NITROGEN ADVECTION AND DENITRIFICATION LOSS IN SOUTHEASTERN NORTH CAROLINA SALT MARSHES. Matthew D. Lettrich

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1 NITROGEN ADVECTION AND DENITRIFICATION LOSS IN SOUTHEASTERN NORTH CAROLINA SALT MARSHES Matthew D. Lettrich A Thesis Submitted to the University of North Carolina Wilmington in Partial Fulfillment of the Requirements for the Degree of Master of Science Center for Marine Science University of North Carolina Wilmington 211 Approved by Advisory Committee Craig R. Tobias Lawrence B. Cahoon Eric J. Henry Chair Accepted by Dean, Graduate School

2 TABLE OF CONTENTS ABSTRACT...v ACKNOWLEDGEMENTS... vi LIST OF TABLES... vii LIST OF FIGURES... viii OVERVIEW...1 CHAPTER 1. Advective Marsh-Estuary Exchange - Hydrologic and Chemical Fluxes...5 INTRODUCTION...6 METHODS...9 Site Description...1 Site Development...1 Hydraulic Parameters...12 Porewater Transport...17 Chemical sampling...17 Marsh Scale Drainage - Whole Creek Tidal Flood vs. Ebb Flux Studies...18 RESULTS...19 Tidal and Watershed Boundary Conditions...19 Porewater Drainage Drivers...27 Creekbank Drainage and Tides...3 Creekbank Drainage and Upland Water Table...3 Interior Drainage and Tides...34 Interior Drainage and Upland Water Table...44 Porewater Drainage Flux Summary...44 ii

3 Porewater Nitrogen Flux from Marsh to Open Water...47 Comparison of Darcy N Flux with Tidal N Exchange Estimates...49 DISCUSSION...55 Water and N Movement...55 Water Movement-Creekbank...55 Water Movement-Interior...56 N Export from Marsh to Open Water...58 Controls on the Advective Fluxes...59 Predicting Drainage...61 Marsh Scale N fluxes - Darcy vs Tidal Approaches...63 Spatial and Temporal Variance - Darcy...63 Spatial and Temporal Variance Tidal Flux...64 Incomplete Capture of All Routes of N Exchange - Darcy...65 Incomplete Capture of All Routes of N Exchange Tidal Flux...66 Scaling - Darcy...66 Scaling Tidal Flux...67 CHAPTER 2. Marsh-Estuary Exchange Attenuation of DIN Fluxes by Denitrification...68 Introduction...69 Methods...71 Results...75 Direct Denitrification...75 Coupled Denitrification 15 NO 3 Addition...76 Coupled Denitrification 15 NH 4 Addition...8 iii

4 Direct Denitrification Capacity...82 Discussion...85 Seasonal and Spatial Differences...85 Comparisons to other systems...86 Direct vs. Coupled Denitrification...88 Denitrification Capacity and Anammox...89 SUMMARY AND SYNTHESIS...91 Scaling Magnitudes of Drainage and Denitrification...92 Importance of Marsh Drainage and Denitrification Relative to Other Sources and Sinks Implications for Marsh N export in a Changing Climate...94 LITERATURE CITED...95 iv

5 ABSTRACT Coastal wetlands serve as sources and sinks of nitrogen to surrounding estuarine waters through advective drainage and denitrification. The advective nitrogen flux of three intertidal estuary wetlands in the New River Estuary in North Carolina was determined using Darcy-derived drainage measurements and calculating the difference between tidal ebb and tidal flood flux. The magnitude of drainage was greatest and most closely linked to tidal elevation in the most down-estuary site and was least in the up-estuary site ranging from a daily mean drainage of.34 L m shoreline -1 day -1 in the up-estuary site to 87 L m shoreline -1 day -1 in the down-estuary site. Nitrogen concentrations in the marsh porewaters peaked in late 29. N flux was determined as a function of drainage (water flux) and porewater N concentration. Advective N flux showed a seasonal pattern that increased in the summer and the winter. Drainage was found to be correlated to tidal elevation within each site and trended with tidal amplitude within the estuary, providing proxies for estimating advective N flux at other sites when given those easily measured parameters combined with porewater N concentration. Marsh denitrification was found to be generally greater in the creekbank than in the interior and greater in May than in February. Coupled denitrification dominated direct denitrification as ambient nitrate concentrations remained low. Denitrification capacity showed that with a nearly 1-fold increase in nitrate concentration, NRE marshes could maintain the rate of denitrification relative to ambient nitrate concentration. v

6 ACKNOWLEDGEMENTS This research was conducted under the Defense Coastal/Estuarine Research Program (DCERP), funded by the Strategic Environmental Research and Development Program (SERDP). vi

7 LIST OF TABLES Table Page 1. Hydraulic conductivity (Kh) of Traps Bay, French Creek, and Freeman Creek Quarterly porewater concentrations of total dissolved nitrogen (TDN) based on location within each marsh Denitrification rates reported of other coastal systems...87 vii

