The current rate of change of salinity in freshwater ecosystems may be much faster than freshwater biota can evolve or adapt. Although lowland river biota may have mechanisms that allow survival during periods of extreme salt concentrations, upland rivers potentially have experienced lower natural variation in salinity and therefore biota in these systems may be less salt-tolerant. Induced changes in salinity in upland systems may be too rapid for taxa to adapt, suggesting that freshwater taxa may be lost and communities will become dominated by salt-tolerant taxa. Pulses of salt into freshwater ecosystems will influence survival of a range of biota and although such increases in salt may be rapid and short-lived, the consequences to the freshwater biota are unknown.
We need to know how salinity changes ecosystem functioning through alteration of biotic and abiotic processes: do changes to ecological processes change community composition? Managers need to know more about the relationship between flow patterns, salt concentrations and environmental damage to predict t consequences of management actions. How a combination of changes in flow and salt affect river and wetland communities is also relevant to management predictions. We have many systems that are naturally variable in both salinity and hydrology, yet we do not know how increasing salinity will affect the biota or ecosystem integrity. Linking salinity levels directly to mortality or recruitment potential of aquatic biota is not sufficient to predict the outcome of increasing salinity on freshwater ecosystems. Second- and third-order effects must also be taken into account in describing the full effect of salinity on aquatic ecosystems. Of particular interest are the effects of increasing salinity on primary and secondary production, nutrient dynamics and food-web structure. Once we understand these interactions, links and the flow on consequences, managers and researchers will be in a better position to predict the condition of aquatic ecosystems under modified salinity and move towards focusing on effective rehabilitation. For example, the use of environmental water allocations (environmental flows) could be considered as a tool in managing salinity in aquatic ecosystems, once the relationships between hydrology, salinity and environmental damage are further delineated. Use of this relationship could enhance effective disposal of salt-contaminated water, with minimal damage to the environment.
If ecosystem health and salt can be related, then tools such as water allocations, river operation, engineering intervention and catchment management programs can be designed to manipulate salt lo ads to increase the health of aquatic ecosystems. Innovative experimental science, together with imaginative predictive management can work together to underpin salinity management issues on both broad and local scales.