, Volume 4, Issue 2, pp 159–181 | Cite as

Major element chemistry, weathering and element yields for the Caura River drainage, Venezuela

  • William M. LewisJr
  • Stephen K. Hamilton
  • Sandra L. Jones
  • Donald D. Runnels


The Caura River, a major tributary of the Orinoco River, was sampled at bi-weekly intervals for two years. Because the watershed is covered with undisturbed forest overlying a Precambrian shield, the water of the Caura River has low conductivity (mean, 15 uS/cm), contains small amounts of particulate material (mean, 11 mg/1), and is slightly acidic (median pH, 6.8). Concentrations of total dissolved solids vary less than two-fold in response to the seasonal ten-fold variation in discharge; concentrations of particulate material vary more (ten-fold) and are lowest at the time of peak discharge. Seasonal changes in concentrations of Si, major metal cations, and hydrogen ion are complementary to each other and indicate regular seasonal changes in weathering rates. Measurements of bulk atmospheric deposition and the observed runoff yield of Cl and S were used in estimating the basin-wide atmospheric deposition of major elements, which were as follows (kg/ha/yr): Ca, 1.3; Mg, 0.29; Na, 8.2; K, 1.0; Cl, 12; S, 2.8; P, 0.14. Element ratios show that terrestrial sources contribute strongly to the atmospheric deposition of Ca, K, S, and P. From the atmospheric contributions and runoff yields, watershed retention was computed for major elements with reference to Si. The watershed accumulates Al, Fe, and P, whereas losses of Ca, Mg, Na, K, and S originating from non-atmospheric sources exceed the relative loss rates of Si. The rock weathering rate based on Si is 1.8 cm/ 1,000 years. Although significant amounts of Ca, Mg, Na, and K are found in atmospheric deposition, the dominant influence on the mass balances of these elements is weathering rather than deposition. Weathering has a trivial influence on the cycles of Cl and S. Both atmospheric deposition and weathering are important in the mass balance of P. The ecosystem does not effectively conserve most biologically active elements (Ca, Mg, Na, K). The ecosystem conserves significant amounts of phosphorus (31% of total input), but probably by abiotic mechanisms.

Key words

nutrient mass balance tropical forest element cycling atmospheric deposition weathering erosion 


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Copyright information

© Martinus Nijhoff/Dr W. Junk Publishers 1987

Authors and Affiliations

  • William M. LewisJr
    • 1
  • Stephen K. Hamilton
    • 1
  • Sandra L. Jones
    • 2
  • Donald D. Runnels
    • 2
  1. 1.University of Colorado Center for Limnology, Department of Environmental, Population and Organismic BiologyUniversity of ColoradoBoulderUSA
  2. 2.Department of Geological SciencesUniversity of ColoradoBoulderUSA

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