Efforts are underway globally to improve water quality and other ecosystem services in watersheds impacted by urbanization. Excess nutrients (i.e., nitrogen and phosphorus) create eutrophic conditions that threaten water supply for human consumption as well as ecological health. It has long been recognized that the interfaces between terrestrial and aquatic ecosystems are locations where nutrient processing and removal is maximized. However, in urban landscapes these dynamic connections are often severed resulting in a shunting of water and materials to urban stream networks. A range of urban stormwater practices are used to reestablish these connections, including restoration of floodplain wetlands and creation of stormwater wetlands throughout the watershed. The assumption is that the structural changes to the watershed that mimic natural systems will allow the development of functional equivalencies as well. To test this, we studied six restored stream-floodplain systems and 4 stream-wetland/pond confluences in Charlotte, NC. We measured monthly nutrient loading, net nutrient mineralization via 30-day in situ core incubations and denitrification via laboratory bottle assays. Measurements were made in floodplains and stream sediments to quantify microbial process and the environmental factors that control them.
Results/Conclusions
Our research shows that restoring stream-floodplain connectivity leads to greater trapping of sediments and nutrients in the floodplain and creates dynamic biogeochemical feedbacks that impact water quality. Denitrification was closely correlated with sedimentation, while in situ N and P mineralization rates were positively correlated with restoration age. These results underscore the importance of designing for connectivity, but also allowing time for vegetation and soil organic matter to develop. While stormwater ponds and wetlands can be effective at storing water and retaining nutrients within the watershed, we show that they also alter outflow of nutrients and carbon to the stream during storms. In watersheds with low imperviousness, denitrification increased downstream of the confluence of the wetland outflow and the stream. We hypothesize that elevated outflow concentrations of carbon and nitrogen promoted microbially driven processes within the stream. However, at higher levels of imperviousness this pattern was reversed, suggesting a limited capacity for these practices to match the scale of the impacts of urbanization. Our results fill an important gap in identifying the potential for cascading positive effects on nutrient biogeochemistry when stormwater management practices are combined in urban landscapes.