The relationship of macroinvertebrate assemblages to water chemistry in a polluted Colorado plains stream

Macroinvertebrate assemblages and 27 water chemistry variables were investi gated at 8 sampling sites over a 4-year period (1992-1995) in the plains se ction of the Cache la Poudre River, north central Colorado, influenced by u rbanization and agricultural activities. Relationships between macroinverte brate assemblages and water chemistry variables were examined along a longi tudinal chemical gradient using ordination techniques (principal component analysis (PCA), redundancy analysis (RDA), and detrended canonical correspo ndence analysis (DCCA)). PCA ordination diagrams provide a reasonable two-d imensional representation of both the macroinvertebrate and environmental d ata. Concentrations of cations (e.g. Ca, Mg, Na, and K ions) exhibited the strongest relationship with macroinvertebrate assemblages along the river s ection. The inorganic nutrients (e.g. ammonia, total Kjeldahl nitrogen, and total phosphorus), suspended solids, and turbidity were also associated wi th macroinvertebrate assemblage structure. Temporal trends of macroinverteb rate assemblages, investigated using DCCA, showed that interannual and seas onal variations of macroinvertebrate assemblages were related to increases in concentrations of heavy metals and to decreases in concentrations of ino rganic nutrients, suspended solids, and turbidity. Interannual variations w ere less important than seasonal changes in explaining temporal patterns of macroinvertebrate assemblages. The highest species turnover and the highes t interannual and lowest seasonal variation of macroinvertebrate assemblage s were found at the reference site.