8 LIST OF FIGURES Figure Page 1. Site map showing Marine Corps Base Camp Lejeune (a), French Creek (b), Traps Bay (c), and Freeman Creek (d). Images are not to the same scale Aerial photo of French Creek study site. White points represent piezometer clusters Aerial photo of Traps Bay study site. White dots represent piezometer clusters Aerial photo of Freeman Creek study site. White dots represent piezometer clusters Conceptual model of salt marsh nitrogen linkages. Tidal recharge supplies particulate nitrogen (PN), dissolved inorganic nitrogen (DIN), and dissolved organic nitrogen (DON). Groundwater (GW) inputs supply DIN. Burial removes PN. Denitrification results in gaseous loss of N 2. Porewaters drain to the adjacent estuary resulting in export of DIN, DON, and PN from the salt marsh system Daily maximum observed high tide (black) and low tide (grey) for French Creek (a), Traps Bay (b), and Freeman Creek (c). Creekbank marsh surface elevation is indicated by the dashed line Upland water table elevations for French Creek (a), Traps Bay (b), and Freeman Creek (c). Freeman Creek and Traps Bay showed responses to precipitation events while French Creek predominantly responded to tidal elevation Short duration fluctuations in upland water table elevation (grey), tidal elevation (black), and precipitation (bar) for French Creek (a), Traps Bay (b), and Freeman Creek (c) Representative daily fluctuation of tidal elevation (grey), creekbank gradient (triangle), and interior gradient (circle) for French Creek (a), Traps Bay(b), and Freeman Creek (c). D h represents the difference in hydraulic head between either the tide gauge and creekbank piezometer (creekbank gradient), or the upland-marsh border piezometer and the creekbank piezometer (interior gradient). D x represents the distance between the points where d h is determined. A positive gradient indicates a slope from the marsh towards open water Creekbank porewater elevation (triangles) and tidal elevation (black line) through a daily tidal series for French Creek (a), Traps Bay (b), and Freeman Creek (c). Once the marsh is flooded, tidal elevation is greater than porewater elevation, creating a negative gradient. Shaded grey areas represent times of vertical recharge Vertical gradient for the creekbank (triangles) and marsh interior (circles) for French Creek (a), Traps Bay (b), and Freeman Creek (c) during flooded periods of a representative tidal cycle. Tidal elevation is indicated by the black line. Negative gradients indicate vertical recharge of the marsh viii

9 12. Creekbank drainage versus tidal elevation for French Creek transect 3 (a), transect 5 (b), and transect 7 (c). A two-phase response was seen with increased drainage at elevations below the marsh surface (black ) and recharge for tidal elevations greater than the marsh surface (grey). Grey points greater than zero occur as the marsh is draining on the falling tide and grey values less than zero occur as the marsh is flooding on the rising tide Creekbank drainage versus tidal elevation for Traps Bay transect 1 (most upstream) (a), transect 3 (b), transect 5 (c), and transect 7 (most downstream) (d). A two-phase response was seen with increased drainage at elevations below the marsh surface (black ) and recharge for tidal elevations greater than the marsh surface (grey). Grey points greater than zero occur as the marsh is draining on the falling tide and grey values less than zero occur as the marsh is flooding on the rising tide Creekbank drainage versus tidal elevation for Freeman Creek transect 1 (a), transect 3 (b), and transect 5 (c). A two-phase response was seen with increased drainage at elevations below the marsh surface (black ) and recharge for tidal elevations greater than the marsh surface (grey). Grey points greater than zero occur as the marsh is draining on the falling tide and grey values less than zero occur as the marsh is flooding on the rising tide Daily maximum creekbank drainage response to upland water table slope for French Creek transect 3 (a), transect 5 (b), and transect 7 (c) Daily maximum creekbank drainage response to upland water table slope for Traps Bay transect 1 (a), transect 3 (b), transect 5 (c), and transect 7 (d) Daily maximum creekbank drainage response to upland water table slope for Freeman Creek transect 1 (a), transect 3 (b), and transect 5(c) Interior marsh drainage response to tidal elevation for French Creek transect 3 (a), transect 5(b), and transect 7 (c). A two-phase response was seen for tidal elevations below interior marsh surface (black ) and above interior marsh surface (grey) Interior marsh drainage response to tidal elevation for Traps Bay transect 1 (a), transect 3 (b), transect 5 (c), and transect 7 (d). A three-phase response was seen for tidal elevations below creekbank marsh surface (black ), between creekbank marsh surface and interior marsh surface (light grey), and above interior marsh surface (dark grey) Interior marsh drainage response to tidal elevation for Freeman Creek transect 1 (a), transect 3 (b), and transect 5 (c). A three-phase response was seen for tidal elevations below creekbank marsh surface (black ), between creekbank marsh surface and interior marsh surface (light grey), and above interior marsh surface (dark grey) Daily maximum interior drainage response to upland water table slope for French Creek transect 3 (a), transect 5 (b), and transect 7(c) ix

10 22. Daily maximum interior drainage response to upland water table slope for Traps Bay transect 1 (a), transect 3 (b), transect 5 (c). and transect 7(d) Daily maximum interior drainage response to upland water table slope for Freeman Creek transect 1 (a), transect 3 (b), and transect 5(c) All sites daily maximum drainage versus daily tidal amplitude. French Creek (open circles), Traps Bay (black circles), and Freeman Creek (triangles) Mean monthly drainage corrected for cross sectional drainage area for French Creek (solid triangles), Traps Bay (open boxes), and Freeman Creek (solid diamonds) Monthly average nitrogen drainage for French Creek, Traps Bay, and Freeman Creek. Freeman Creek drains two orders of magnitude more N per day than Traps Bay and French Creek Monthly average Darcy-derived nitrogen flux for French Creek, Traps Bay, and Freeman Creek. Freeman Creek drains two orders of magnitude more N per day than Traps Bay and French Creek. Rates were scale up to the whole marsh using shoreline length estimates of 13m for French Creek, 9m for Traps Bay, and 96m for Freeman Creek Traps Bay discharge L s -1 (gray area) (a,c), TDN concentration (black line) (a,c), and tidal signature (b,d). Discharge out of the marsh is negative, discharge into the marsh is positive. Tidal flux studies from September 1, 29 (a,b) and May 3, 21 (c,d) are shown Freeman Creek discharge L s-1 (gray area) (a,c), TDN concentration (black line) (a,c), and tidal signature (b,d). Discharge out of the marsh is negative, discharge into the marsh is positive. Tidal flux studies from September 11, 29 (a,b) and May 4, 21 (c,d) are shown Mean direct denitrification rates from 4 hour incubations for French Creek (a, b), Traps Bay (c,d), and Freeman Creek (e, f). Black represents creekbank, grey represents interior Comparison of direct denitrification rates of May versus February (a) and marsh interior versus creekbank (b). Triangles represent French Creek, boxes represent Traps Bay, diamonds represent Freeman Creek. For May versus February (a), solid symbols represent creekbank, open symbols represent interior. For interior versus creekbank (b), solid symbols represent February, open symbols represent May Mean coupled denitrification rates from 4 hour incubations for French Creek (a-d), Traps Bay (e-h), and Freeman Creek (i-l). Black represents creekbank sediment incubations, grey represents interior sediment incubations Comparison of coupled denitrification rates of marsh May versus February (a,b) and interior versus creekbank (c,d). Triangles represent French Creek, boxes represent x

