Ammonium Fluxes from Channel Deposits in the Neuse River Estuary, North Carolina: Implications for Ammonium Increase in Estuarine Waters

Abstract

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 directly by using 222Rn as a tracer and by using seepage meters. Maximum SGD occurred during spring at a rate of 13.6 cm d-1 and was variable depending upon the sampling location. Porewaters exhibited seasonal variations in NH4+ concentrations down to a depth of 210 cm, which produced temporal changes in advective flux of NH4+ from the sediment. High concentrations and production of seasonal trends in groundwater seepage rates and NH4+ concentration suggest that groundwater is an important mechanism advecting nutrients from porewaters to surface waters, which is comparable to riverine NH4+ discharge in magnitude. 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 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 mmol 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 NH4+ production rates. The nearshore site demonstrated increasing NH4+ production with depth, down to 35 cm (0.004 mmol NH4+ cm-3 d-1), whereas highest NH4+ production at the mid-channel site (0.001 mmol NH4+ cm-3 d-1) occurred in the 0-10 cm interval. Submarine groundwater discharge contributes significantly more NH4+ to the overlying water in the nearshore environment than 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 variation in NH4+ sources and 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 submarine groundwater discharge (SGD) and resuspension events, are of greater importance during the winter season. The advection term represents >14% of the NH4+ inputs during the winter season and >4% during the summer. SGD is the dominant advection term during both seasons. The nearshore environment was relatively consistent between summer and winter with regard to diffusive and advective fluxes from SGD, ~22% and ~21% respectively. 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.