MS Thesis Defense: Kathleen Onorevole

Restoration of estuarine habitats supports changes in N cycling and removal over time

Habitat restoration is a valuable way to curtail environmental degradation.  In estuarine systems in North Carolina, oyster reefs and salt marshes are frequently restored together as living shorelines to limit erosion and restore other ecosystem functions.  A common goal of estuarine restoration is to reduce available nutrients, which can be accomplished by augmenting denitrification (DNF).  DNF converts N from a bioavailable form (NO₃^–) to a non-bioavailable form (N₂ gas).  NC salt marshes and oyster reefs have been previously shown to increase rates of DNF.  Since DNF is a valuable ecosystem service, it is useful to identify whether it develops in a predictable way following restoration.  This study used a chronosequence design spanning 0-20 years to study N cycling in restored oyster reefs and salt marshes near Morehead City, NC.  Sediment cores were collected from each habitat and from adjacent sandflats.  Cores were incubated and analyzed with a MIMS to assess DNF rates.  Supporting data were also collected, including sediment characteristics, ambient nutrient concentrations, and physical site properties.  Results indicate that the relationship between DNF and restored site age varied across seasons, prohibiting identification of a single restoration trajectory.  DNF rates were not significantly different between habitats, suggesting that the restored system was functioning as a unit and impacting adjacent areas.  DNF was significantly correlated with sediment oxygen demand (SOD) annually and with sediment organic matter (SOM) in the summer.  Measurement of SOD may be a viable proxy for DNF, which is too expensive and time-consuming for most restoration practitioners to measure directly.  During the summer, when overall biological activity is high, DNF may be SOM-limited.  Analysis of DNF efficiency and N₂O gas production indicated that DNF was not producing high fluxes of inorganic N or N₂O.  Overall, results indicate that a single restoration trajectory for DNF cannot be proposed.  However, restoration of oyster reefs and salt marshes does augment N removal over time, and the benefits of using a living shoreline design may increase ecosystem functions across habitat gradients.

Presented at UNC IMS seminar room 222, streamed to Venable/Murray G3204

Kathleen Onorevole (M.S. student in the Piehler Lab)