Trace Element Distributions in Aquatic Insects: Variations among Genera, Elements, and Lakes

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Abstract
The distribution patterns of trace element contaminants (As, Cd, Cu, Pb, Zn) among the tissues of freshwater insects (Chironomus, Clinotanypus, Procladius, Hexagenia, Sialis) are both taxon and element specific. In quantity terms (micrograms per animal), the gut is the major organ of Cd accumulation, while most of the As, Cu, Pb, and Zn accumulates in the body (i.e. animal minus gut and other organs). In concentration terms (micrograms per gram), Cd, Cu, and Zn are often highest in the gut, while Pb and As are not. Much of the As, and in some genera the Pb, appears to be bound externally. Respiratory and excretory structures generally contain small proportions of the total trace element burdens, except for Cu. Trace elements are not uniformly distributed along the gut but tend to accumulate either in the mesenteron (midgut) or in the proctodeum (hindgut), depending on the element and insect concerned. The distribution patterns of most of the trace elements vary little among populations of the mayfly Hexagenia from different lakes with the exception of Cd, for which the proportion of the total Cd burden localized in the gut increases markedly with increasing Cd concentrations in the whole nymph.