11 Traps Bay, diamonds represent Freeman Creek. For May versus February (a,b), solid symbols represent creekbank, open symbols represent interior. For interior versus creekbank (c,d), solid symbols represent February, open symbols represent May Direct denitrification versus coupled denitrification for all samplings. Triangles represent French Creek, boxes represent Traps Bay, diamonds represent Freeman Creek. Closed symbols represent February samples, open symbol represent May samples Denitrification and ANAMMOX capacity for French Creek creekbank (a) and interior (b), Traps Bay creekbank (c) and interior (d), and Freeman Creek creekbank (e) and interior (f). Solid symbols denote denitrification, open symbols denote ANAMMOX.undetectable at ambient concentrations and ranged from -<2% of the denitrification at ambient concentrations and at higher NO 3 treatments, respectively xi

12 OVERVIEW Rates of nutrient loading and turnover impact water quality, fisheries, and recreational uses of coastal waters (Valiela et al. 199, Cloern 21). Dissolved inorganic nitrogen (DIN) is the principal nutrient limiting estuarine primary production, but in excess leads to eutrophication, hypoxia, algal blooms, and altered ecosystem structure (Howarth et al. 2). The availability of DIN is controlled by external rates of loading from the watershed and rates of DIN recycling/removal within and between ecosystem components within the estuarine landscape. Specific habitats that can modify both incoming loads of DIN from the watershed and also affect turnover of DIN once it enters the estuary play a central role in regulating overall DIN availability on an estuarine scale. Intertidal marshes represent such a habitat. They are situated between the nitrogen rich upland and the nitrogen-limited estuary; well-positioned to impact the DIN budget through marsh-upland interactions and through marsh-estuary interactions. Much effort has been exerted over the past 35 years to generally characterize the source and sink nature of intertidal marshes with respect to nitrogen (Valiela and Teal 1979, Correll 1981, Childers et al. 1994). Past studies have relied on whole-creek tidal exchange or flume approaches, which are limited to certain marsh geomorphologies (Spurrier and Kjerfve 1988, Dame 1994, Childers et al. 2). No clear unifying consensus has emerged over whether marshes uniformly behave in all locations at all times as a source or sink. There is some evidence that marsh import or export of N depends on N speciation, tidal range, and marsh age. There remains considerable inter- and intra-marsh spatial variability in the direction and magnitudes of marsh N fluxes. It is possible that alternate approaches that operate on scales smaller than whole tidal creeks could prove valuable for addressing the source or sink dynamics

13 on the scales at which they vary and extend the work to other marsh geomorphologies not amenable to whole-system approaches. Groundwater, tidal exchange, burial, and denitrification represent significant pathways through which marshes may regulate N speciation and availability in adjacent estuarine waters (Tobias and Neubauer 29). Specifically for DIN, groundwater discharge and tidal delivery during flooding serve as principal sources of N to the marsh, while denitrification and porewater drainage serve as the primary modes of N export from the marsh (Howes et al. 1996). DIN exchanges regulated by groundwater, tidal infiltration, and drainage are integrally linked to fluxes of water (i.e. advection). Groundwater discharge and tidal infiltration both contribute to the total volume of porewater available for drainage. Quantifying the total water flux of porewater through the rhizoshpere is the requisite first step for estimating the chemical fluxes of DIN and dissolved organic nitrogen (DON). The drainage flux of porewater integrates contributions from groundwater, tidal infiltration, and precipitation, with water loss from macrophyte evapotransipiration. During drainage through the rhizosphere, porewater may acquire additional DIN/DON owed to high rates of organic matter mineralization, which leads to the typically N-rich character of marsh porewaters. Coupling the porewater drainage water flux with measurements of the N chemical composition of the porewater permits calculation of the DIN/DON chemical flux from the marsh to adjacent open water. The magnitude of both the drainage water and N fluxes varies temporally from hours to months as the magnitude of the components of the water budget changes in response to tidal cycles, precipitation patterns, and temperature, and mineralization rates change with growing season. Second only to accretion of marsh sediment, denitrification represents one of the largest marsh sinks for total nitrogen and is typically on par with rates of plant uptake for DIN removal. 2

