An assessment of the chemical and biological methods of river water-quality classification in the Foyle River catchment
Data for ten chemical parameters, obtained from the routine chemical monitoring programme for 49 sites in the Foyle River catchment, were assessed for temporal trends.
Results suggest: A downward trend in dissolved oxygen concentration (%DO) at a majority of sites; An indication of a widespread upward trend in nitrate concentration; Evidence that increases in nitrate concentration occur a sites after rainfall following dry periods; No corresponding upward trend in Biochemical Oxygen Demand (BOD); Eutrophication as the main water-quality problem in the Foyle River catchment.
Biological and chemical General Quality Assessment of the Foyle River sites classifies only 7 sites to the same classification band. Ordination of 10 sites was carried out using the seasonal averages of BOD, %DO, and NH4-N for each site and repeated using Spring, Summer and Autumn macroinvertebrate abundance data. Spearman Rank correlation suggests that BOD, %DO, and NH4-N have little influence in structuring the community composition at the sites.
Comparison of species and family-level ordinations appears to support family-level identification in a monitoring programme although species of the same family show different correlations to the determinants. Family-based scoring may not reflect the complexes of species level responses to these variables. Multivariate analysis indicates that some pollution-sensitive species are correlated with greater than average BOD concentration. The removal of such species results in an increase of 3.1% in the % variance in the Spring data explained by the three variables, while Autumn is unchanged. The variance of the species-based data and the family-based ‘explained’ by the three variables in each season is essentially unchanged.
Recalculation of Ecological Quality Indices (EQIs) for the 10 sites after these species are removed indicates that the number of sites classified to the same band is increased.