Controls on Nitrous Oxide Production In, and Fluxes from a Coastal Aquifer in Long Island, Ny, Usa

Nitrous oxide (N 2 O) has 265 times greater greenhouse potential than carbon dioxide and its atmospheric concentration has increased by about 20% since industrialization; however, N 2 O production and emissions from aquatic systems are poorly constrained. To evaluate N 2 O fluxes associated with meteoric groundwater discharge to coastal zones, we measured N 2 O concentrations in May and October 2011 from two discharge points of the Upper Glacial aquifer on Long Island, NY, USA. One coastal zone contains only fresh water and the other contains an upper saline zone. N 2 O concentrations decreased by around 40% for the fresh water and a factor of two for the salt water from May to October, 2011. Fluxes were around 100 to 200 times greater from the freshwater (246 to 448 µmol/m shoreline/day) than saltwater aquifer (26.1 to 26.5 µmol/m shoreline/day). N 2 O concentrations correlate positively with NO 3 − and dissolved oxygen concentrations and negatively with salinity, dissolved organic carbon (DOC) and N 2 denitrification concentrations. The smaller saltwater N 2 O export resulted from DOC enrichment in the upper saline zone, which appears to have driven denitrification to completion, removed N 2 O, and increased N 2 denitrification. DOC concentrations should be considered in global N 2 O flux estimates for coastal aquifers.