14 Unlike plant uptake or accretion that stores N in the marsh on varying timescales, denitrification converts N into N 2 gas and facilitates its complete removal from the coastal landscape to the atmosphere. Unlike the drainage N flux, denitrification is uncoupled from advective water transport. While denitrification can be affected by patterns of tidal infiltration and drainage, rates of denitrification can be calculated independently of water movement. Marsh denitrification operates either directly on nitrate supplied by flooding water, precipitation, and groundwater (Seitzinger et al. 26) or indirectly on nitrate produced from the nitrification of ammonium produced during mineralization of organic matter in marsh sediments ( coupled denitrification ; Hammersley and Howes 25, Seitzinger et al. 26). It therefore has the potential to attenuate N availability by acting on allochthonous N loads and by decreasing the amount of N internally recycled within the marsh that would otherwise be available for drainage. The net effect of the groundwater nitrogen inputs, tidal infiltration and drainage, and denitrification ultimately modify the availability of N to the adjacent estuary and resultant water quality. The overall balance of these processes determines the role and magnitude of intertidal marshes within the landscape as a source or sink for DIN. The body of research contained herein examines two fundamental pathways and processes through which intertidal marshes modify nitrogen cycling in the coastal marine landscape. First, chapter 1 considers the combined interactions of the marsh, watershed, and estuary in the context of advective fluxes of water from the marsh to the estuary. Focus is given to the role of porewater drainage as a primary conduit for DIN/DON transport to estuarine waters. Second, chapter 2 considers N removal in the marsh via denitrification; a process independent of water exchanges yet a typically important DIN removal process in marshes. Determining the balance of these source and sink terms is necessary for assessing the importance 3

15 of intertidal marshes on response and resilience of coastal ecosystems to exogenous nitrogen inputs. 4

16 CHAPTER 1. ADVECTIVE MARSH-ESTUARY EXCHANGE - HYDROLOGIC AND CHEMICAL FLUXES

17 INTRODUCTION Intertidal marshes are characterized by alternating tidal wetting and draining. Marsh porewaters are derived from a mixture of groundwater, tidal infiltration (recharge), and precipitation, and are modified by water losses due to evapotranspiration. Porewater chemistry is controlled by contributions from these various sources of water and by in-situ N cycling in the subsurface, most notably organic matter mineralization which results in typically high concentrations of both DIN and DON (Valiela et al. 1978, Anderson et al. 1997). Porewaters can drain to the adjacent surface waters through seepage during periods of low water or diffuse into surface waters when the marsh is flooded (Harvey et al. 1995, Gardner 25). Groundwater input is focused at the marsh-upland interface, being driven by the water table height in the watershed (Harvey and Odum 199). Typically the shallow aquifer is the primary source (Tobias et al. 21b), however there are exceptions in which local geology permits the deep aquifer to contribute. While previous studies show that seasonal effects vary by location (Valiela et al. 1978, Tobias et al. 21b), groundwater input is a function of water table height regardless of location. Groundwater discharge to surrounding waters has been found to be low when compared to freshwater runoff, accounting for ~1% of the total freshwater input (Portnoy et al. 1997, Bowen et al. 27). However, if the shallow aquifer is high in N, it can supply the salt marsh with large amounts of allochthanous N. With elevated nutrient concentrations, groundwater discharge may play a large part in the landscape level loading of nutrients to the adjacent estuary (Gardner 1975, Valiela et al. 1978, Corbett et al. 1999). In some cases the shallow aquifer is N-poor, but groundwater input still serves as a major component in the water budget that promotes drainage by raising the hydraulic gradient from the upland border to the creekbank (Tobias et al. 21b). 6

18 Tidal infiltration occurs at the marsh-estuary interface and is derived from a pattern of tidal flooding and draining. Along the east coast of the United States, the tidal signature is dominated by a semi-diurnal lunar tide and a semimonthly spring-neap signature that modifies the daily tides. Tidal signatures are further modified stochastically by storm systems and Ekman tides propagated by offshore wind patterns (Bacopoulos et al. 29). Due to these variations, the magnitude of infiltration deviates from a regular periodicity but does not deviate seasonally like groundwater inputs. In addition to the frequency of flooding, magnitude is also a function of the thickness of the unsaturated zone and the specific yield of the marsh rhizosphere (Harvey and Odum 199, Tobias et al 21b). Infiltration of precipitation follows the same functions but occurs only during periods of heavy rainfall and is typically a small component of the subsurface water budget. The accepted model for infiltration is one of vertical recharge when the marsh is flooded (Harvey et al. 1987). The model is based on the premise that hydraulic conductivity of the saturated zone inhibits horizontal infiltration and that once the rising tide overtops the marsh surface, vertical infiltration of the unsaturated zone rapidly occurs. Groundwater discharge along the upland marsh edge mixes with infiltrating tidal water to facilitate the drainage of nutrient-rich porewaters to surrounding surface water (Harvey and Odum 199, Howes and Goehringer 1994, Howes et al. 1996, Tobias et al. 21b). The marsh drains on two functional timescales. First, there is slow and constant drainage from the back of the marsh to the creekbank represented by the slope of the water table within the marsh (Harvey et al. 1987, Tobias et al. 21b). Second, there is rapid drainage in the creekbank zone resulting from draining and filling with each flooding event (Harvey et al. 1987, Howes and Goehringer 1994, Harvey et al. 1995). The two timescales combine to create a system that includes slow transport of porewater from the back of the marsh to the creekbank zone which is then rapidly 7

19 drained and recharged. While input may be horizontal (groundwater) or vertical (groundwater, tidal infiltration, precipitation), drainage is a horizontal export flux (Harvey et al. 1987, Howes and Goehringer 1994, Tobias et al. 21b). Several approaches have been used to estimate marsh N export ranging from wholesystem approaches to small scale measurements. Radium balance approaches that estimate water flux based on the half life of radium isotopes have been successfully implemented (Krest et al. 2, Charette et al. 23) for large marshes but are expensive, do not necessarily account for other nutrient sources, and do not correlate directly to the water or nutrient budgets. Whole creek studies that sample incoming and outgoing waters throughout a tidal cycle (Valiela et al. 1978, Woodwell et al. 1979, Roman 1984, Wolaver et al. 1988) produce a net flux for the marsh but are reliant on the presence of a centralized tidal creek. The studies are labor-intensive and can only be performed at specific times, providing fine temporal resolution but for only a very brief time and limiting transferability to other systems and times. Smaller scale approaches focus on transects or plots within the marsh. The salt balance approach (Harvey and Odum 199, Tobias et al. 21b) works well in areas of fresh groundwater and has less error in calculation than other approaches but is less effective when used in brackish waters that are common in intertidal marshes. Flume studies (Childers 1994, Childers et al. 2) and flux chamber studies (Chambers et al. 1992, Windham-Myers 25) offer integrated measurements of a specific marsh surface and the ability to isolate individual aspects of the marsh. The protocols are easily transferred between sites but require intensive intstrumentation to characterize a single site. Hydrogeologic techniques applied to marsh hydrology have also been used to provide estimates for porewater drainage. Hydraulic head (Darcy) calculations use direct measurements 8

