Ammonium fluxes from channel deposits in the Neuse River Estuary, North Carolina: Implications for ammonium increase in estuarine waters

Sediment and porewater samples were collected from three nearshore sites and one mid-channel site in the Neuse River Estuary (NRE), North Carolina, USA to investigate advective and diffusive inorganic N fluxes from sediments. Ammonium (NH4+) fluxes were used to determine the significance of submarine groundwater discharge (SGD) in nearshore environments to the overall NH4+ dynamics in the water column. 222Rn and NH4+ were measured in interstitial water at NRE nearshore sites to determine the advective flux of NH4 + from sediments to the overlying water column. Porewater samples were collected over an annual cycle from multi-level piezometers installed in nearshore sites. SGD was measured indirectly by using 222Rn as a tracer and directly by using seepage meters. Maximum SGD occurred during spring at a rate of 13.6 +/- 0.2 cm d-1 and was variable depending upon the sampling location. Shallow porewaters exhibited seasonal variations in NH4+ concentrations, which produced temporal changes in advective flux of NH4+ from the sediment. Seasonal trends in groundwater seepage rates and NH4 + concentrations suggest that groundwater is an important mechanism advecting nutrients from porewaters to the overlying water column, and that groundwater inputs are comparable to riverine NH4+ discharge.;NH4+, nitrate (NO3-), and dissolved oxygen (DO) diffusive fluxes across the sediment-water interface were measured in the shallow nearshore environments with sandy sediments and one mid-channel site with organic rich, fine-grained sediments. NH4 + was the major form of inorganic N in sediment porewaters and in the flux to the overlying water; NO3- fluxes were small or not detected. NH4+ and DO fluxes showed significant seasonal variations at all sites. NH4+ diffusive flux was highly variable and ranged from --29.1 to 811 mumol m-2 hr-1 among the three sites, with the negative flux indicating flux into the sediments from the water column. Sediment experiments were also conducted at a nearshore and mid-channel site to measure the NH 4+ production rates. The nearshore site demonstrated increasing NH4+ production with depth, down to 35 cm (0.004 mumol NH4+ cm-3 wet sediment d-1), whereas the highest NH4+ production at the mid-channel site (0.001 mumol NH4+ cm -3 wet sediment d-1) occurred in the 0-10 cm interval. SGD contributed significantly more NH4+ to the overlying water in the nearshore environment than the diffusive flux from organic rich mid-channel sediments at these sampling locations.;Data compiled from this study and multiple other studies conducted in the NRE were applied to a mass balance model to analyze seasonal variations in NH4+ sources and to investigate the increase of NH4+ concentration in the NRE water column. Many of the sources of NH4+ dynamics in the NRE are driven by temperature and climate, and therefore, demonstrate seasonal variability. Advection terms and other climate-related parameters, including SGD and resuspension events, were found to be of similar importance during the winter and summer periods. The advection term, including SGD, porewater exchange, and resuspension, represented >23% of the NH4+ inputs during the winter season and >21% during the summer. SGD and permeable porewater exchange were the dominant advection terms during both seasons. Sediments played a crucial role in NH4+ supply to the overlying water column. When sandy sediments and muddy sediments are included in the budget and the role of advection is considered, sediments may contribute more than 70% of the NH4+ to the water column. The nearshore environment was relatively consistent between summer and winter when diffusive fluxes and advective fluxes from SGD were considered, 35% for summer and 24% for winter.;This study demonstrated that nearshore sediments of the NRE are sites with significant NH4+ production that can be important to benthic primary production and overall water column concentration in shallow waters. This study quantifies NH4+ production and flux from sediments, which is poorly understood in nearshore environments, and may help to explain an overall increase in water column NH4 + concentrations that has been documented in the NRE and certain other shallow, eutrophic estuaries. The data can provide valuable information important to management practices and future eutrophication mitigation studies.