Water quality of a stream recently fenced‐off from deer

Abstract

Water quality was monitored on a tributary of the Dow Stream on the AgResearch Invermay deer farm near Mosgiel. The aim was to see if fencing‐off an area of the stream channel with a known contaminant source (a wallow) and riparian planting improved water quality as measured by the two‐weekly concentrations and annual loads of nitrogen (NH4 +‐N, NO3 ‐‐N), phosphorus (P; dissolved reactive P, participate P and total P) species, suspended sediment (SS) and the faecal indicator bacteria, Escherichia coli (E. coli). Measurements were taken 2 years before and after fencing‐off and riparian planting. Analysis of the data indicated that the mean concentrations of N03 ‐‐N, NH4 +‐N, SS and TP improved after fencing‐off and planting compared to those beforehand. As a result, loads of these constituents decreased by 78% to 98%. Mean concentrations of E. coli and DRP showed no significant difference with fencing‐off and planting. Mitigation effects were attributed to (i) the settling of SS and associated NH4 +‐N and PP in an area formerly used by deer for wallowing and (ii) the absence of direct excretal input by deer. Hence, fencing‐off and riparian planting is recommended for areas where known sources of contaminant loss occur (e.g., wallows or degraded areas of a stream channel). However, this must be balanced with the cost of fencing and potential loss of land from production and the suitability of the stream, or area, for fencing. For instance, the potential mitigation performance of fencing‐off and planting is likely to be poorer in areas without large contaminant sources or already good water quality.