Longitudinal and temporal variation in the hydrochemistry of streams in an Irish conifer afforested catchment

A study of the spatio-temporal variation in hydrochemistry in the afforested catchment of the River Douglas, in the Araglin Valley, Co. Cork, Ireland, was undertaken over a two year period. The aim of the study was to examine the influence of afforestation on stream water quality both spatially and temporally. The catchment, one of the most westerly in Europe, with low atmospheric pollution, allowed the analysis of the interactions between conifer afforestation per se on stream chemistry. In contrast to most other studies, there was a general trend of increasing pH and related variables with distance from headwater despite increasing levels of catchment afforestation. In one tributary, pH and related variables increased rapidly as the stream entered the forest, with pH rising by 1.67 units over a distance of 1.2 km. Temporal fluctuations in most physico-chemical variables were minor and no acid pulses were noted during spate. Thus, the current level of afforestation within the River Douglas catchment does not appear to have negatively affected stream chemistry. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stream macroinvertebrate communities in a conifer‐afforested catchment in Ireland: relationships to physico‐chemical and biotic factors

1. The influence of land use and physico-chemical factors on stream macroinvertebrates was analysed at fifteen sites over a 2-year period in a single conifer-afforested catchment in Ireland, in an area subject to very low levels of atmospheric pollution. 2. Macroinvertebrate assemblages were classified using two-way indicator species analysis into five major groupings that were related to distance from headwaters and land use. Trends in macroinvertebrate community composition were related to changes in physico-chemical and biotic characteristics of the river and its tributaries using canonical correspondence analysis. 3. Local ecological factors (e.g. acid water, moss, shading or agricultural runoff), longitudinal trends in stream physico-chemistry (related to distance from headwaters, geology and land use) and season (related to life history patterns of the invertebrates) were the explanatory variables of spatio-temporal patterns in macroinvertebrate community composition in the catchment. 4. Spatial variation in macroinvertebrate density, taxon richness, diversity and evenness was investigated in relation to environmental characteristics of the study sites using Spearman’s rank correlation, principal components analysis and stepwise multiple regression. Invertebrate density and richness increased with distance from the headwater and associated increases in pH, water hardness and nutrients. Macroinvertebrate density and richness also increased with increasing moss weight. Invertebrate diversity and evenness increased with shading of the channel. 5. The increase in macroinvertebrate density and richness and changes in community composition were particularly marked over a relatively short (1.2 km) distance in one tributary, and were concurrent with a rapid increase in stream pH of 1.7 units. 6. Although macroinvertebrate communities at conifer-afforested sites were not impoverished in the same way as those in some other parts of Europe, they differed from the communities found above and below the plantation. This appeared to be owing to the primary importance of local ecological factors and the effect that the longitudinal position of these forest sites within the river system had on their physico-chemical and biotic nature.