Development of biotic indices for establishing and monitoring ecosystem health of the Swan-Canning Estuary

F.J. Valesini; C.S. Hallett; A. Cottingham; S.A. Hesp; S.D. Hoeksema; N.G. Hall; T.E. Linke; A.J. Buckland

The Swan-Canning Estuary is highly valued for its ecological, recreational, commercial and indigenous importance (e.g. Seddon 1972, Swan River Trust 2008, 2009). It supports a diverse range of fish species (several of which complete their life cycles in the system and/or are recreationally or commercially important, e.g. Loneragan et al. 1989, Kanadjembo et al. 2001, Hoeksema and Potter 2006), migratory and resident waterbirds (Bamford et al. 2003), submerged and fringing vegetation (e.g. Hillman et al. 1995, Astill and Lavery 2001, McMahon 2001) and a dolphin population (Lo 2009). The Swan-Canning Estuary and its large (ca 125 000 km2) catchment have been subjected to substantial anthropogenic change since European settlement in the early to mid 1800s, and the system is now classified as highly modified (Commonwealth of Australia 2002). These artificial modifications, combined with the ongoing effects of local population growth and climate change, continue to have a wide range of implications for the water quality of this system. For example, reduced river flow due to damming or diversion of the major tributaries and the effects of climate change, increased tidal exchange through widening and deepening of the estuary mouth and extensive clearing of catchment vegetation, have all contributed to rising salinity throughout this system (Hamilton et al. 2001, Thomson et al. 2001, Chan et al. 2002, CSIRO 2009). Changes in the volumes of marine vs riverine flow have also exacerbated the stratification of salinity and dissolved oxygen concentration within the water column, particularly in the upper estuarine reaches where bottom waters become hypoxic during drier periods of the year (Hamilton et al. 2001, Thomson et al. 2001, http://www.swanrivertrust.wa.gov.au/science/river/Content/plots.aspx). This lack of dissolved oxygen has become so extensive that remedial oxygenation of both the Swan and Canning rivers is now undertaken mechanically (http://www.swanrivertrust.wa.gov.au/ science/river/content/oxygenation.aspx). Widespread land clearing, shoreline modification and the growth of surrounding urban and agricultural activity have also resulted in increased surface runoff from the catchment, and thus also of the sediment, nutrient and pollutant loads entering the estuary. These loadings have also risen due to the vast network of drains servicing residential, farming and industrial areas that discharge into the system, and their impacts are further compounded by the reduced flushing of the estuary due to diminishing rainfall (Jakowyna et al. 2000, Swan River Trust 2003, 2009, Foulsham 2009). The system, and particularly its upper reaches, is now considered to be eutrophic to hypereutrophic (Swan River Trust 2009), and the levels of various non-nutrient contaminants in the sediment exceed ANZECC and ARMCANZ Interim Sediment Quality Guideline Trigger Values at several locations throughout the estuary (Nice 2009).

Development of biotic indices for establishing and monitoring ecosystem health of the Swan-Canning Estuary

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