20 of the forcing functions but assume sediment homogeneity which may increase error (Tobias et al. 21b) and the calculations may not capture processing at the discharge interface or diffusion at the marsh surface. Porewater solute concentrations are highly variable between and within marshes due to vegetation regimes (Windham-Myers 25), microbial processes (Seitzinger et al. 26), and source waters (Harvey et al. 1995). In general, with the exception of very young marshes (Osgood and Zieman 1993a, Osgood and Zieman 1993b), porewaters are richer in DIN and DON than tidal flooding water. Consequently, marsh drainage constitutes a net flux of dissolved reactive N to adjacent coastal waters. The objective of this study was to assess the marsh to estuary flux of dissolved N. The work relies primarily on small-scale, high-frequency hydraulic head/gradient measurements coupled to porewater chemistry characterization. A long-term continuous record of porewater drainage from three sites within an estuary provides temporal resolution on a monthly, daily, and hourly scale. Fine temporal and spatial resolution was used to calibrate porewater drainage to easily measured physical drivers at the marsh-estuary and marsh-upland interfaces. Small-scale hydraulic gradient-based drainage N flux estimates are scaled up and compared to marsh-scale tidal N flux of select marshes. METHODS Two approaches were used to quantify marsh export of DIN and DON. First, small-scale hydraulic gradient induced fluxes were calculated using measurements of hydraulic gradient and conductivity (the Darcy method). Second, whole-creek tidal flood and ebb flux studies were used on a subset of marshes. In all, three marshes were studied in the New River Estuary, NC that span a gradient in salinity and tidal range. 9

21 Site Description Freeman s Creek, Traps Bay, and French Creek are three intertidal marshes located on Marine Corps Base Camp Lejeune in Jacksonville, NC along an estuarine salinity gradient in the New River Estuary (Fig. 1a). Freeman s Creek is a large (~45, m 2 ) polyhaline marsh located on the Intracoastal Waterway near Brown s Inlet with a large tidal amplitude (~1m) and a centralized tidal creek. Smooth cordgrass (Spartina alterniflora) dominates the flora of Freeman s Creek. Traps Bay is a pocket-type polyhaline marsh located near the mouth of the New River Estuary at the New River inlet. Traps Bay is a small marsh (~14, m 2 ) dominated by black needlerush (Juncus roemerianus) with a small, centralized tidal creek and median (4cm) tidal amplitude. French Creek is a fringing-type marsh (~25, m 2, 2 to 4m wide) dominated by black needlerush with no centralized drainage and a small tidal amplitude (2cm). Unlike the other sites, French Creek is bordered on the upland by a steep slope with a developed watershed and by oligohaline adjacent waters in the estuary. Site Development Each site was developed with a series of piezometers for hydraulic and chemical sampling, a tide gauge for estuary monitoring, and boardwalks to facilitate access and minimize marsh disturbance. Piezometers were constructed of 3.175cm diameter PVC with a 4cm long slotted screen at the base. Upland piezometers were constructed with 18cm long slotted screen. A 1.96cm diameter auger was used to create the cavity in the marsh in which the piezometers were set. Shallow piezometers were screened within the rhizoshpere (~.5m deep) and deep piezometers were screened below the rhizosphere 1

22 a c b c d b d Figure 1. Site map showing Marine Corps Base Camp Lejeune (a), French Creek (b), Traps Bay (c), and Freeman Creek (d). Images are not to the same scale. 11

23 (~3m deep). Upland piezometers were screened ~4m below the surface. The slotted screen was surrounded by a 1.96cm diameter sand pack, capped with grout and bentonite. French Creek (Fig. 2) consisted of 15 shallow piezometers, 1 deep piezometers, and 2 upland piezometers. Traps Bay (Fig. 3) consisted of 16 shallow piezometers, 9 deep piezometers, and 2 upland piezometers. Freeman s Creek (Fig. 4) consisted of 12 shallow piezometers, 12 deep piezometers, and 3 upland piezometers. Hydraulic Parameters and Water Level Measurements Hydraulic parameters and water levels were measured in piezometers to determine the water flux in each marsh (fig. 5). Slug tests were performed to determine hydraulic conductivity (K) according to the Hvorslev method (Hvorslev 1951). The initial depth to water (DTW) from the top of the casing (TOC) was taken using a handheld water level meter. An In-Situ Level Troll1 pressure transducer recording depth at 1-second intervals was then placed in the well at a known depth from the TOC. Immediately, a volume of water (the slug) was added to the well. After ~5 minutes, the terminal DTW was read using a handheld water level meter and the transducer was removed. The data was downloaded and a plot of the ratio of water level (h) to maximum water level (h ) versus time was generated with the y-axis on a log scale. Hydraulic conductivity was calculated according to: K = [r 2 ln(l e /R)]/[2L e t 37 ] (1) Where K is hydraulic conductivity, r is the radius of the well casing, R is the radius of the well screen, L e is the length of the well screen, and t 37 is the time required for the water level to fall to 37% of the initial rise. Hydraulic head, dh, in selected piezometers was monitored with an In-Situ Level Troll1 pressure transducer recording water level at fifteen-minute intervals. A separate 12

