Developing sediment targets to prevent excessive sedimentation in agriculturally-dominated watersheds

Of the many land-disturbing activities contributing to increased sedimentation of aquatic ecosystems, agriculture is considered one of the most common and widespread sources in the US and Canada.  As part of the “National Agri-Environmental Standards Initiative”, the Government of Canada developed non-regulatory environmental performance standards that represent thresholds of total suspended (TSS) and deposited sediments to protect ecological condition of agricultural streams.  Analysis of data from long-term monitoring programs and contemporary sampling of networks with 8-15 streams showed that agricultural land use increased sedimentation in all six agricultural regions examined. In addition, five empirical approaches used for developing water quality guidelines were applied to these data to produce protection thresholds for TSS and deposited sediments.  Because increased sedimentation in agricultural watersheds resulted in greater abundance of pollution tolerant taxa and significant reductions in several benthic macroinvertebrate metrics of ecological quality (i.e., diversity, total richness, EPT richness, EPT relative abundance), we also cross-calibrated these calculated physical TSS and deposited sediment targets with regression-tree analysis that incorporated information for both sediments and benthic metrics of ecological condition.  Although analysis of TSS and deposited sediment data alone was useful for setting interim sediment targets, future methods development should focus on the use of biological endpoints (i.e., benthic macroinvertebrate composition and metrics) to establish standards that confer good ecological condition.