BENTHIC MACROINVERTEBRATE COMMUNITY ORGANIZATION IN 2 CONTRASTING RIVERS - BETWEEN-SITE DIFFERENCES AND SEASONAL PATTERNS

Between-site differences and within-site seasonal patterns were studie d in benthic macroinvertebrate communities from two physically and che mically contrasting streams in the River Blackwater catchment (the Riv er Awbeg and the Glenfinnish River). Between-site differences occurred in species composition (related to water hardness, pH and stream orde r), total macroinvertebrate densities (related to hardness and riparia n 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, n umbers of shredders, filterers and deposit collectors and relative abu ndance of filterers were greatest in the higher-order, hard-water Awbe g. Diversity and relative abundance of grazers and predators were grea test in the lower-order, softer-water Glenfinnish. Seasonal changes in total invertebrate numbers, diversity, rank/abundance distributions a nd 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 abundan ce in the Awbeg were dominated by Simuliidae. peaking in late summer. Rank abundance distributions varied from log series to geometric patte rns over the year. Taxon richness and abundance of shredders and filte rers increased in late summer in both rivers in accord with seasonal p atterns predicted under the River Continuum Concept. Other seasonal pa tterns, e.g. in grazers, did not; this was related to the local habita t conditions (e.g. shading).