24 Figure 2. Aerial photo of French Creek study site. White points represent piezometer clusters. 13

25 Figure 3. Aerial photo of Traps Bay study site. White dots represent piezometer clusters. 14

26 Figure 4. Aerial photo of Freeman Creek study site. White dots represent piezometer clusters. 15

27 Tidal PN, DIN DON Denitrification N 2 Watershed GW DIN Burial PN Drainage DIN / DON, PN New River Estuary Figure 5. Conceptual model of salt marsh nitrogen linkages. Tidal recharge supplies particulate nitrogen (PN), dissolved inorganic nitrogen (DIN), and dissolved organic nitrogen (DON). Groundwater (GW) inputs supply DIN. Burial removes PN. Denitrification results in gaseous loss of N 2. Porewaters drain to the adjacent estuary resulting in export of DIN, DON, and PN from the salt marsh system. 16

28 transducer was left exposed solely to the atmosphere to record barometric pressure at fifteenminute intervals. The data from the pressure transducers in the wells was corrected for barometric effects. A GPS survey was performed to determine the coordinates of each well and elevation with regard to NAD83 at the TOC for each well. Porewater elevations were calculated as:: h PW = H TOC DTT + D (2) where h PW is the porewater elevation, H TOC is the elevation of the TOC, DTT is the depth to the transducer, and D is the barometric pressure corrected depth of water recorded by the transducer. Horizontal distance between water porewater elevation measurements, dx, was determined using GPS coordinates. Porewater Transport Horizontal porewater drainage between any two given piezometers or a piezometer and the tidal creek was calculated using measured hydraulic head and hydraulic conductivity values according to Darcy s Law: q = -K dh / dx (3) where Q is the drainage flux (L m -2 d -1 ), dh / dx is the hydraulic gradient, and K is the hydraulic conductivity. Porewater drainage was then brought to the marsh-scale by multiply the marsh shoreline length by the per meter shoreline drainage. Chemical sampling Porewater chemistry was sampled seasonally to be combined with porewater drainage to calculate solute export fluxes. Porewaters were sampled from all piezometers at all sites for a range of inorganic and organic analytes. Sampling occurred seasonally, taking place in 17

29 February, May, August, and November in 29 and February, May, and August 21. All collections were made at low tide. At each piezometer, water level was recorded using a handheld water level meter and the piezometer was pumped dry with a peristaltic pump and then given ~2 minutes to recharge. The porewater was then pumped with a peristaltic pump through an in-line glass fiber filter and a.45μm filter into the appropriate water chemistry vessels for subsequent analysis of nitrate, nitrite, ammonium, phosphate, dissolve organic carbon/nitrogen, sulfate, ferrous iron, and hydrogen sulfide. Redox sensitive species SO 4, Fe + 2, and H 2 S were fixed immediately with acetic acid and BaCl 2, ferrozine, and diamine reagents in the field, respectively. DOC/DON samples were H 3 PO 4 acid preserved and refrigerated prior to analysis. DIN/DIP were frozen and stored until analysis. Marsh Scale Drainage - Whole Creek Tidal Flood vs. Ebb Flux Studies At Freeman s Creek and Traps Bay, duplicate whole creek tidal flux studies were performed to get a marsh integrated dissolved N export estimate. A tidal flux study could not be performed at French creek because it is a fringing marsh and lacks a defined tidal creek. Creek depth profile, current velocity (V), and water chemistry were sampled at 2 to 3 minute intervals beginning at low tide and finishing at the following low tide. The depth profile was used to calculate cross-sectional area (A). Discharge (D) was determined using the following equation: D = AV (4) Samples for water chemistry were collected at the middle of the water column in Traps Bay and at 4 locations (2 deep, 2 shallow) in Freeman Creek. The same analytes were measured as those for the seasonal porewater sampling as well as particulate N. Particulate N was sampled by forcing sample water through a Whatman GFF using a peristaltic pump until the filter was 18

30 clogged. The volume of sample filtered was recorded and the filter was sandwiched in aluminum foil and frozen. Analysis was performed on an Isotope Ratio Mass Spectrometer (IRMS) elemental analyzer. Analyte concentrations (specifically DIN and DON) at each sampling time point were multiplied by the corresponding discharge to generate a nutrient flux for each sampling time point. Integrating the fluxes over the sampling period generated a marsh scale flux for the tidal cycle. RESULTS Tidal and Watershed Boundary Conditions Marsh elevation, tidal stage, and water table elevation of the shallow aquifer in the adjacent watershed served as boundaries for the marsh porewater volume. Marsh elevation relative to mean high water differed by a factor of 3 between the up-estuary and down-estuary sites. French Creek, Traps Bay, and Freeman Creek marshes were 14, 18, and 41 cm below mean high water, respectively. Semi-diurnal tidal patterns observed at all sites were overprinted with stochastically distributed periods of extended flooding caused by wind-driven high water events lasting from 1-6 days in duration (Fig. 6). Mean tidal amplitudes (i.e. tidal range) increased with distance down-estuary from a minimum of 21cm (min = 5. cm; max = 51.4cm) at French Creek, to an intermediate amplitude of 34.3cm (min = 8.4cm; max = 62.1cm) at Traps Bay. The Freeman Creek marsh exchanged directly with the Intracoastal Waterway (ICW) and was not influenced by any of the tidal constrictions in the New River Estuary proper. It was the farthest down-estuary and had the largest mean tidal amplitude of 95.3cm (min = 41.9cm; max = 138.5cm). The combined effects of marsh elevation relative to mean high water and differences 19

