A LABORATORY STUDY OF THE BIOGEOCHEMICAL CYCLING OF FE, MN, ZN AND CUACROSS THE SEDIMENT-WATER INTERFACE OF A PRODUCTIVE LAKE

Laboratory incubation experiments were carried out on sediment cores c ollected from Esthwaite Water, U.K., during April 1987, when the sedim ents displayed a characteristic surface (1.5 to 2 cm) oxide floc. The experiments were undertaken at 10 degrees C, in the dark, under variab le redox and pH conditions for periods of similar to 720 h (30 d). In some cases, realistic amounts of decomposing lake algae were added to simulate the deposition of an algal bloom. Pore waters and overlying w aters were obtained from the incubated sediment cores at various time intervals and the samples analysed for pH and dissolved Fe, Mn, Zn and Cu by AAS. The results demonstrated that trace metal concentrations a t the sediment-water interface can show rapid, pulsed responses to epi sodic events associated with controlling factors such as algal deposit ion and mixing conditions. The variations in dissolved Fe and Mn conce ntrations could generally be explained by their well known redox behav iour. Appreciable loss of Mn from solution under conditions of well-de veloped anoxia was consistent with adsorption of Mn2+ by FeS. Cu and Z n were both rapidly (24 h) released into solution during incubation of sediment cores prior to the development of anoxia in the overlying wa ters. Their most Likely sources were the reductive remobilization of M n oxides and the decomposition of organic matter. The addition of deco mposing algae to a series of cores resulted in even higher interfacial dissolved concentrations of Cu and Zn, probably through acting as a s upplementary source of the metals and through increased oxide dissolut ion. Switching from anoxic to oxic conditions also rapidly increased d issolved Cu and Zn concentrations, possibly due to their release durin g the oxidation of metal sulphides. The enhanced releases of dissolved Cu and Zn were generally short-lived with removal being attributed to the formation of sulphides during anoxia and to adsorption by Fe and Mn oxides under oxic conditions.