Seasonal variation of the microbially regulated buffering capacity at sediment-water interfaces in a freshwater lake

Abstract:

The consequences of microbial mineralization activities on the water chemistry of the sediment-water interface was studied at two sampling sites in the southern basin of the eutrophic Lake Lugano (Lago di Lugano). Water samples were collected with the aid of dialysis pore water samplers during three seasonally different time periods: in spring, after complete mixing of the water column had occured, and in summer and fall, while the water masses were stratified.¶The concentration profiles of dissolved inorganic carbon, methane, acid-neutralizing capacity, pH and total dissolved calcium in the interstitial water showed strong seasonal as well as spatial variations as deep as 10 to 15 cm below the sediment surface. Gradients of dissolved inorganic carbon and of methane indicate intense release of CO₂ and methane respectively in the deeper layers of the sediment. During thermal stratification, fluxes of dissolved calcium from the sediment into the lake and release of calcium from sediment surface layers are suggested by the slope of the calcium gradients. In contrast, the gradients of total dissolved magnesium in the sediment as well as in the sediment-near water showed no significant seasonal variation at both sampling sites. The changes in acid-neutralizing capacity and pH in the top sediment layers are consequences of intense microbial mineralization processes, which occur in the sediment surface layers and in the hypolimnion adjacent to it. Larger fluctuations between spring overturn and fall stratification are partially buffered by calcium carbonate dissolution.