Hydrobiologia

, Volume 197, Issue 1, pp 35–50 | Cite as

The evolution of saline lake waters: gradual and rapid biogeochemical pathways in the Basotu Lake District, Tanzania

  • Peter Kilham
  • Paul L. Cloke
Article

Abstract

The biogeochemical evolution of solutes markedly alters the chemistry in the closed-basin maar lakes that comprise the Basotu Lake District (Tanzania, East Africa). Examination of 11 (out of 13) lakes in the Basotu Lake District identified two distinct evolutionary pathways: a gradual path and a rapid path. During the course of biogeochemical evolution these waters follow either the gradual path alone or a combination of the gradual and rapid paths. Solute evolution along the gradual path is determined by all of the biogeochemical processes that for these waters appear to be tightly coupled to evaporative concentration (e.g. mineral precipitation, sorption and ion exchange, C02 degassing, and sulfate reduction). Rapid evolution occurs when mixing events suddenly permit H2S to be lost to the atmosphere. The chemistry of waters undergoing rapid evolution is changed abruptly because loss of every equivalent of sulfide produces an equivalent permanent alkalinity.

The Basotu Lake District in north central Tanzania is comprised of 13 maar lakes. They range in surface water conductivity from 592 to 24 000 µ S cm −1 (at 20°). Within these lake basins only a few of the variety of geo- and biogeochemical processes known to occur in lakes of this type are actually responsible for the gain and/or loss of individual solutes. For example, potassium appears to be taken up in the formation of illite. Calcium is precipitated as calcite. Magnesium interacts with alumino-silicate precursors to form a variety of clay minerals that contain magnesium (e.g. stevensite). This process is also known as reverse weathering. Sulfate is reduced to sulfide and subsequently lost as H2S and/or metal sulfides. Alkalinity is lost owing to calcite precipitation and as a consequence of reverse weathering. Alkalinity is gained in the form of extra permanent alkalinity when sulfide is lost from these waters (via metal sulfide precipitation or gaseous emission to the atmosphere). Rapid (punctuated) evolution can occur in any lake containing anoxic waters providing that mixing events take place which cause H2S to be lost to the atmosphere.

Key words

Saline lakes biogeochemistry geochemical evolution sulfate reduction CaC03 precipitation maars 

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Copyright information

© Kluwer Academic Publishers 1990

Authors and Affiliations

  • Peter Kilham
    • 1
  • Paul L. Cloke
    • 2
  1. 1.Department of Biology and the Center for Great Lakes and the Aquatic SciencesThe University of MichiganAnn ArborUSA
  2. 2.Department of Geological SciencesThe University of MichiganAnn ArborUSA

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