Seasonal variation of heavy metals in Mytilus edulis, Fucus vesiculosus and sediment from the Shannon Estuary

Samples of Mytilus edulis, together with samples of sediment and Fucus vesi culosus, were taken monthly from each of five shores on the south coast of the Shannon Estuary from March 1992 to December 1993 inclusive. Samples wer e digested individually and the levels of iron, zinc, manganese, cobalt, ch romium and copper were determined using atomic absorption spectrophotometry . Levels of metals were lower than previously published values from Irish e ast coast estuaries, including Dublin, except for a few high levels in M. e dulis, which did not persist in the tissues and were probably lost through excretion, spawning or detoxification. Of the metals analysed iron showed t he highest levels. This was attributed in part to the background levels of iron in sediment. On some occasions high metal levels ill sediment did not lead to higher levels either in F. vesiculosus or in M. edulis, suggesting that metals in M. edulis may come from other sources, including anthropogen ic inputs, in the estuary. On an international scale the levels in M. eduli s of the six metals studied were relatively low in the Shannon Estuary. The longer-term monthly sampling regime followed in this Study is important si nce it shows that occasional high levels, which may be detected by once-off studies, are not always maintained.