Excess nutrient loading to streams and rivers are major environmental and human health concerns in many regions. Methods to assess effects of nutrient loading to these systems are needed to prioritize monitoring and management. Although measuring instantaneous nutrient concentrations is straightforward, resulting data often do not reflect chronic effects of excess nutrients. Measures of biological community composition provide an integrative way of examining effects of nutrients by capturing long term ecosystem effects.
As part of the US Environmental Protection Agency’s Nutrient Scientific Technical Exchange Partnership and Support (N-STEPS) program, we evaluated how well the composition of attached algal communities (periphyton) in Pacific Northwest streams and rivers correspond to nutrient concentrations, including total nitrogen (TN) and total phosphorus (TP). We hypothesized that across the landscape, changes in periphyton communities would correspond to variation in TN, TP, or both. We compared metrics describing periphyton communities, including percent diatoms and abundance of nutrient-sensitive species, with paired nutrient concentration data from 450 streams and rivers across eight Level-III Ecoregions in the Pacific Northwest. We isolated correlations between periphyton and nutrients from correlations with other water quality parameters, abundance of macroinvertebrate grazers, and watershed land use/land cover (percent non forest) using structural equation modeling (SEM).
SEMs successfully allowed us to isolate specific correlations between periphyton community metrics and nutrient concentrations. Nearly all periphyton community metrics correlated strongly with TP concentration in full SEMs that included all water quality and watershed parameters. Fewer showed strong correlation with TN concentration. Watershed land use/land cover correlated with periphyton metrics indirectly through correlations with nutrient concentrations (TN and TP), substrate size, and riparian canopy cover. The abundance of grazing macroinvertebrates did not correlate with periphyton metrics after accounting for nutrient concentrations and other parameters. This suggests that bottom-up processes related to growth limitation had a larger effect on periphyton community structure among streams than top-down processes that can select for specific species through herbivory. Overall, our results demonstrate that periphyton community data can be used to indicate the degree of nutrient loading in Pacific Northwest streams and rivers. This should allow organizations to focus monitoring and other actions to address nutrient-related water quality impairments in the region.