amazon region. Possible causes for the absence of a large 13C enrichment with depth are: (1) dominance of C3 rather than C4 grasses in mid-Holocene savannahas, (2) soil profiles did not preserve organic matter derived from mid-Holocene plants, (3) the retreat of forest areas did not occur on a regional scale, but was a much more localized phenomenon.
RESUMO
In the lower Amazon River, suspended sediment is stored during rising stages of the river and resuspended during falling river stages. The storage and resuspension in the reach are related to the mean slope of the flood wave on the river surface; this slope is smaller during rising river stages than during falling stages. The pattern of storage and resuspension damps out the extreme values of high and low sediment discharge and tends to keep them near the mean value between 3.0 x 10(6) and 3.5 x 10(6) metric tons per day. Mean annual discharge of suspended sediment in the lower Amazon is between 1.1 x 10(9) and 1.3 x 10(9) metric tons per year.
RESUMO
Summary Plankton and chemical studies conducted during a synoptic cruise of the Amazon in May-June 1977 indicate that inorganic and organic substances of terrestrial origin could influence the microbial activity in the mainstem, tributaries, and varzea (floodplain) lakes. The major limiting factors to plankton primary production (PPR) were light attenuation by high levels of suspended particulate matter (SPM) in the mainstem and tributaries (average 69 mg SPM liter-1), and low nutrient levels in river mouthbays and várzea lakes (average 0.12 mole PO4-P liter-1 and 1.6 mole NO3-N liter-1). In riverine systems, PPR averaged 4.04 mg C m-3 hr-1, and electron transport system measures of respiration (Re) averaged 0.67 mg C m-3 hr-1; and in the more productive mouthbays and várzea lakes, PPR averaged 26.37 mg C m-3 hr-1, and Re, averaged 2.30 mg C m-3 hr-1. :Re ratios of 8 for the riverine systems and 17 for the mouthbays and varzea lakes indicated that Re was not as limited as PPR in the rivers. Bacterial densities, 14C-acetate rate constants for uptake, and particulate organic C:N of 20, implied that microflora was influenced by terrestrial carbon. Shifts of CO2 components due to more free CO2 and H2CO3 in surface waters of some lacustrine environments and tributaries were suggested by high partial pressures of carbon dioxide (~ 5,000 to ~ 15,000 x 10-6 atm). It is suggested that carbon dioxide oversaturation of some of the water with respect to the atmosphere was partly due to decomposing allochthonous matter. Calculations imply that respiration balanced by evasion is sufficient to explain the high carbon dioxide vapor pressures.
Resumo Estudos planctônicos e químicos efetuados durante um cruzeiro sinótico do Amazonas, em maio-junho de 1977, indicam que substâncias inorgânicas e orgânicas de origem terrestre podem influenciar a atividade microbiana no canal principal, tributários e lagos de várzea. Os principais fatores limitantes da produção primária planctoniana (PPR) foram a diluição da luz por altos níveis de matéria particulada em suspensão (SPM) no canal principal e tributários (média 69mg SPM litro-1), e baixos níveis de nutrientes nas desembocaduras de rios nos lagos de várzea (média 0,12 mole PO4-P litro-1 e 1,6 mole NO3-N litro-1). Em sistemas fluviais PPR tem em média 4,04mg C m-3 hr-1, e as medidas de respiração (Re) por sistema de transporte de eléctron têm em média 0,67 mg C m-3 hr-1; e nas desembocaduras e lagos de várzea mais produtivos, PPR teve em média 26,37 mg C m-3 hr-1, e a Re teve em média 2,30 mg C m-3 hr-1. As proporções PPR:Re de 8 para os sistemas fluviais e 17 para as desembocaduras e lagos de várzea indicaram que a Re não era tão limitante quanto o PPR nos rios. Densidades bacterianas, taxas constantes de 14C-acetado para assimilação, e partículas orgânicas C:N de 20, implicam que a microflora era influenciada pelo carbono terrestre. Trocas de componentes CO2 devidas a mais CO2 livre e H2CO3 nas águas superficiais de alguns ambientes lacustres e de tributários foram sugeridos por altas pressões parciais de dióxido de carbono (~5.000 a ~15.000 x 10-6 atm). É sugerido que a supersaturação de dióxido de carbono de algumas das águas com relação à atmosfera era parcialmente devida à matéria alóctone em decomposição. Os cálculos concluem que a respiração equilibrada pela evasão é suficiente para explicar as altas pressões do vapor de dióxido de carbono.
