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

Neuse Estuary;submarine groundwater discharge;ammonium fluxes;nitrogen

Null, Kimberly Ann ; Dave DeMaster, Committee Chair,JoAnn Burkholder, Committee Co-Chair,Carrie Thomas, Committee Member,Robert Reed, Committee Member,Reide Corbett, Committee Member,Null, Kimberly Ann ; Dave DeMaster ; Committee Chair ; JoAnn Burkholder ; Committee Co-Chair ; Carrie Thomas ; Committee Member ; Robert Reed ; Committee Member ; Reide Corbett ; Committee Member

University:North Carolina State University

关键词: Neuse Estuary;    submarine groundwater discharge;    ammonium fluxes;    nitrogen;   

Others  :  https://repository.lib.ncsu.edu/bitstream/handle/1840.16/6164/etd.pdf?sequence=1&isAllowed=y

美国|英语

来源: null

【 摘 要 】

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.