Effects of logging on stream environments and faunas in Nelson

Abstract

The effects of various logging practices on stream environments and faunas were studied at the Golden Downs State Forest in Nelson. Comparisons were made between the features of a control stream with an unmodified forest catchment and three streams whose catchments had been affected by different logging practices. Measurements were made of stream flow, water temperature, stream bed sedimentation, suspended sediment and dissolved solids concentrations, and the abundance of benthic invertebrates and fishes. Clearfelling to the stream's edge, together with inappropriate roading and bridging techniques, caused great changes in stream environments and faunas. Excessive amounts of waste timber and soil entered streams and stream bed loads, and suspended sediment and dissolved solid concentrations increased. In comparison to the control stream water temperatures increased in summer by up to 6.5^cc and decreased in winter by as much as 2.5°c. In one stream the benthic invertebrate fauna was greatly modified; there was a reduction in the abundance of Plecoptera and certain Ephemeroptera nymphs, and an increase in the abundance of oligochaetes. chironomids, and Deleatidium nymphs. Fishes, Galaxias divergens and Anguilla dieffenbachi, were also reduced in numbers in this stream. In January 1971 numerous brown trout (Salmo trutta) and other fishes died in the Motueka River, and there are indications that this was due in part to low dissolved oxygen concentrations following excessive sedimentation of the river bed caused by unsatisfactory logging practices. In general, the physical and chemical changes found in the stream environments were similar to those found overseas, but those noted in the invertebrate and fish fauna were rather different, e.g., logging had different effects on the abundance of Diptera, Ephemeroptera, and Trichoptera, and on trout survival and migration. The data collected will be of some value in assessing the effects of this type of logging on other streams and their biotas in the Nelson region. A protective buffer strip of unlogged vegetation was left alongside one stream and the remainder of the catchment was clearfelled. In comparison to the other streams there was relatively little change in the aquatic environment and fauna. Although stream flows and nitrate concentrations were considerably higher than in the control stream, these differences may have been natural and not a consequence of the logging operations. Similar results have been found overseas, and it is concluded that provided measures are taken to reduce erosion, buffer strips will be effective in reducing the effects of logging operations on stream environments and their faunas in other forests throughout New Zealand.