Introduction
Nitrous oxide (N 2 O) is a powerful greenhouse gas with a greenhouse potential approximately 265 times that of CO 2 [1].Atmospheric N 2 O concentrations have increased from 270 ppb during pre-industrial times to the current value of around 327 ppb, and yet its global budget, particularly atmospheric fluxes from estuaries, rivers and groundwater, is still poorly understood.The increase in atmospheric N 2 O concentrations largely results from anthropogenic activities that increase reactive N, including fixation of atmospheric N 2 through the Haber-Bosch process, increased cultivation of biological nitrogen fixers, such as legumes and rice, and burning of fossil fuels [2][3][4].This rate of increase is accelerating; N 2 O concentrations increased faster from 2013 to 2014 than average rates of increase during the preceding decade [5].
About two thirds of the uncertainty of the N 2 O budget originates from limited information about N 2 O emissions from agricultural leaching and runoff, human sewage, and atmospheric deposition, according to the Intergovernmental Panel on Climate Change (IPCC) 5th assessment report [6].According to this methodology leaching and runoff comprises about half of the uncertainty in calculating the total agricultural source.Indirect N 2 O sources are defined as the sum of N 2 O lost from agricultural applications through gas volatilization (N 2 O (G)), human consumption of crops followed by municipal sewage treatment (N 2 O (S)), nitrogen leaching and runoff (N 2 O (L)) [1].Nitrogen leaching (N 2 O (L)) contributes ~75% of the indirect N 2 O source and is derived from estuaries, rivers and groundwater [7].The amount of N 2 O lost from these settings is quantified by emission factors, termed EF5, which are calculated as the ratio of N 2 O produced to the initial amount of inorganic nitrogen (NO x and NH 4 + ) in the system.Each of the three hydrologic pathways, which are referred to as EF5-e (estuaries), EF5-r (rivers), and EF5-g (groundwater) has an assigned EF5 value of 0.0025 by the IPCC 5th assessment report.Previous IPCC assessment reports had varying EF5 for each of the hydrologic pathways, due to uncertainty surrounding the calculation of emission factors.Uncertainty around the emission factors from estuaries, rivers, and groundwater stems from (1) small number of studies; (2) variability in study methodology and (3) differences in biogeochemical reactions within each setting.N 2 O production by biogeochemical reactions is well understood in terrestrial systems, but less so in coastal systems.In terrestrial systems experiments in which addition of five different N-bearing chemical fertilizers were correlated with N 2 O emission in both aerobic and anaerobic agricultural and non-agricultural systems found denitrification of NO 3 − contributed the most to total N 2 O emissions [8].
Coastal systems should also be an important for N 2 O production because the interface between fresh and salt water is a biogeochemical hot spot [9][10][11] where N 2 O forms as an intermediate product in both nitrification and denitrification processes.The oxidation of ammonium (NH 4 + ) to nitrate yields N 2 O as a byproduct of nitrification under aerobic conditions in soils and open water (1).
In hypoxic environments denitrification usually results in the formation of N 2 , a net sink of N from the environment, but can also terminate in N 2 O (2) [12].
These reactions indicate that N 2 O production in aquatic systems depends on the amount of total dissolved inorganic nitrogen (DIN; including NO 3 − , NO 2 − and NH 4 + ) [13], as well as the oxidation states of mixed fresh groundwater and saline surface water in coastal aquifers [14,15].Oxidation of NH 4 + promotes N 2 O production in mangrove sediments [16,17] and in estuarine water columns [18].Denitrification of NO 3 − produces N 2 O in intertidal sediments [19][20][21] which have low oxygen concentrations when they flood during high tide, and increased oxygen concentrations when exposed at low tide [22].Intertidal fluxes of N 2 O from muddy and sandy sediments are strongly correlated to NO 3 − availability and therefore extent of denitrification in the porewater [11,23].
Land use in the catchment area of coastal aquifers affects the amount of nitrogen available in porewater, which should in turn impact porewater N 2 O concentrations.In Port Jefferson Harbor, a Long Island Sound embayment, land use in the coastal aquifer catchment area ranges from undeveloped to high density development.As a result, submarine groundwater discharge (SGD) derived nitrate loading varies by two orders of magnitude along the embayment coastline, with catchment areas containing high density development contributing more than 70% of the nitrate load to surface water [24].In West Falmouth Harbor, MA the highest measured N 2 O fluxes from the coastal aquifer were located in a nitrogen rich wastewater plume [11].A summary of N 2 O fluxes from temperate coastal sediments presented by Moseman-Valtierra et al., (2015) found average values range from 0.1 µmol•m −2 •h −1 to 10.1 µmol•m −2 •h −1 , with the highest fluxes occurring when sediments received nitrogen amendments.These studies highlight the need for better understanding of how nitrogen additions to groundwater will impact N 2 O fluxes to coastal waters, particularly as anthropogenic nitrogen degrades coastal waters worldwide [25].
In this paper, we describe intertidal sediment porewater N 2 O concentrations from two discharge sites for a coastal aquifer on Long Island, NY, USA, which have similar sediment compositions.Each aquifer has distinct N 2 O concentrations and distributions in time and space, which we interpret here in terms of the biogeochemical factors that control N 2 O production.We calculated N 2 O emission factors to place our measurements in a broader context and compared our values with those used by the Intergovernmental Panel on Climate Change (IPCC) [7].We describe how dissolved organic carbon (DOC) in porewater impacts N 2 O concentrations and consequently N 2 O flux to surface waters.This work provides a framework for how DOC and salinity can act as a proxy for N 2 O concentrations in shallow porewater, which will help refine N 2 O flux at the embayment and coastline scale.

