Biomonitoring of the Derwent River at Derwent Bridge: 1995 - 2003

Abstract

This report describes the cumulative results to March 2003 of the biological monitoring conducted in the Derwent River at Derwent Bridge prior to and following the commencement of discharge of the wastewater treatment plant (WWTP) constructed to treat waste waters from developments at Cynthia Bay at Lake St Clair. Monitoring of the biota of the ambient waters receiving discharge from the WWTP is required under the conditions of license to operate by the Department of Primary Industry Water and Environment (DPIWE). As part of those conditions, monitoring at two sites was required during a period prior to commencement of WWTP discharge (1995) and thereafter (1996, 1997, 1998, 2000). Sampling was conducted three times per year in 1995-1997 (once each in summer, autumn and winter), was decreased to twice a year in 1998 (once in autumn and winter). In 1998, the frequency of sampling was further decreased to once every second year, in autumn, with sampling conducted in April 2000 and March 2003. Pre-discharge biological monitoring commenced in early 1995. Commencement of WWTP discharge commenced in August 1996. There are therefore, three sampling events conducted prior to discharge. These samples represent the 'pre-discharge' conditions, while all samples after late 1996 represent the ‘post-discharge’ conditions. The two sampling sites are situated 100 m upstream and downstream of the discharge point, located approx. 1 km downstream of Derwent Bridge. Monitoring consists of regular quantitative sampling of benthic (bottom dwelling) macroinvertebrates (principally aquatic insects and crustaceans) and algae in the Derwent River. The discharge point is immediately upstream of Lake King William. At the point of proposed discharge, the Derwent River consists of a sequence of long runs interspersed with occasional deep pools. The runs are characterised by a gravel and small cobble substrate, grading to fine gravel and sand on the channel margins. Flows are partially controlled by HEC gates downstream of Lake St Clair, but the river hydrology is essentially natural, reflecting the discharge from Lake St Clair, whose storage characteristics have not been substantially altered by HEC operations and whose catchment is essentially pristine. The reach in which the WWTP discharge occurs, is, however, frequently inundated by Lake King William waters when that lake is within 1 - 2 m of full supply level (FSL). Prolonged periods of inundation lead to a decrease in the abundance and diversity of macroinvertebrates at both sites, to a similar extent (see Davies and Cook 1998a, 1998b, 2000).