Abstract

Salinisation of freshwater habitats is an issue with global implications that can have serious detrimental effects on the environment resulting in an overall loss in biodiversity. Whilst increases in salinity can occur naturally, such anthropogenic actions as the disposal of industrial and urban effluents and the disturbance of natural hydrological cycles can also result in the salinisation of freshwater habitats. The Water Framework Directive (WFD) requires Member States to restore all freshwater habitats to “good ecological status” and to prevent any further deterioration. Macro-invertebrates are widely used as indicators of river condition for a wide range of reasons and have been designated a key biological element in the assessment of aquatic habitats by the WFD. A review of the available literature, however, found no macro-invertebrate-based biotic indices have been developed for the detection and determination of salinity increases in freshwater habitats that are suitable for application in the United Kingdom for the purposes of the WFD. To this end, a biotic index based on the aquatic macro-invertebrate community response to changes in salinity, termed the Salinity Association Group (SAG) index, was developed. The potential of the SAG index for assessing water quality in terms of salinity in freshwater systems was investigated using data collected from survey sites in Lincolnshire and Norfolk, England, and the results compared to several published salinity indices. Whilst the SAG index was found to show both geographic and seasonal dependence, as is common among many biotic indices, the proposed metric exhibited a stronger relationship to salinity than macro-invertebrate indices employed in Europe for the purposes of the WFD show to their specific pressure. Furthermore, the SAG index was found to be highly selective to only salinity concentration, was significantly related to salinity when used with less detailed information and significantly discriminated between the salinity classes defined by the WFD. It is also highlighted that application of the SAG index with such predictive models as the River InVertebrate Prediction And Classification System (RIVPACS) can resolve the exhibited geographical and seasonal dependence.
In a comparison of the SAG index with the published indices, it was found that the SAG index was the superior metric in terms of recognising abundance as required by the WFD, reliably indicating changes in salinity, compatibility with sampling protocols employed by England’s regulatory authority and producing a linear output. Consequently, it was concluded that the SAG index surpasses other published metrics for the detection and determination of salinity increases in freshwater habitats and is a viable biomonitoring tool suitable for use in England for informing aquatic habitat management decisions, research application and the purposes of the WFD. It is proposed, however, that more rigorous sampling protocols for both macro-invertebrate and environmental data may result in more accurate metric scores and reveal further issues or benefits associated with the SAG index and could also be used to further refine the metric. It is also suggested that adaptation and examination of the SAG index at a larger geographical scale would further demonstrate the validity of the proposed metric and illustrate the potential of the SAG index for worldwide application. Furthermore, intercalibration of the SAG index to harmonise WFD reference conditions and class boundaries across Europe would allow the application of the SAG index throughout Europe for the purpose of the WFD.