SPECIATION AND CYCLING OF TRACE-METALS IN ESTHWAITE WATER - A PRODUCTIVE ENGLISH LAKE WITH SEASONAL DEEP-WATER ANOXIA

The chemical speciation of Cu, Ni, and Cr and the dissolved and unfilt ered concentrations of Mo, Ti, Co, Fe, Mn, and Zn were determined in t he water column of a lake with a seasonally anoxic hypolimnion (Esthwa ite Water, Cumbria, UK) during three surveys in the summer of 1991. Ca thodic stripping voltammetry (CSV) with ligand competition was used fo r this speciation study. The metal data, in conjunction with physical and nutrient data, were used to assess the biogeochemical processes co ntrolling the distribution and speciation of the trace metals in the l ake and their seasonal variation. Thermodynamic calculations were used to assess the dissolved trace metal speciation and the saturation sta te of metal-sulphide phases. The development of anoxia in the lake had an important influence on the redox speciation of dissolved Cr, with Cr(III), forming the major Cr species under anoxic conditions. Cobalt showed enhanced concentrations in the hypolimnion during anoxia, coinc iding with enhanced Fe and Mn levels. Dissolved Cu, Ni, and Zn concent rations in the hypolimnion decreased dining anoxia, which could possib ly be explained by particle scavenging (iron sulphides) and a decline in hypolimnetic supply as the season develops. Regeneration processes of sunken organic detritus on the sediment-water interface were the mo st likely causes for enhanced hypolimnetic concentrations of Ni, Cu, a nd Zn, during periods when the lake was completely oxic. Substantial o rganic Ni and Cu complexation was observed in Esthwaite Water, which w as attributed to the eutrophic character of the lake. Between 25 and 8 0% of the dissolved Ni in Esthwaite Water was strongly complexed by na tural ligands. The highest proportions of Ni complexation were observe d in the epilimnion and hypolimnion, with a minimum in the thermocline . Strong Cu complexing ligands were observed in the lake, with dissolv ed ligand concentrations between 6.8 and 29.4 nM and conditional stabi lity constants between 10(11.1) and 10(14.4). More than 94% of the dis solved Cu in Esthwaite Water was complexed by the natural ligands. Cop yright (C) 1997 Elsevier Science Ltd.