Living shorelines enhance nitrogen removal capacity over time

Abstract

Living shorelines are nature-based solutions to coastal erosion that can be constructed as salt marshes with fringing oyster reefs. Each of these habitats can decrease the potential for eutrophication through increased nitrogen (N) removal via denitrification. However, the development of N cycling over time has not been studied in living shorelines. This research measured denitrification rates in a chronosequence of living shorelines spanning 0–20 years in age in Bogue Sound, NC. Analyses were conducted seasonally from summer 2014 to spring 2015 along an elevation transect through the salt marsh, oyster reef, and adjacent sandflat at all sites. Gas fluxes (N2 and O2) from sediment core incubations were measured with a membrane inlet mass spectrometer (MIMS) to assess denitrification and sediment oxygen demand. Fluxes of dissolved nutrients and the greenhouse gas N2O were measured. Sediment properties, inundation frequency, oyster filtration rates, and marsh grass stem density were also quantified. There was no significant difference in denitrification rates among habitats. N removal consistently increased from the 0- to 7-year-old sites. Denitrification efficiency was always greater than 50% and positive N2O fluxes were negligible. Our results suggest that living shorelines increase net N removal within a relatively short time period following construction, without introducing deleterious greenhouse gas emissions. This demonstrates that living shorelines can play an important role in estuarine N cycling and management.