The influence of stream nutrients and habitat on three biological assemblages

This study determined the relative influence of nutrients and stream habitat on three biological assemblages in five regions of the USA. Nutrient samples were collected over a four-week period, followed by habitat assessment and the collection of biological samples. Biological sampling included diatoms, invertebrates and fish, with three assemblage metrics used for each taxonomic group. We developed boosted regression tree (BRT) models for each biological assemblage metric within each region. Diatom BRT models indicated that nutrients, primarily orthophosphate or total phosphorus, played a larger role than habitat. Nutrients and habitat were approximately equal for invertebrate models with ammonia nitrogen and total organic nitrogen dominant. For fish, habitat had greater importance than nutrients, with total organic nitrogen and total phosphorus the dominant nutrients. Invertebrates had the highest model performance with average CV (cross-validation) R² at 0.47, with diatoms at 0.31 and fish at 0.26. Strongest individual metrics included low phosphorus diatom taxa, tolerant invertebrate abundance and fish multimetric (fish MMI). These findings suggest that the influence of nutrient species varies regionally and by metric, with the relative importance of nutrients and habitat changing as one moves up the trophic level.