Measures of Ecosystem Function Complement Water Quality in a Polluted, Spring-influenced River

Ecosystem functions, which integrate physical, chemical, and biological processes, may be useful indicators of the biological integrity of streams and rivers. The Portneuf River, southeast Idaho, is impacted by human land use and consequently experiences high nutrient and sediment supply to some reaches, and simultaneously experiences a range of hydrologic conditions due to varied spring input. We tested how turbidity, nutrient concentrations, and spring input interacted to control three ecosystem functions (leaf decomposition, nutrient limitation, and open channel metabolism) at three study sites in Aug 2006. In addition, we compared these to changes in ecosystem structure by examining invertebrate communities at the same study sites. Leaf decomposition rates were 50% slower at a site without spring input than at two sites with spring input, despite 3 100 fold differences in nutrient concentrations between the two spring influenced sites. Similarly, invertebrate community composition was most similar at the two spring influenced sites and different from the site without spring influence. Stream biofilms were limited by N at the two sites with lowest nutrient concentrations (one spring influenced and one non spring influenced), and not nutrient limited at the site with the highest nutrient concentration, suggesting that nutrient limitation is sensitive to in stream nutrient concentrations. Finally, estimates of open channel metabolism showed that primary production was highest at the site with lowest turbidity, intermediate at a site with varied turbidity, and lowest at the site with highest turbidity. Respiration, however, was greatest at the site with the highest nutrient concentration, and neither primary production nor respiration responded to differences in spring inputs. Based on our findings, we propose that biofilm nutrient limitation and particularly open channel metabolism might be the most useful indicators of water quality in this river, while invertebrate community structure and leaf decomposition, which respond more closely in-stream conditions created by spring input, might be less useful water quality indicators