Consistency and sensitivity of stream periphyton community structural and functional responses to nutrient enrichment

Eutrophication remains one of the foremost impacts of industrialization and population expansion on aquatic ecosystems worldwide. Selecting metrics for assessing the manner in which communities and biogeochemical processes respond to nutrient fertilization is an ongoing management challenge critical both for detecting changes and for monitoring recovery of impaired environments. A key limitation to the selection of response variables is the lack of consistent evaluation of metrics under the same conditions in multiple systems across ecoregions. Here we report the results of nutrient-diffusing agar fertilization experiments conducted simultaneously in 30 streams in two distinct ecoregions of California, USA: the mountainous Sierra Nevada and the coastal chaparral of Santa Barbara county. In each experiment we evaluated algal community shifts across five nutrient delivery rates using multiple response variables at the ecosystem process (respiration and primary production), community (biomass and diversity metrics), functional group (nitrogen fixing, growth form, nutrient adaptation), and taxonomic group (indicator species, genera, families) levels of ecological organization. We used mixed-effect general linear models to quantify the magnitude, sensitivity, and consistency of responses among streams within and across ecoregions to provide an objective assessment of the potential for each variable to describe and detect significant changes in algal community characteristics. Our results indicate that ecosystem- and community-level variables showed significant and consistent nutrient responses among diverse streams and across the two ecoregions, while indicator taxa and functional groups were less likely to respond consistently to nutrient enrichment. We discuss the relevance of our findings to the ongoing development of monitoring and bioassessment strategies for aquatic eutrophication.