Effect of spatial variation on salinity tolerance of macroinvertebrates in Eastern Australia and implications for ecosystem protection trigger values

Abstract

Salinisation of freshwater has been identified as a serious environmental issue in Australia and around the world. Protective concentrations (trigger values) for salinity can be used to manage salinity impacts, though require locally relevant salinity tolerance information. 72-h acute salinity tolerance values were determined for 102 macroinvertebrates collected from 11 locations in four biologically distinct freshwater bio-regions in Northeast Australia and compared with sensitivities observed in Southeast Australia. The salinity tolerance of individual taxa was consistent across Northeast Australia and between Northeast and Southeast Australia. However, two distinct communities were identified in Northeast Australia using distributions of the acute tolerance values and a calculated index of salinity sensitivity. Salinity trigger values should therefore be representative of local or regionally relevant communities and may be adequately calculated using sensitivity values from throughout Eastern Australia. The results presented provide a basis for assessing salinity risk and determining trigger values for salinity in freshwater ecosystems at local and regional scales in Eastern Australia.

Introduction

Salinisation is one of Australia's most serious environmental issues. Although dissolved salts are natural components of freshwater and some inland aquatic systems have naturally high salinity levels, it is now well recognised that impacts from excessive anthropogenic related increases in concentrations of dissolved salts can have profound and measurable effects on freshwater aquatic ecosystems (Hart et al., 1991, Williams et al., 1991, James et al., 2003, Kefford et al., 2003, Marshall and Bailey, 2004, Dunlop et al., 2005). In Australia, an estimated 5.6 million hectares of land is at high risk from induced dryland salinity and by the year 2050, estimates indicate that 17 million hectares of land may be salt affected (National Land and Water Resources Audit, 2000). In areas already affected, salinity has resulted in social, economic and environmental impacts (Land and Water Australia, 2002). It is therefore clear that there is an urgent need to manage salinity impacts in Australia.

Establishment of trigger values (guidelines) for salinity can provide a basis for its management but there are currently no widely acceptable, biologically based guidelines applicable at local or regional scales. Probabilistic risk assessment techniques can be used to determine salinity concentrations or trigger values protective of particular taxa or a proportion of taxa (Hart et al., 2003). This can be achieved by modelling a distribution of laboratory derived salinity sensitivity values (species sensitivity distribution (SSD) or taxa sensitivity distributions (TSD)) from which a percentile, or ecosystem protection value (trigger value) can be calculated to ensure the protection of a pre-defined proportion of taxa. Salinity sensitivity data can be derived from either concentration–response laboratory experiments (resulting in ≤96-h LC₅₀ values) using a standard composition of dissolved salts and/or the maximum salinity at which taxa have been observed to occur in the field (resulting in F_max values). Laboratory experiments provide a direct causal link between a concentration of exposure and a measured effect (Goetsch and Palmer, 1997, Berezina, 2003, Kefford et al., 2003, Kefford et al., 2004) and are preferred for use in the determination of water quality guidelines (ANZECC/ARMCANZ, 2000). Despite a clear need to determine biological effects based ecosystem protection values, there is limited salinity tolerance information available from freshwater organisms in Australia, and in particular there is a lack of such data in Northern Australia as most studies have been conducted either outside of Australia or in Southeastern Australia (Kefford et al., 2003, Kefford et al., 2005a, Kefford et al., 2006a). It is also not known whether the existing salinity tolerance information provides adequate representation of the variation in salinity tolerance in Eastern Australia. This study evaluates the sensitivity of a broad range of macroinvertebrates in Northeast Australia to a standard synthetic marine salt and compares these tolerance values with those observed across Eastern mainland Australia.