Mechanisms of alkalinity generation in acid-sensitive soft water lakes
In-lake alkalinity generation (IAG) is important in regulating alkalinity in lakes that have large water residence times (tw), particularly seepage lakes, which receive most of their water from direct precipitation and thus receive little alkalinity from watershed weathering processes. Major IAG processes include nitrate assimilation, sulfate reduction, and cation production; conversely, ammonium assimilation consumes alkalinity. Nitrate and ammonium are both efficiently retained by lakes with tw \textgreater 1 year, and net alkalinity generation by immobilization of these ions depends upon their relative inputs. Ion budgets show that in-lake cation production occurs in some lakes, but the relative importance of possible mechanisms (e.g., cation exchange, mineral weathering, or biological recycling) is not well understood. Sulfate reduction is the dominant IAG process in most seepage lakes, and sulfate is the major acid anion in precipitation. Sulfate reduction does not require an anoxic water column but occurs by diffusion into anoxic sediments. Sulfate retention in acid-sensitive lakes can be predicted by a simple input-output model with a first-order rate constant (kSO4 0.5 m/year). Further research is needed to assess the relative importance of sestonic sulfur deposition and dissimilatory reduction, to determine environmental factors that control reduction rates, and to determine the extent of recycling. Techniques used to measure IAG processes are described, and limitations of each method are discussed.
American Chemical Society