SELECTIVE SCAVENGING OF COPPER, ZINC, LEAD, AND ARSENIC BY IRON AND MANGANESE OXYHYDROXIDE COATINGS ON PLANKTON IN LAKES POLLUTED WITH MINEAND SMELTER WASTES - RESULTS OF ENERGY-DISPERSIVE X-RAY-MICROANALYSIS

Energy dispersive X-ray micro-analyses of the remains of individual or ganisms in plankton samples, together with more conventional analyses of sediments and water, were performed for the purpose of investigatin g the accumulation of heavy metals by plankton in three Canadian Shiel d lakes polluted with Cu, Zn, Cd, Pb, As, and SO4(2-) from a base meta l mine and smelter. The results showed that appreciable quantities of Cu and Zn were bound to crustacean exoskeletons, loricae of the rotife r Keratella, and loricae of the euglenophytes Trachelomonas and Stromb omonas; euglenophyte loricae also contained As and Pb, but an associat ed chrysophyte cyst, though richer in Cu, Zn, and Pb, had no detectabl e As. In all specimens except the chrysophyte cyst, the trace elements were associated with Fe and Mn, implying that Fe and Mn oxyhydroxides deposited on the organisms' armour had scavenged them from the water. Euglenophyte loricae were richest in Fe and Mn and associated trace e lements. Moreover, the inferred FeOOH and MnOOH phases discriminated b etween different elements, Zn being selectively bound by MnOOH whereas Cu and As were bound by FeOOH. Adsorbed Cu and Zn were associated wit h S to a significant degree only where the sediments were poorest in s ulfide, suggesting that the metals were complexed by thiol compounds s ecreted by organisms as a defense against metal toxicity wherever envi ronmental sulfide levels were too low to provide protection. The Fe, M n, Zn, and Cu concentrations and Mn/Fe and Zn/Cu ratios of plankton ha rd parts decreased with the E(h) of the sediments. Dissolved Fe and Mn released into the water column from anoxic but sulfide-poor sediments probably precipitated as FeOOH and MnOOH on surfaces of organisms whe rever the water had sufficient dissolved O2, the MnOOH/FeOOH ratio and hence the Zn/Cu ratio increasing with O2 concentration; but strongly reducing conditions and H2S production in sediments interfered with th e formation of both FeOOH and MnOOH, Fe precipitating as FeS in the se diments whilst Mn remained in solution. Sulfides inhibited bio-accumul ation of Cu and Zn by hindering the formation of FeOOH and MnOOH coati ngs and by suppressing the release of dissolved Cu and Zn (Cu in parti cular) from the sediments. The Cu, Zn, Fe, and Mn concentrations and Z n/Cu and Mn/Fe ratios of plankton hard parts and the concentrations of bio-available (DTPA-extractable) Cu and Zn in the sediments were lowe st in a lake whose sediments had anomalously low E(h) and high sulfide levels caused by sewage effluents, algal blooms, and SO4(2-), even th ough these sediments had the highest total Cu and Zn content. Thus, bi o-accumulation of metals depended on the bio-availability, not the tot al supply, of the metals. The results of the research are consistent w ith the following generalisations: (1) FeOOH and MnOOH are commonly de posited on exposed surfaces of a wide range of plankton hard parts in different freshwater environments; (2) the FeOOH and MnOOH coatings sc avenge heavy metals and metalloids from the water and are largely resp onsible for the passive accumulation of these elements by plankton; (3 ) FeOOH and MnOOH have different trace element affinities, thus affect ing the proportions as well as the quantities of the elements bound to the plankton surfaces; (4) the accumulation of metals by plankton har d parts is controlled by environmental variables such as E(h) and sulf ide and O2 levels, as these factors regulate the precipitation of FeOO H and MnOOH surface films, determine the MnOOH/FeOOH ratio, and contro l the speciation, solubility and bio-availability of metals stored in bottom sediments, whereas total metal concentrations in sediments may have no appreciable effect on the bio-accumulation of the metals; and (6) heavy metal bio-availability is limited mainly by sulfides generat ed in sediments; where sulfides are lacking, thiol compounds are relea sed into the water by the plankton to prevent toxic effects of the met als. These conclusions have significant implications for the cycling a nd ecological effects of toxic and nutritional trace elements and for the monitoring, abatement, and prevention of water pollution due to mi ning, smelting, and other industrial activities.