Like most ecosystems on the planet, streams and rivers are experiencing a barrage of physical, chemical, and biological stresses from both natural and anthropogenic sources. These stressors often reduce the diversity of stream periphyton, which play important roles in primary production, nutrient cycling, and decomposition. Given limited time and funding, resource managers cannot possibly address all sources of stress to a given environment. So to determine which stressors have the greatest impacts on biological communities, we need 'environmental triage' - that is, quantitative rankings of the influence of different stressors on biological diversity.
We conducted a mesocosm experiment in which we manipulated eight of the most common stressors being imposed on freshwater habitats (increased temperature, species extinctions, species invasions, sedimentation, nitrogen, phosphorus, herbicide, and road salt). We inoculated 13-L recirculating stream mesocosms with water and periphyton from Michigan's Huron River. Periphyton were given 14 days to acclimate, after which, we imposed six increasing levels of each stressor on different streams (48 treatments x 2 streams each = 108 streams). Twelve mesocosms were left unaltered to serve as controls. After the algal communities reached a steady-state biomass (circa 50 days), we collected samples to determine community composition in each mesocosm, and quantified effects of each stressor on algal richness, abundance and evenness using regression.
We found 26 different algal taxa present in our mesocosms, of the 48 that were initially inoculated. Flumes were dominated by the diatoms Navicula, Nitzschia, Synedra, and the filamentous green algae Rhizoclonium, comprising 84% of the cells counted. Of the eight stressors tested, only the herbicide showed a significant (negative) relationship between increasing stress level and taxa richness (p < 0.05). Phosphorus and nitrogen increased taxa richness at all levels when compared to controls, though the slope of stress level versus richness was not significantly different from zero (p > 0.05). Results suggest that algae respond more strongly to chemical stressors than physical or biological stressors. More exhaustive analyses of samples will reveal whether shifts in dominant taxa occurred as stress increased, and ongoing research is addressing whether these changes in community composition translate to changes in ecosystem functions. This research will help facilitate the difficult environmental management decisions we face regarding where to focus our limited time and resources.