Nutrients and nitrophiles: Effects of treated wastewater on dryland riparian plant communities

Background/Question/Methods

During the twentieth century, nutrient inputs to rivers worldwide increased dramatically, leading to changes in community composition and ecosystem function.  For riparian ecosystems of the southwestern United States, declines in surface water flows coupled with the release of treated municipal wastewater have resulted in the emergence of ‘novel ecosystems’ typified by nutrient-rich perennial base flows.  Little is known about the attributes of the riparian plant communities created and sustained by the effluent. Our objectives were to determine how nutrient enrichment (increased N and P in treated wastewater) shapes wetland and riparian plant communities in arid and semi-arid regions, and to examine interactions between site water availability and nutrient enrichment.  To address these objectives, we conducted field  and  greenhouse experiments. In the field, streamside herbaceous vegetation was sampled for two years along the effluent-dominated Santa Cruz River and along a non-effluent control river.  In the greenhouse, soil seed banks collected from ephemeral, perennial, and effluent-dominated rivers were monitored  to address how varying concentrations of nitrogen and phosphorus influenced richness, biomass, density and composition.  

Results/Conclusions

Using a modified Ellenberg index to assess composition we found that effluent-dominated systems had greater abundance of nitrophilic species (i.e. those with higher nitrogen affinity),  in both greenhouse and field settings. Many of the nitrophiles, such as Conium maculatum, are introduced species that are designated by managers as undesirable species.  Biomass and plant height increased while stem density and richness decreased with increasing nutrient concentrations in all experiments.  Site flow permanence also influenced community response, as biomass and density were significantly higher in the seed banks from perennial and effluent-dominated streams.  Overall, this study revealed that water quality and hydrologic setting both influence herbaceous plant community development and population-level processes in waterways receiving treated wastewater. As anthropogenic nitrogen inputs currently equal or exceed natural N inputs in many ecosystems and as larger scale restoration is planned for many riparian landscapes, knowledge of the drivers of community change is important for informing management and restoration actions.