Research Program Area: Ecosystem & Multimedia Effects
The importance of sediments in buffering lake water from acidic deposition was assessed by using a diffusion based approach to calculate the flux of alkalinity, and other chemical constituents, from sediments to lake water in three subalpine Sierran lakes. The approach requires the measurement or estimation of temperature, concentration gradients, deposition velocities, diffusion coefficients, porosity, and tortuosity within the sediments and their pore waters. Concentration gradients were measured by sampling sediment pore waters with in situ samplers. The other parameters needed for the alkalinity flux calculations were either determined from sediment analyses or from values obtained in the literature. Calculations of fluxes from the deep sediments show that they are dominated by products of organic matter decomposition and mineral weathering. Calculations based on measured gradients indicate that diffusive transport in freshwater sediments is significantly affected by activity coefficient gradients, coulomb forces, and complex formation. These calculated fluxes represent a low estimate, because materials produced at or near the sediment-water interface are not included. Annual average base cation fluxes from deep sediments (the major source of permanent alkalinity contributions to the lake waters) are roughly 8, 5, and 12 neq cm-2day-1 for Eastern Brook bake, Emerald Lake, and Mosquito Lake. Fluxes of base cations from the deep sediments represent a permanent contribution of alkalinity to the water column. Calculated for the entire sediment area, these fluxes are 600, 200 and 400 eq yr-1. Normalizing to lake volume, they are 4, 1, and 10 ueq l-lyr -1 . We estimate that the "confidence interval" for these numbers ranges from one half to double the stated value. Assuming the hydrologic residence times are roughly equal, the sediments in Mosquito Lake, the shallowest lake, have the most influence on lakewater chemistry, and the sediments of Emerald Lake, the deepest lake, have the least influence. We estimate that the sediments are contributing less than 100 of the lake's total alkalinity. During periods when water is rapidly moving through the lakes, the sediments have little influence on water chemistry. However, during periods of the year when inflows are minimal, sediments exert a greater control on the lake's water.
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