Abstract
Since the mid-1900s, humans have significantly accelerated the nitrogen (N) cycle by more than doubling N input into natural systems. Common sources of enhanced N include fertilizers, animal waste, septic systems, sanitary sewers, fossil fuel combustion, untreated stormwater, and atmospheric deposition. There is particular concern about increased nitrogen loads in streams and their link to riverine and estuarine eutrophication. Stream restoration and stormwater control measures (SCMs) are common practices to reduce nitrate loads in streams. This research explores the downstream trends in inorganic N concentrations, loads, and retention in seven restored and unrestored stream sections of the Meadow Creek watershed in Charlottesville, Virginia, in addition to adjacent riparian pond and wetland. We further track the dissolved inorganic nitrogen (DIN) sources to these sites through isotopic analysis, and analyze stream metabolism patterns through seasons and sites of different restoration status.
Water samples for nitrate + nitrite, and dual isotope analysis of δ15NO3 and δ18O were collected. Photosynthetically active radiation, dissolved oxygen, conductivity, water temperature, water level, and atmospheric pressure sensors were deployed. These variables were used as inputs in the Bayesian Single-Station estimation (BASE) model to generate gross primary productivity and ecosystem respiration.
Restoration was found to be effective at sequestering nitrogen with an enhanced effect in the summer. Pond and wetland studies also showed a water treatment effect through inlet and outlet nitrate + nitrite concentration and load comparisons. However, enhanced denitrification was not supported by the isotopic analysis of these samples. Sanitary effluent was the dominant nitrate source to our sites at baseflow conditions, yet this signal seems to be diluted at hurricane scale flows. Some stream metabolic activities had seasonal variability with peaks in the summer, which is the reverse pattern to nutrient concentration seasonality. These results have implications on the potential of stream restoration and SCMs to decrease excess nutrient loads into downstream systems, as well as to maintain or recover the various ecosystem services provided by streams.
