Benthic Macroinvertebrate Community Organisation in Two Contrasting Rivers: Between-Site Differences and Seasonal Patterns

Between-site differences and within-site seasonal patterns were studied in benthic macroinvertebrate communities from two physically and chemically contrasting streams in the River Blackwater catchment (the River Awbeg and the Glenfinnish River). Between-site differences occurred in species composition (related to water hardness, pH and stream order), total macroinvertebrate densities (related to hardness and riparian vegetation), diversity and functional group organisation (partially related to water chemistry, but also to riparian vegetation and stream order and distance from headwaters). Total density, taxon richness, numbers of shredders, filterers and deposit collectors and relative abundance of filterers were greatest in the higher-order, hard-water Awbeg. Diversity and relative abundance of grazers and predators were greatest in the lower-order, softer-water Glenfinnish. Seasonal changes in total invertebrate numbers, diversity, rank/abundance distributions and functional group organisation were also observed. Overall densities were greatest in late summer/early autumn and lowest in early summer in the Glenfinnish River. Seasonality patterns in invertebrate abundance in the Awbeg were dominated by Simuliidae, peaking in late summer. Rank abundance distributions varied from log series to geometric patterns over the year. Taxon richness and abundance of shredders and filterers increased in late summer in both rivers in accord with seasonal patterns predicted under the River Continuum Concept. Other seasonal patterns, e.g. in grazers, did not; this was related to the local habitat conditions (e.g. shading).