15N natural abundance in plants of the Amazon River floodplain and potential atmospheric N2 fixation.

The15N natural abundance values of various Amazon floodplain (várzea) plants was investigated. Samples of young leaf tissues were collected during three different periods of the river hydrography (low water, mid rising water and high water) and during one period in the Madeira River (high water). A large variation of15N abundance was observed, both among the different plant types and between the different flood stages. This variation probably, reflected, in part, the highly variable nature of the floodplain, sometimes dry and oxygenated and at other times inundated and anaerobic and, in part, changes in plant nitrogen metabolism. Comparison of the nitrogen isotopic composition of leguminous plants with that of non-leguminous plants showed that, on average, the15N abundance was lower in the legumes than non-legumes, suggesting active N-fixation. Also, the15N natural abundance in aquatic grasses of the generaPaspalum, was in general, lower than the15N abundance of aquatic grasses of the generaEchinochloa. As both of these grasses grow in the same general habitat, it appears thatPaspalum grasses may also be nitrogen fixers.

Amazon river discharge and climate variability: 1903 to 1985.

Reconstruction of an 83-year record (1903 to 1985) of the discharge of the Amazon River shows that there has been no statistically significant change in discharge over the period of record and that the predominant interannual variability occurs on the 2- to 3-year time scale. Oscillations of river discharge predate significant human influences in the Amazon basin and reflect both extrabasinal and local factors. Cross-spectrum analyses of Amazon flow anomalies with indicators of the El Niño-Southern Oscillation phenomenon suggest that the oscillations in the hydrograph are coupled to the tropical Pacific climate cycle.

Organic carbon-14 in the Amazon river system.

Coarse and fine suspended particulate organic materials and dissolved humic and fulvic acids transported by the Amazon River all contain bomb-produced carbon-14, indicating relatively rapid turnover of the parent carbon pools. However, the carbon-14 contents of these coexisting carbon forms are measurably different and may reflect varying degrees of retention by soils in the drainage basin.

Storage and remobilization of suspended sediment in the lower Amazon river of Brazil.

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.

Fetal rat hepatic ultrastructural changes associated with organ culture and glucocorticoid treatment.

Fetal hepatic ultrastructure was examined from 18 days gestation to term. Such structure was compared to that of tissue obtained at 18 days gestation which had been maintained in organ culture for 24, 48, and 72 h. In addition, the effects of maternal glucocorticoid administration in vivo and in vitro organ culture glucocorticoid exposure upon fetal hepatic morphogenesis were examined. Enhanced fetal hepatocyte ultrastructural maturation, characterized by a reduction in hematopoietic percursors, an increased frequency of hepatocyte-hepatocyte contact, a complete compliment of intracellular organelles, and the development of bile canaliculi by either maternal in vivo or in vitro fetal organ culture glucocorticoid exposure, was demonstrated. These studies extend previous observations concerning bile salt synthesis and secretion by fetal hepatic tissue both in vivo and in vitro by providing an ultrastructural basis for the functional changes observed and reported to occur in response to steroid therapy

Taurocholate pool size and distribution in the fetal rat.

Taurocholate concentrations in fetal and neonatal rats were determined by radioimmunoassay. Total body taurocholate pool size varied from 0.0049 +/- 0.0008 to 203 +/- 8 nmol/g body weight from day 5 of gestation to 5 d after birth. A 50-fold increase in taurocholate pool size was observed between days 15 and 19 of gestation. The distribution of taurocholate between liver, intestine, and the remainder of the carcass was determined for rats of gestational age 19 d to 5 d after birth. The major fraction of total body taurocholate was in the liver and intestine, with less than 15% in the remainder of the carcass. The ratio of taurocholate in intestine to taurocholate in liver, which was 1:17 at 19 d of gestation, had altered substantially to a ratio of 6:1 by 5 d after birth. Treatment of pregnant rats with 60 microgram/d of dexamethasone from gestational day 9 until sacrifice increased fetal taurocholate pool size by 80% at 15 d, 40% at 19 d, and 16% at 1 d after birth. Administration of dexamethasone to the mother also changed the ratio of taurocholate in intestine to taurocholate in liver. At 19 d of gestation, dexamethasone-treated mothers had fetuses with approximately equal amounts of taurocholate in intestine and liver. This suggested that adrenocorticosteroids stimulate the early maturation of factors controlling taurocholate pool size and tissue distribution in the rat fetus.

Carbon flow in four lake ecosystems: a structural approach.

Direct and indirect carbon fluxes in lakes Marion (British Columbia), Findley (Washington), Wingra (Wisconsin), and Mirror (New Hampshire) are compared, using budgets and input-output analysis. Overall differences in carbon flow between the lakes are shown with cycling indices of .031, .108, .572, and .661, respectively. The results suggest that lake ecosystems may be considered unique aggregatins of similar components.