Deliverable 2.24

Abstract: Changes in ecological status at RIvPACS reference condition sites

The WFD requires Member States to determine the ecological status of rivers and streams with respect to deviation from a type-specific reference condition. It is essential that Member States can demonstrate that the biological datasets used to define reference conditions meet the criteria of the WFD. The approach requires that reference sites be at their ecological optima, and are assumed to not change because by definition they are not impacted. We used RIvPACS reference site data and UK Environment Agency monitoring data to identify 81 RIvPACS reference sites that had subsequent monitoring data, and analysed seasonal patterns in ASPT and Ntaxa. Autumn ASPT increased over time in both data sets, but not Ntaxa, indicative of a shift in reference conditions and species replacement.

The trend was site dependent, indicating that long term climatic cycles, or shifts in climate, are an unlikely cause. Deviation from the perceived reference condition was common for ASPT and Ntaxa at most sites, as a majority of subsequent samples did not fall within +/- 5% of the RIvPACS reference values. The ASPT and Ntaxa values of the RIvPACS reference samples for a site did not lie within the standard error range of the overall mean ASPT and Ntaxa for 70 and 80% of the sites respectively. ASPT was generally higher in upland areas of the UK and lowest in lowland agricultural areas. Rates of change in ASPT were highest at sites with intermediate ASPT scores. Low and high values of Ntaxa were more dispersed across the UK, though Ntaxa correlated to mean air temperature indicating a north/south gradient. Rates of change in Ntaxa were also highest at sites with intermediate Ntaxa scores, and rates of change were higher for spring samples. These results demonstrate that the fixed reference condition concept may not be realistic and that selection of reference sites should consider long term variability.


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