STREAM MACROINVERTEBRATE COMMUNITIES IN A CONIFER-AFFORESTED CATCHMENT IN IRELAND - RELATIONSHIPS TO PHYSICOCHEMICAL AND BIOTIC FACTORS

1. The influence of land use and physico-chemical factors on stream ma croinvertebrates 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 asse mblages 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 rela ted to changes in physico-chemical and biotic characteristics of the r iver and its tributaries using canonical correspondence analysis. 3. L ocal ecological factors (e.g. acid water, moss, shading or agricultura l runoff), longitudinal trends in stream physico-chemistry (related to distance from headwaters, geology and land use) and season (related t o life history patterns of the invertebrates) were the explanatory var iables of spatio-temporal patterns in macroinvertebrate community comp osition in the catchment. 4. Spatial variation in macroinvertebrate de nsity, taxon richness, diversity and evenness was investigated in rela tion to environmental characteristics of the study sites using Spearma n's rank correlation, principal components analysis and stepwise multi ple regression. Invertebrate density and richness increased with dista nce from the headwater and associated increases in pH, water hardness and nutrients. Macroinvertebrate density and richness also increased w ith increasing moss weight. Invertebrate diversity and evenness increa sed with shading of the channel. 5. The increase in macroinvertebrate density and richness and changes in community composition were particu larly marked over a relatively short (1.2 km) distance in one tributar y, and were concurrent with a rapid increase in stream pH of 1.7 units . 6. Although macroinvertebrate communities at conifer-afforested site s were not impoverished in the same way as those in some other parts o f Europe, they differed from the communities found above and below the plantation. This appeared to be owing to the primary importance of lo cal ecological factors and the effect that the longitudinal position o f these forest sites within the river system had on their physicochemi cal and biotic nature.