The impact of copper in a contaminated stream with particular reference to Plectrocnemia conspersa (Curtis) (Trichoptera)
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An investigation of a copper contaminated mine drainage stream, the Darley Brook, revealed that the water quality was relatively constant over time, with an exceptionally high copper concentration (mean 0.89 mg lˉ¹ ) and low pH (mean 5.2) at its source. Virtually all of the copper was in a 'soluble' form with upto 29% as the free cupric ion. Overall mean leachable copper concentrations of 2139 µg gˉ¹ and 101 µg gˉ¹ were recorded in the uppermost reaches of the Darley Brook and a control stream respectively, with little temporal variation. Biotic communities in the uppermost sampling stations were of reduced diversity. Excessive growth and high copper concentrations (maximum plant tissue concentration of 3664 µg gˉ¹) were recorded in Jungermannia atrovirens and Microspora sp. in the riffles and the pool-dwelling Juncus bulbosus. In these tolerant plants copper concentrations were higher in the summer than in the preceding winter. Macroinvertebrates were reduced to chironomid larvae, Coleoptera and the net-spinning caddis Plectrocnemia conspersa. This species was univoltine and larvae were more aggregated and occurred at higher densities (upto a mean of 126 mˉ²) in the contaminated stream than in the control stream. Each larval instar of P. conspersa from the Darley Brook contained significantly more copper than the same instar in the control stream. There was an exponential decrease in larval copper concentration with increasing weight for both populations, and maximum concentrations were always recorded in the summer. An histological and ultrastructural investigation of P. conspersa larvae revealed the presence of copper containing granules in the cells of the malpighian tubules and in the subcuticular region. These may be primarily pigment granules, which serve to immobilise excess copper within the larva. The metal tolerance of P. conspersa was confirmed by transfer experiments and discussed in relation to the occurrence of this species in metal rich waters.
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