Dry Wetlands: Nutrient Dynamics in Ephemeral Constructed Stormwater Wetlands
Constructed stormwater wetlands (CSWs) are used to address contaminants in urban stormwater such as nitrogen (N) and phosphorus (P), but their performance is variable. Ephemeral CSWs tend to be less effective than perennial CSWs at removing N and P. We asked: How does wetland vegetation and sediment affect nutrient cycling/release from sediment and vegetation in ephemeral CSWs? We focused on two ephemeral urban CSWs in Pocatello, ID, USA, one densely vegetated and the other nearly bare. We rewetted intact cores of dry wetland sediments and, separately, senesced vegetation for 1 week at the end of the summer dry period to assess whether wetland sediments and vegetation acted as sources or sinks of N and P. For both CSWs, there was a pulse of nutrients immediately following rewetting, but the magnitude of that pulse and subsequent changes in nutrient concentrations suggest different processes dominate at each wetland, driven by differences in wetland vegetation and associated sediment characteristics. There was evidence of denitrification between and during events at the vegetated wetland, but larger fluxes of P at this site suggests a tradeoff between denitrification and P release. While the experimental results suggested specific biogeochemical controls, CSW nutrient concentrations during three events were more dynamic and suggested more biogeochemical complexity than that represented in our experiment, both within events and seasonally. Ephemeral CSWs may create unique biogeochemical conditions and require careful design to ensure N and P retention. Managers will also need to consider whether perennial water sources would improve CSW function.
KeywordsConstructed stormwater wetland Nitrogen Phosphorus Stormwater Ephemeral wetland Urban
This work was supported by NSF EPSCoR grant IIA 1301792 as part of the Idaho EPSCoR Program. We thank the City of Pocatello for allowing the sampling and instrumentation of the studied sites, and particularly Hannah Sanger, whose interest sparked this project and who has been helpful and supportive from the project’s infancy. Dr. Keith Reinhardt provided both equipment and technical advice critical to the vegetation components to this project. Sarah Stalder, Sophie Hill, Alyssa Millard, Zach Fishburn, Kyndra Hawkes, and James Guthrie all provided valuable field and lab assistance.
Compliance with Ethical Standards
Conflict of Interest
The authors declare that they have no conflict of interest.
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