RESUMO
Summary Plankton and chemical studies conducted during a synoptic cruise of the Amazon in May-June 1977 indicate that inorganic and organic substances of terrestrial origin could influence the microbial activity in the mainstem, tributaries, and varzea (floodplain) lakes. The major limiting factors to plankton primary production (PPR) were light attenuation by high levels of suspended particulate matter (SPM) in the mainstem and tributaries (average 69 mg SPM liter-1), and low nutrient levels in river mouthbays and várzea lakes (average 0.12 mole PO4-P liter-1 and 1.6 mole NO3-N liter-1). In riverine systems, PPR averaged 4.04 mg C m-3 hr-1, and electron transport system measures of respiration (Re) averaged 0.67 mg C m-3 hr-1; and in the more productive mouthbays and várzea lakes, PPR averaged 26.37 mg C m-3 hr-1, and Re, averaged 2.30 mg C m-3 hr-1. :Re ratios of 8 for the riverine systems and 17 for the mouthbays and varzea lakes indicated that Re was not as limited as PPR in the rivers. Bacterial densities, 14C-acetate rate constants for uptake, and particulate organic C:N of 20, implied that microflora was influenced by terrestrial carbon. Shifts of CO2 components due to more free CO2 and H2CO3 in surface waters of some lacustrine environments and tributaries were suggested by high partial pressures of carbon dioxide (~ 5,000 to ~ 15,000 x 10-6 atm). It is suggested that carbon dioxide oversaturation of some of the water with respect to the atmosphere was partly due to decomposing allochthonous matter. Calculations imply that respiration balanced by evasion is sufficient to explain the high carbon dioxide vapor pressures.
Resumo Estudos planctônicos e químicos efetuados durante um cruzeiro sinótico do Amazonas, em maio-junho de 1977, indicam que substâncias inorgânicas e orgânicas de origem terrestre podem influenciar a atividade microbiana no canal principal, tributários e lagos de várzea. Os principais fatores limitantes da produção primária planctoniana (PPR) foram a diluição da luz por altos níveis de matéria particulada em suspensão (SPM) no canal principal e tributários (média 69mg SPM litro-1), e baixos níveis de nutrientes nas desembocaduras de rios nos lagos de várzea (média 0,12 mole PO4-P litro-1 e 1,6 mole NO3-N litro-1). Em sistemas fluviais PPR tem em média 4,04mg C m-3 hr-1, e as medidas de respiração (Re) por sistema de transporte de eléctron têm em média 0,67 mg C m-3 hr-1; e nas desembocaduras e lagos de várzea mais produtivos, PPR teve em média 26,37 mg C m-3 hr-1, e a Re teve em média 2,30 mg C m-3 hr-1. As proporções PPR:Re de 8 para os sistemas fluviais e 17 para as desembocaduras e lagos de várzea indicaram que a Re não era tão limitante quanto o PPR nos rios. Densidades bacterianas, taxas constantes de 14C-acetado para assimilação, e partículas orgânicas C:N de 20, implicam que a microflora era influenciada pelo carbono terrestre. Trocas de componentes CO2 devidas a mais CO2 livre e H2CO3 nas águas superficiais de alguns ambientes lacustres e de tributários foram sugeridos por altas pressões parciais de dióxido de carbono (~5.000 a ~15.000 x 10-6 atm). É sugerido que a supersaturação de dióxido de carbono de algumas das águas com relação à atmosfera era parcialmente devida à matéria alóctone em decomposição. Os cálculos concluem que a respiração equilibrada pela evasão é suficiente para explicar as altas pressões do vapor de dióxido de carbono.