Solute transfer across the sediment surface of a eutrophic lake: I. Porewater profiles from dialysis samplers

Abstract

Porewater profiles often are used to identify and quantify important biogeochemical processes occurring in lake sediments. In this study, multiple porewater profiles were obtained from two eutrophic Swiss lakes using porewater equilibrators (peepers) in order to examine spatial and seasonal trends in biogeochemical processes. Variability in profile shapes and concentrations was small on spatial scales of a few meters, but the uncertainty in calculated diffusive fluxes across the sediment surface was, on average, 35%. Focusing of Fe and Mn oxides toward the lake center resulted in systematic increases in porewater concentrations and diffusive fluxes of Fe 2+ and Mn 2+with increasing water depth; these fluxes are postulated to be regulated by the pH-dependent dissolution of reduced-metal phases. Despite higher concentrations of inorganic carbon, NH 4 + , Si and P in pelagic compared to littoral sites, diffusive fluxes of these substances across the sediment surface increased only slightly or not at all with increasing water depth. Porewater profiles did reveal temporal changes in Fe 2+ , Mn 2+ , Ca 2+ and Mg 2+ that were an indirect result of the large, seasonal changes in seston deposition, but no clear seasonal variations were found in diffusive fluxes of nutrients across the sediment surface. The intense mineralization occurring at the sediment surface was not reflected in the porewater profiles nor in the calculated diffusive fluxes. Calculated diffusive fluxes across the sediment surface resulted from decomposition occurring primarily in the top 5-7 cm of sediment. Diffusive fluxes from this subsurface mineralization were equal to the solute release from mineralization occurring at the sediment-water interface. Buried organic matter acts as a memory of previous lake conditons; it will require at least a decade before reductions in nutrient inputs to lakes fully reduce the diffusive fluxes into the lake from the buried reservoir of organic matter.

Seasonal variations in pore water and sediment geochemistry of littoral lake sediments (Asylum Lake, MI, USA)

“…More compressed redox stratification (thinner zones, with the onset of the sulfidic zone nearer to the sediment surface) is favored by warmer temperatures, more dense vegetation and less bioturbation activity. Similarly, sulfur cycling and pore water composition in lake [5,11,13,16] and freshwater marsh [12] sediments and overlying waters have been shown to vary with season. This study demonstrates that redox stratification in the littoral sediments of a freshwater kettle lake also varies strongly with season.…”

“…Seasonal variations in sediment geochemistry, particularly with respect to redox zonation, have been demonstrated in freshwater marsh [10-12] and lake [3-5,13] sediments. More rapid depletion of terminal electron acceptors generally occurs in spring and summer, when labile organic matter is readily available and temperatures are highest.…”

Nitrate Flux into the Sediments of a Shallow Oligohaline Estuary during Large Flood Pulses of Mississippi River Water

“…The NO 3 − and NH 4 + gradients across the sediment–water interface were examined at the peeper station (PS, 4.9 km from spillway inflow) shown in Fig. 1 using dialysis porewater samplers (“peepers”; Urban et al, 1997) deployed in duplicate on 4 June 2011. During deployment, the peepers were within the freshwater Mississippi River plume entering the lake via the Bonnet Carré Spillway (salinity, 0.17–0.18 PSU).…”

“…Fickean diffusive flux of NO 3 − ( F ) across the sediment–water interface was estimated as where D eff is the effective areal sediment diffusion coefficient (cm 2 d −1 ) and Δ C /Δ z is the concentration gradient of NO 3 − (Urban et al, 1997). The value used for Δ C /Δ z was determined based on linear regression using points along the sharp decrease in NO 3 − observed at the sediment–water interface (Urban et al, 1997).…”

“…Fickean diffusive flux of NO 3 − ( F ) across the sediment–water interface was estimated as where D eff is the effective areal sediment diffusion coefficient (cm 2 d −1 ) and Δ C /Δ z is the concentration gradient of NO 3 − (Urban et al, 1997). The value used for Δ C /Δ z was determined based on linear regression using points along the sharp decrease in NO 3 − observed at the sediment–water interface (Urban et al, 1997). Following Urban et al (1997), D eff was calculated as where ϕ is the porosity, n is a constant set here to equal 1 (Urban et al, 1997), and D is the molecular diffusion coefficient for NO 3 − in water (1.64 cm 2 d −1 at 25°C) (Li and Gregory, 1974).…”