Salinisation of rivers: An urgent ecological issue

Abstract

Secondary salinisation of rivers and streams is a global and growing threat that might be amplified by climate change. It can have many different causes, like irrigation, mining activity or the use of salts as de-icing agents for roads. Freshwater organisms only tolerate certain ranges of water salinity. Therefore secondary salinisation has an impact at the individual, population, community and ecosystem levels, which ultimately leads to a reduction in aquatic biodiversity and compromises the goods and services that rivers and streams provide. Management of secondary salinization should be directed towards integrated catchment strategies (e.g. benefiting from the dilution capacity of the rivers) and identifying threshold salt concentrations to preserve the ecosystem integrity. Future research on the interaction of salinity with other stressors and the impact of salinization on trophic interactions and ecosystem properties is needed and the implications of this issue for human society need to be seriously considered.

Introduction

Salinity refers to the total concentration of dissolved inorganic ions in water or soil (Williams and Sherwood, 1994) and is therefore a component of all natural waters. Dissolved ions can be also expressed as the ionic activity of a solution in terms of its capacity to transmit electrical current (electrical conductivity (EC), measured in Siemens per meter). Therefore, EC is routinely used to measure salinity, and the relation between them is a function of water temperature. Surface waters can be classified according to their salt content as follows (Venice system, 1959): freshwater < 0.5 g L⁻¹; oligohaline 0.5–4.0 g L⁻¹; mesohaline water 5–18 g L⁻¹; polyhaline water 18–30 g L⁻¹; euhaline water 30–40 g L⁻¹; hyperhaline water > 40 g L⁻¹. In inland waters, salinity may vary from 10's of mg L⁻¹ to 100's of g L⁻¹ and is a major factor limiting the distribution of biota (Williams, 1987). In fact, aquatic biota have been commonly grouped according to their salinity preferences; i.e. freshwater fauna, brackish-water fauna and marine fauna (Remane and Schlieper, 1971). While in principle salinity can refer to any inorganic ions, in practise it is mostly the result of the following major ions: Na⁺, Ca²⁺, Mg²⁺, K⁺, Cl⁻, ${{\text{SO}}_4}^{2-}$, ${{\text{CO}}_3}^{2-}$ and ${{\text{HCO}}_3}^{-}$ (Williams, 1987).

In the absence of anthropogenic influences, salinity and the proportions of the above ions originate from three sources. (1) Weathering of the catchment, which is a function of both geology of the catchment and precipitation. (2) Sea spray, although this is only an important source of salts in coastal locations. (3) Small amounts of salts dissolved in rainwater as a consequence of evaporation of seawater. This third source can be a significant source of salt in the terrestrial landscapes distant from the sea (Herczeg et al., 2001). Regardless of its source, dissolved ions can be concentrated by evaporation and transpiration, and this is particularly important in semi-arid, arid and regions with seasonally hot dry climates. The ability of plants to extract small amounts of soil moisture and the depth that their roots can capture soil moisture play an important role determining the resultant salinity of soil, groundwater and runoff to rivers (and thus dilution of salts). Furthermore salts can be stored in soils, sub-soils and groundwater as a result of previous periods of aridity and then subsequently be released. Especially in regions with flat topography, stored salts can move very slowly and remain in the landscape for an extended period. Such storage and release of salts can occur at various time-scales from seasons, decadal scale climate variations (McNeil and Cox, 2007) to 100,000's years (Herczeg et al., 2001). So natural salinity of rivers is a complex and dynamic function of the climate (recent and past), the geology of its catchment, the distance from the sea, topography and vegetation.

Anthropogenic increases in salinity are referred to as secondary salinisation. This contrasts with primary salinisation, which involves the accumulation of salts originating from natural sources at a rate unaffected by human activity as outlined above. Secondary salinisation is a global and growing threat that poses a risk of causing severe biodiversity losses and compromising the ecosystem goods and services that rivers, wetlands and lakes of the world provide. Therefore, salinisation has been rated as one of the most important stressors for freshwater ecosystems in the Millennium Ecosystem Assessment (2005). Moreover, in the United States salinisation is among the top 15 causes of impairment of streams being equally important as pesticide input (US Environmental Protection Agency, 2012). In Australia a survey of river managers rated salinity in the top three most important environmental contaminants (Lovett et al., 2007). Although there are some review papers dealing with Australian rivers (Hart et al., 1991; James et al., 2003; Dunlop et al., 2005), no comprehensive review integrating different regions of the world has been done yet. This paper reviews the major causes of secondary salinisation, its impacts on the biota of rivers, management of rivers subject to secondary salinisation and identifies future research needs in these areas under a global perspective.