31 15 a Jan-9 Mar-9 Jun-9 Aug-9 Nov-9 Feb-1 Apr-1 Tidal Elevation (cmnad83) Jan-9 Mar-9 Jun-9 Aug-9 Nov-9 Feb-1 Apr-1 15 c Jan-9 Mar-9 Jun-9 Aug-9 Nov-9 Feb-1 Apr-1 b Figure 6. Daily maximum observed high tide (black) and low tide (grey) for French Creek (a), Traps Bay (b), and Freeman Creek (c). Creekbank marsh surface elevation is indicated by the dashed line. 2

32 in tidal amplitudes among the sites resulted in different flooding durations in the marsh depending on position within the estuary. While Freeman Creek marsh was flooded approximately 4% of the time during the yearlong monitoring period, Traps Bay marsh was flooded 38%, and French Creek was flooded 62% of the time. Upland water table elevations adjacent to all marshes were characterized by seasonal fluctuations of approximately 5 cm, and were interspersed with short duration excursions of almost equal magnitude that were attributable to tidal effects and/or precipitation events (Figs. 7, 8). Seasonal drawdown of the water table was seen in all watersheds during summer 29 and summer 21. The absolute elevation of the water table was largely a function of the distance between the location of the upland well and the marsh-upland edge (French Creek=15m; Traps Bay=15m; Freeman Creek=1m). The slopes of the water table towards the marsh were similar between French Creek and Traps Bay, but a factor of 4 smaller at the lower topographical gradient and larger Freeman Creek site. A dampened tidal effect and rapid response of the water table to precipitation events were encountered in the Freeman Creek and Traps Bay uplands (Fig. 7,8). A tidal signature in the Traps Bay upland was only evident during periods of low water table. Short-duration fluctuations in the French Creek water table were small in comparison to Traps Bay and Freeman Creek, were wholly tidally driven, and showed little to no response to precipitation events (Fig. 7a). Hydraulic Properties of the Rhizosphere Horizontal hydraulic conductivity (K h ) of the rhizosphere (-5cm depth) determined by in-situ slug tests were on the order of that typically found in fine sands (Table 1). K h averaged 2.3 ± cm s -1 for French Creek, 4.1 ± cm s -1 for Traps Bay, and 1.7 ± cm s -1 for Freeman Creek. Mean K h for the interior of French Creek was twice 21

33 a 15 1 Watershed Water Table Elevation (cm NAD83) 5-5 Apr-9 Jul-9 Sep-9 Dec-9 Mar-1 May-1 Aug-1 b Oct-1 Jul-9 Sep-9 Dec-9 Mar-1 May-1 Aug-1 c Oct-1 Jul-9 Sep-9 Dec-9 Mar-1 May-1 Aug-1 Oct Apr Apr-9 Figure 7. Upland water table elevations for French Creek (a), Traps Bay (b), and Freeman Creek (c). Freeman Creek and Traps Bay showed responses to precipitation events while French Creek predominantly responded to tidal elevation. 22

34 15 a /1 12/6 12/11 12/16 12/21 12/26 12/31 Water Elevation (cm NAD83) b 4 12/1 12/6 12/11 12/16 12/21 12/26 12/31 15 c /1 12/6 12/11 12/16 12/21 12/26 12/ Precipitation (cm) Figure 8. Short duration fluctuations in upland water table elevation (grey), tidal elevation (black), and precipitation (bar) for French Creek (a), Traps Bay (b), and Freeman Creek (c). 23

35 that of mean creekside K h. With the exception of the most upstream transects at Traps Bay dominated by Typha spp, mean interior K h was 2-3 times greater in the interior of the marshes than in the creekbanks for all sites (Table 1). There were no significant inter-marsh differences between interior K h or creekbank K h values. Vertical hydraulic conductivity (K v ) values determined from falling head permeameter measurements on rhizosphere cores were on average 1% of K h values for the same marsh locations. The horizontal hydraulic gradient was considered separately for the creekbank and interior zones of the marsh. The creekbank gradients were calculated from the difference in hydraulic head between the creekbank piezometer and tidal stage, and the marsh interior gradient was calculated from the difference in hydraulic head measured at the upland-marsh border and the creekbank piezometers. The daily maximum hydraulic gradient towards open water (i.e. drainage) in the creekbank exceeded the interior gradient by a factor of 1-1. The creekbank gradient was dynamic and exhibited hourly variation throughout the tidal cycle, while fluctuations on hourly to daily scales in the interior gradient were barely detectable at all sites (Fig. 9). The gradient from the creekbank to open water was maximized at low tide for all sites. Consistent with the different marsh elevations relative to mean high water and the different tidal amplitudes, the low tide creekbank hydraulic gradients among sites differed by a factor of 1 (Fig. 9) with a maximum and minimum occurring at Freeman Creek and French Creek, respectively. For all marshes, as the tide rose the creekbank gradient decreased until the tidal elevation matched or exceeded the porewater elevation (Fig. 9). When tidal elevation was greater than porewater elevation, either no gradient existed, or a negative gradient was created (Fig. 9 a,b), indicating periods of recharge. Mean positive creekbank gradient increased with distance down-estuary from French Creek (.21) and Traps Bay (.25) to Freeman Creek 24

36 .45 a : 4:48 9:36 14:24 19:12 : 4:48 9:36 d h /d x.45 b :24 19:12 : 4:48 9:36 14:24 19:12.45 c : 4:48 9:36 14:24 19:12 : 4:48 9:36 Tidal Elevation (cm NAD83) Figure 9. Representative daily fluctuation of tidal elevation (grey), creekbank gradient (triangle), and interior gradient (circle) for French Creek (a), Traps Bay(b), and Freeman Creek (c). D h represents the difference in hydraulic head between either the tide gauge and creekbank piezometer (creekbank gradient), or the upland-marsh border piezometer and the creekbank piezometer (interior gradient). D x represents the distance between the points where d h is determined. A positive gradient indicates a slope from the marsh towards open water. 25

