Sediment pore‐water dynamics of Little Rock Lake, Wisconsin: Geochemical processes and seasonal and spatial variability

Abstract

The nature of sediment alkalinity generation processes and the temporal and spatial variability of the pore-water chemistry of an experimentally acidified seepage lake (Little Rock Lake, Wisconsin) were determined. Analysis of vertical gradients of solutes near the sediment-water interface indicates that sulfate reduction and base cation production were the major mechanisms of alkalinity generation. A comparison of surficial accumulation rates and burial rates indicates that the major source of cations to the pore water occurred by release of organically bound and exchangeable cations through decomposition. Pore-water measurements also reveal significant seasonal changes in solute fluxes, including a sudden change in sediment metabolism following a springtime algal bloom. Spatial differences in particle deposition caused pore-water fluxes of ammonium and alkalinity to be almost an order of magnitude higher at a hypolimnetic site than at epilimnetic sites. After 2 yr of acidification, pore-water gradients of sulfate, calcium, and alkalinity showed only minor changes, and the pore-water pH in the acidified basin remained within 0.5 pH units of preacidification pH.

Sulfate reduction and diffusion in sediments of Little Rock Lake, Wisconsin

“…At a given site, fluxes were higher in summer than in winter and spring. Also, the depth at which sulfate depletion began in porewater profiles changed seasonally; depletion began higher above the sediment surface in summer than in winter (Sherman et al 1994). The factor accounting for the most variance (20%) in the magnitude of the fluxes was the sulfate concentration in the overlying water column (Fig.…”

“…Sulfate profiles from pore-water equilibrators-All profiles of sulfate in pore water showed depletion of sulfate below the sediment surface (see figure 3 of Sherman et al 1994). Over a depth interval of l-10 cm, concentrations of sulfate decreased from ambient lake values (13-45 pmol liter -l) to concentrations between 2 and 14 pmol liter-l.…”

“…Diffusion coefficients (from Li and Gregory 1974) were corrected for temperature and porosity (measured by weight loss of sediments upon drying). A discussion of the concentrations and fluxes of all measured ions in these same pore-water profiles is given by Sherman et al (1994).…”

“…Pore-water profzles -Profiles of sulfate in pore water were measured frequently over the 5-yr period from 1983 to 1988 (Sherman et al 1994;Weir 1989;Perry 1987). Pore-water equilibrators (Hesslein 1976) were installed at five locations in the lake on multiple occasions.…”

Organic carbon mass accumulation rate regulates the flux of reduced substances from the sediments of deep lakes

“…creased with lake depth caused by the focusing of labile particulate OC into the deeper part of the lake. A study at oligotrophic Little Rock Lake also showed elevated NH + 4 concentrations at the deepest site due to a greater supply of finegrained organic particles caused by sediment focusing (Sherman et al, 1994). In eutrophic Lake Zug, Maerki et al (2009) found that NH + 4 fluxes increased proportionally with the sediment contents of TOC and total nitrogen (TN), indicating a link between fluxes of reduced substances and TOC-MAR.…”