Site Description
Porewater samples were collected from the Upper Glacial aquifer where it discharges to two embayments (Stony Brook Harbor, SBH and Port Jefferson Harbor, PJH) of Long Island Sound (Figure 1).The Upper Glacial aquifer consists of medium to coarse grain sands interbedded with clay lenses and contains elevated NO 3 − concentrations due to septic/sewage and lawn fertilizer loading [26].The two field sites have different land use; Port Jefferson Harbor has medium to high density housing whereas in Stony Brook Harbor has low density housing or is forested [24].These different land uses affect NO 3 − discharge via submarine groundwater discharge (SGD), with greater fluxes from more urbanized settings [24,27].Both the fresh groundwater and saline coastal water endmembers are well oxygenated, limiting the extent of denitrification in the inland aquifer [28].
Both harbors have an ~2 m daily tidal range and undergo periodic dredging to maintain shipping channels [29].Port Jefferson Harbor receives no fresh surface water, but undergoes tidal exchange with Long Island Sound and two small adjacent harbors; Conscience Bay and Setauket Harbor.The narrow (75 m) inlet of Stony Brook Harbor is bounded by West Meadow Creek, which provides limited amount of freshwater to the southern lobe of the harbor during high tide [29].Inland topography at the Port Jefferson field site is steep.Consequently large hydraulic gradient between the groundwater and adjacent surface water drives freshwater towards the coast and prevents intrusion of saline coastal water during tidal inundation [27].This work provides a framework for how DOC and salinity can act as a proxy for N2O concentrations in shallow porewater, which will help refine N2O flux at the embayment and coastline scale.

Site Description
Porewater samples were collected from the Upper Glacial aquifer where it discharges to two embayments (Stony Brook Harbor, SBH and Port Jefferson Harbor, PJH) of Long Island Sound (Figure 1).The Upper Glacial aquifer consists of medium to coarse grain sands interbedded with clay lenses and contains elevated NO3 − concentrations due to septic/sewage and lawn fertilizer loading [26].The two field sites have different land use; Port Jefferson Harbor has medium to high density housing whereas in Stony Brook Harbor has low density housing or is forested [24].These different land uses affect NO3 − discharge via submarine groundwater discharge (SGD), with greater fluxes from more urbanized settings [24,27].Both the fresh groundwater and saline coastal water endmembers are well oxygenated, limiting the extent of denitrification in the inland aquifer [28].Both harbors have an ~2 m daily tidal range and undergo periodic dredging to maintain shipping channels [29].Port Jefferson Harbor receives no fresh surface water, but undergoes tidal exchange with Long Island Sound and two small adjacent harbors; Conscience Bay and Setauket Harbor.The narrow (75 m) inlet of Stony Brook Harbor is bounded by West Meadow Creek, which provides limited amount of freshwater to the southern lobe of the harbor during high tide [29].Inland topography at the Port Jefferson field site is steep.Consequently large hydraulic gradient between the groundwater and adjacent surface water drives freshwater towards the coast and prevents intrusion of saline coastal water during tidal inundation [27].

Field Sampling and Observations
SGD rate measurements were taken using ultrasonic seepage meters in May and October 2011 and by manual seepage meter at each site in August 2012 [30].Details of seepage meter results are presented elsewhere [31] and summarized below.Porewater was collected in May and October of 2011 simultaneous with the seepage meter sampling.These dates were chosen to minimize differences in porewater and atmospheric temperatures during sampling and to assess temporal variability in nitrous oxide dynamics.Porewater was sampled using an AMS Retract-A-Tip vapor probe [32] connected to a peristaltic pump with low gas permeability Viton tubing.Temperature, conductivity, pH, and dissolved oxygen were measured in the field by YSI 556 multi-parameter probe connected to an overflow cell.Nutrient samples were filtered (Whatman GF/B 0.45 µm), kept on ice while in the field, and frozen within 8 h of collection.Dissolved N 2 /Ar samples were collected into 12 mL gastight vials (LabCo, Lampeter, UK), inverted and stored under deionized water at 4 • C until analysis.Dissolved N 2 O samples were collected into pre-evacuated 9 mL vials (Grace-Discovery Sciences) with Teflon butyl septa crimp caps (MicroLiter Analytical, Suwanee, GA, USA).Samples were collected into glass syringes and loaded into vials with no headspace with the addition of 100 µL 0.1 M sodium azide to prevent microbial alteration of the sample.Field duplicates were collected for 10% of the total number of porewater sampling depths.

