Shredders and leaf breakdown in streams polluted by coal mining in the South Island, New Zealand
Leaf breakdown and macroinvertebrate colonisation of artificial leaf packs were investigated in six streams experiencing coal mine discharge and metal deposition near Reefton, South Island, New Zealand. Nine sites; three with metal precipitates prominent on their beds, three with abundant metal bacterial flocs and three control sites with no mine related discharges were investigated over a nine week period during summer. pH was lowest in streams dominated by metal precipitates (pH 3.8 – 4.6), which also had the highest concentrations of dissolved iron (1.16 ± 0.44 g / m³) and dissolved aluminium (1.83 ± 0.96 g / m³). Leaf breakdown of American sycamore (Platanus occidentalis) was slow in all streams (-k/day = 0.0007 – 0.007) and did not differ significantly among stream types. Leaf toughness took longer to decline in streams dominated by metal precipitates than in streams with metal flocs suggesting that leaves would have taken longer to decompose in precipitate streams. Shredder densities were low in all streams (< 4 per leaf pack) and precipitate streams had the lowest macroinvertebrate densities per leaf pack (< 6). Our findings indicate that organic matter processing may be reduced only marginally in streams dominated by metal flocs, however metal precipitates that can form on leaves in mine drainage streams may increase the time taken for leaves to soften and hinder the processing of organic matter.