The biogeochemical cycling of Zn, Cu, Fe, Mn, and dissolved organic C in a seasonally anoxic lake

Abstract

Detailed time-depth distributions of dissolved Zn, Cu, Fe, Mn, and dissolved organic C (DOC), together with pH, were obtained for the water column and near-surface sediment pore waters of a seasonally anoxic lake (Esthwaite Water, U.K.) over a 1 -yr period. Dissolved Fe and Mn followed well-known patterns linked to redox recycling. The variation in DOC resulted mostly from its coupling with Fe, DOC being coprecipitated with Fe oxide and released into solution on the reductive remobilization of' the oxide. This process produced a > 7-fold seasonal variation in DOC concentrations in the pore waters, with a maximum of -7 mg liter-l. Dissolved Zn and Cu exhibited coherent and similar distribution patterns unrelated in any simple way to the distributions of dissolved Fe and Mn, pH, and DOC. The most ubiquitous feature of the distribution of dissolved Zn, and to a lesser degree that of dissolved Cu, was a concentration maximum at the sedimentwater interface, which disappeared completely only when anoxia was maximal in mid-and late summer. The interfacial maxima were attributed to rapid release from freshly deposited particles allied to sulfide precipitation in the sediments. The timing of events, associated with an increase in interfacial dissolved Cu and Zn concentrations before and during a period of ice cover, indicated that the Cu was derived from Fe and possibly Mn oxides and Zn from planktonic algae. The cumulative downward diffusive fluxes of Zn and Cu from the interfacial maxima were unimportant (-1% of total) compared to total metal sedimentation rates. Chemical equilibrium calculations indicated that Zn-and Cu-sulfide complexes dominated in the pore waters under reducing conditions, with humic binding being insignificant, and that precipitation as pure sulfides or coprecipitation with FeS were likely major removal processes for Zn and Cu under anoxic conditions.It is apparent from the limited available information that trace metal distributions in lakes are complex and generally do not show the model (e.g. biologically recycled and scavenged) vertical profiles seen in the oceans (see Murray 1987;Hamilton-Taylor and Davison 1995). This complexity has been attributed to various environmental factors, including the highly dynamic nature of lakes where inputs, mixing, and removal processes can vary significantly on a variety of time scales, the greater availability of competing scavenging phases, the varied chemical composition of lake waters, and the compressed depth scales. Insight into biogeochemical processes has been afforded by considering detailed temporal changes in trace metal distributions in the water columns of lakes (e.g. Baccini and Joller 198 1;Balistrieri et al. 1992). We found no comparable temporal data for the highly active site of the sediment-water interface. This paper provides detailed time-depth distributions of dissolved Zn, Cu, Fe, and Mn, together with DOC and pH, in the water column and near-surface pore waters of a circumneutral, productive, seasonally anoxic lake. Acknowledg...

Determination of High Resolution Pore Water Profiles of Trace Metals in Sediments of the Rupel River (Belgium) using Det (Diffusive Equilibrium in Thin Films) and DGT (Diffusive Gradients in Thin Films) Techniques

“…This maximum has already been observed in several studies and can be attributed to the mobilization of these metals from the decomposition of freshly precipitated organic matter (Hamilton-Taylor et al, 1996;. Very sharp surface maxima are found at mm scale which cannot be observed with conventional pore water extraction techniques.…”

Diurnal variations of trace metals and heterotrophic bacterioplankton concentration in a small boreal lake of the White Sea basin

“…Less in known about the fate of trace metals, except those involved in redox cycles linked to photoreduction that has been extensively investigated (McMahon, 1969;Emmenegger et al, 2001). There is a large body of literature on trace element geochemistry in temperate lakes (e.g., Hamilton-Taylor and Willis, 1990;Noel et al, 1990;Balistrieri, 1992;Viollier et al, 1995Viollier et al, , 1997Hamilton-Taylor et al, 1996Nriagu et al, 1996;Falkner et al, 1997;Albe´ric et al, 2000), including detailed seasonal observations. However the diurnal patterns of trace metal concentration remain poorly investigated.…”

Sporadic hotspots for physico-chemical retention of aquatic organic carbon: from peatland headwater source to sea

“…In a field experiment where DOC-rich waters were added to iron (Fe)-rich waters, fulvic acids were preferentially removed at a very rapid rate (of the order of milliseconds; McKnight et al 2002). Adsorption and/or co-precipitation of DOC with Fe may therefore constitute a sink for DOC in upland waters, although re-suspension or dissolution may occur if conditions are favourable, and have been linked to seasonal variation in lake DOC concentrations in Esthwaite Water, Northern England (Hamilton- Taylor et al 1996). Photo-mediated oxidation of aquatic DOC also potentially converts fluvial organic C to CO 2 , and is recognised as an important process in lakes (Anesio and Graneli 2004).…”