Impacts of wetland afforestation on the distribution of benthic invertebrates in acid streams of Westland, New Zealand

Abstract

Inter‐relationships between some physical and chemical parameters of streams, the degree of catchment development, and benthic invertebrate distributions were investigated on two dates at 26 running‐water sites in North Westland, New Zealand. Streams draining catchments recently developed for forestry had higher summer water temperatures, greater total reactive aluminium concentrations (up to 31°C and 911 mg m^‐3, respectively), and lower pH (down to 4.1) than most streams draining undeveloped wetland or native‐forested catchments. Apparent differences in water chemistry were attributed to the most recent development being at the heads of wetland catchments where leaching of organic acids into drainage waters was probably highest, rather than to any effects of development per se. Overall, streams that were very acidic (pH mostly < 5.0 and often < 4.5) and drained wetland catchments supported 1.3–3.3 fewer invertebrate taxa than sites that consistently had pH>4.5 and drained native‐forested catchments. Invertebrate colonisation appears to be limited by the extremeacidity of brownwater streams on pakihi wetlands.

Key words:

• benthic invertebrates
• wetlands
• pakihi
• drainage
• forestry
• acid streams
• TWINSPAN
• DECORANA
• New Zealand

Notes

Present address: Department of Conservation, P.O. Box 10–420, Wellington, New Zealand