37 Table 1. Hydraulic conductivity (K h ) of Traps Bay, French Creek, and Freeman Creek. K h x1-3 cm s -1 Transect Well location Creekside Interior Traps 1 East Traps 1 West Traps 3 East Traps 3 West Traps 5 East Traps 5 West Traps 7 East Traps 7 West French French French French Freeman Freeman Freeman

38 (.13). Creekbank gradient was negative in Freeman creek for 6% of the monitoring period compared to 43% in French Creek and 49% in Traps Bay. Mean negative creekbank gradient was greatest in French Creek (-.19) and decreased with distance down-estuary to Traps Bay (-.15) and was smallest in Freeman Creek (-.12). Interior drainage gradients were on the order of 2% of the creekbank gradient in French Creek, to less than 2% of the creekbank gradient in Freeman Creek. Mean positive interior drainage gradient decreased with distance down-estuary from.4 in French Creek to.3 in Traps Bay and.1 in Freeman Creek. Mean negative interior drainage was smallest in Freeman Creek (-.1, 29% of sampling period) and increased with distance up-estuary to Traps Bay (-.2, 44% of sampling period) and French Creek (-.4, 34% of sampling period). Negative vertical gradient (dh/dz) indicated vertical recharge during flooded conditions. As the marsh flooded and tidal elevation rose above porewater elevation (Fig. 1), vertical gradient became negative (Fig. 11). Mean negative vertical gradient was -.14 in French Creek, -.15 in Traps Bay, and -.1 in Freeman Creek. Porewater Drainage Drivers Using all transducer records aggregated over an annual period, daily drainage of the creekbank to adjacent waters and from the interior of the marsh to the creekbank were calculated using Eq. 3. Overall magnitudes, temporal patterns of change, and responses of each of these drainage fluxes in response to changes in tidal and upland water table elevation were assessed for each site. Drainage in the creekbank was greater than drainage from the marsh interior for all sites. 27

39 4 a 2-2 Water Elevation (cm NAD83) b c : 12: : Figure 1. Creekbank porewater elevation (triangles) and tidal elevation (black line) through a daily tidal series for French Creek (a), Traps Bay (b), and Freeman Creek (c). Once the marsh is flooded, tidal elevation is greater than porewater elevation, creating a negative gradient. Shaded grey areas represent times of vertical recharge. 28

40 a : 4:48 9:36 14:24 19:12 : 4:48 9:36 d h /d z b :24 19:12 : 4:48 9:36 14:24 19:12 Tidal Elevation (cm NAD83) c :12 : 4:48 9:36 14:24 19:12 : 4:48 Figure 11. Vertical gradient for the creekbank (triangles) and marsh interior (circles) for French Creek (a), Traps Bay (b), and Freeman Creek (c) during flooded periods of a representative tidal cycle. Tidal elevation is indicated by the black line. Negative gradients indicate vertical recharge of the marsh. 29

41 Creekbank Drainage and Tides For all creekbanks at all sites, a two-phase response was found between porewater drainage and tidal elevation (Figs ). Drainage increased as a function of decreasing tidal elevation below the marsh surface. Except for the two most up-gradient transects at Traps Bay whose drainage was heavily influenced by groundwater discharge, the largest increase in drainage per unit drop in tidal height was found in Freeman Creek, followed by Traps Bay, and French Creek. For every cm tidal elevation fell below the marsh surface, Freeman Creek, Traps Bay, and French Creek drainage fluxes increased by 3.5, 2., and.85 L m -2 d -1. The higher drainage fluxes per unit tidal change also showed less variability in the range of drainage for a given tidal elevation. This phenomenon was evidenced by the higher correlation coefficients relating tidal stage to drainage for Freeman Creek and Traps Bay relative to French Creek. At tidal elevations above the marsh surface, the drainage flux (either a small residual flux towards open water, or a negative recharge flux) was largely independent of further changes in tidal height so long as the marsh was flooded (Figs ; gray regions). Creekbank Drainage and Upland Water Table The relationship between creekbank drainage and upland water table slope was examined for the lowest daily tidal elevation values. These periods coincided with maximum drainage of marsh porewater, and were assumed to be most indicative of water table effects not confounded by tidal effects. French Creek had a significant daily maximum creekbank drainage response to increased water table slope in all transects (Fig. 15). Traps Bay showed a significant daily maximum creekbank drainage response to increased upland water table slope for all 3

42 1 5 y = -.639x R 2 =.132 p<.1 a Creekbank Drainage (L m -2 day -1 ) y = -1.23x R 2 =.159 p< y = -.719x R 2 =.171 p< Tide Elevation (cm NAD83) Figure 12. Creekbank drainage versus tidal elevation for French Creek transect 3 (a), transect 5 (b), and transect 7 (c). A two-phase response was seen with increased drainage at elevations below the marsh surface (black ) and recharge for tidal elevations greater than the marsh surface (grey). Grey points greater than zero occur as the marsh is draining on the falling tide and grey values less than zero occur as the marsh is flooding on the rising tide. b c 31

43 Creekbank Drainage (L m -2 day -1 ) y = -9.85x R 2 =.399 p< y = -14.x R 2 =.284 p< y = -2.63x R 2 =.655 p< y = -1.56x R 2 =.561 p< Tide Elevation (cm NAD83) a b c d Figure 13. Creekbank drainage versus tidal elevation for Traps Bay transect 1 (most upstream) (a), transect 3 (b), transect 5 (c), and transect 7 (most downstream) (d). A two-phase response was seen with increased drainage at elevations below the marsh surface (black ) and recharge for tidal elevations greater than the marsh surface (grey). Grey points greater than zero occur as the marsh is draining on the falling tide and grey values less than zero occur as the marsh is flooding on the rising tide. 32