Derivation of nutrient guidelines for streams in Victoria, Australia

Human induced increases to nutrient concentrations in streams have led to many agencies developing strategies and criteria for nutrient reduction. National or statewide guidelines are generally inappropriate, due to the natural variability in stream ecosystems within political boundaries. This study used an extant aquatic macroinvertebrate-based regionalisation for the state of Victoria, Australia, as the basis for defining regions of relatively homogeneous environmental character. This enabled the selection of ecologically-based regional reference sites and subsequent characterisation of the nutrient status of these sites. Using an extensive biological and nutrient data base for streams across the State, we calculated 50th and 75th percentile concentrations for reference sites within each region. Using these percentiles in conjunction with 'impact and recovery' studies, we defined nutrient guidelines for each region. Although the nutrient data largely supported the biological regionalisation, patterns in the nutrient data did require some minor modifications for the nutrient regions. Relatively unimpacted regions with reference sites in very good-to excellent-condition were assigned guidelines largely based on the 75th percentiles. The more impacted regions, where 'best available' reference sites were of poorer quality, were assigned guidelines based largely on the 50th percentiles. Professional judgement and known extents of impacts across each region provided important contributions to the decision-making process. The derived guideline concentrations are comparable to several cited in the literature and are proposed for use in monitoring, assessment and restoration targets.