Determining the ecological health of estuaries in NW Tasmania: A case study assessing the status of the Duck, Montagu, Detention and Black River estuaries. NHT Final Report

Identifying suitable reference points is critical for effective monitoring of any natural system. Here we utilize a comparative approach where putative impacts within two estuaries are determined via comparison with two reference estuaries deemed to be relatively free of human impacts. The two putatively impacted estuaries considered in this study were the Duck and Montagu River estuaries situated on the NW coast of Tasmania. The river catchments of these estuaries are typical of the region in that their catchments have become highly modified through the clearance of native vegetation and subsequent replacement with intensive agriculture, in particular dairy farming. Nutrient loadings (nitrogen and phosphorous) for these catchments are amongst the highest in Tasmania. The catchments for the Black and Detention estuaries, by comparison, are located for the most part in State Forest with limited grazing in the lower catchment. This study considered a range of water-column and sediment (benthos) based variables commonly used to monitor estuaries. These included: salinity, dissolved oxygen, turbidity, nutrient and chlorophyll a levels for the water-column; and sediment redox, organic carbon content, chlorophyll a and macroinvertebrate community structure amongst the benthos. In addition to comparing reference with impacted estuaries, comparisons were also made across seasons, commensurate with seasonal changes in freshwater river input, and between regions within estuaries (upper and lower reaches) - previously identified in Hirst et al. (2005). This design enabled us to examine whether the detection of impacts (i.e. differences between reference and impacted systems) was contingent on the time and location of sampling or independent of these factors. Previous work had indicated that impacts may only be manifested at certain times, and in certain regions, within these estuaries.