Nitrate (as NO 3
− + NO 2 − ) concentrations were analyzed on a Lachat Quickchem auto-analyzer with a precision of ±5% for triplicate sample analysis.Dissolved organic carbon (DOC) was analyzed using a Shimadzu TC-total organic carbon analyzer.Dissolved N 2 O concentrations were determined with gas chromatography following headspace extraction.Briefly, 3 mL of helium was injected into the headspace of sample vials, displacing 3 mL of sample.Samples were equilibrated at 25 • C for 2 h on a shaker table.Equilibrated headspace gas was analyzed on a gas chromatograph equipped with an electron capture device (Shimadzu GC-14a) at the Cary Ecosystems Institute.Replicates were analyzed for each sampling site and time period with a mean precision of ±8%.Dissolved N 2 /Ar concentrations were analyzed by membrane inlet mass spectrometry (MIMS) at the University of California, Davis [33].Instrument drift was determined from analyses of deionized water held at a constant temperature (15 • C) which was analyzed every 10-15 samples.Precision was 0.05 for N 2 /Ar, calculated as the deviation of reference water from theoretical solubility values [34].

Calculation of Excess N 2 , Emission Factors (EF5), and N 2 O Flux to Surface Waters
Solubility concentrations of N 2 and N 2 O were calculated using equations of Weiss and Price [34,35].For both gases, dissolved argon was used as an inert tracer to correct for physical effects of temperature, pressure and salinity [36].Dissolved N 2 O concentrations were calculated according to: where N 2 O pw is the concentration of N 2 O in the porewater sample (nmol•L −1 ), K o is the solubility coefficient for N 2 O (mol•L −1 •atm −1 ) derived from Weiss and Price (1980), x is the dry gas mole fraction of N 2 O in the sample headspace, P is the atmospheric pressure (1 atm), V wp is the volume of the water phase (mL), V hs is the volume of the headspace (mL), R is the gas constant (L atm•K −1 •mol −1 ) and T is the temperature (K) [35,37].
The total concentration of dissolved N 2 in a sample is equal to where N 2 total is the total concentration of N 2 in the sample, N 2 atm.eq is the amount of N 2 present due to solubility dependent atmospheric equilibration [34] corrected for salinity and recharge temperature, N 2 EA is the amount of N 2 due to excess air entrainment and N 2 denitrification is the remaining amount of N 2 in the sample, which is assumed to result from denitrification.Excess air entrainment occurs when bubbles trapped at the capillary fringe dissolve, either partially or completely, into groundwater during recharge [38].During complete bubble dissolution the amount of excess air in the sample will have N 2 /Ar ratios representative of atmospheric molar ratios whereas during incomplete bubble dissolution N 2 /Ar ratios will be lower than atmospheric ratios as Ar solubility is greater than N 2 solubility [39].As bubble dissolution approaches completion this lower value will be close to the N 2 /Ar ratio for air-equilibrated water at the recharge temperature [40].As we are unable to determine the extent of bubble dissolution for a given groundwater recharge scenario, we calculate the minimum and maximum amount of excess air and use the average of these to values to calculate the amount of N 2 denitrification in each sample.
N 2 EA min = X Ar T − X Ar Eq × (X N2 EQ /X Ar EQ ) ( where N 2 EA max and N 2 EA min are the maximum and minimum excess air incorporations, X Ar T is the concentration of dissolved argon in the sample and X Ar Eq is the argon concentration in equilibrium with the atmosphere.X N2 atm and X Ar atm are the mole fractions of N 2 and Ar in the atmosphere respectively, reflecting complete dissolution, and X N2 EQ and X Ar EQ are the equilibrium mole fractions of N 2 and Ar respectively.We use Equations ( 5) and (6) where N 2 O is the concentration of N 2 O in a sample and NO 3 − initial is as defined in Equation (7).Fluxes of N 2 O to surface water were calculated by multiplying the N 2 O concentration from the shoreward-most porewater concentrations by the SGD rate for the given sample location and time period.The shoreward most porewater profile lies within the zone of low tide discharge, where seepage meters were placed to measure SGD rates (cm•d −1 ).The N 2 O concentration was taken from the shallowest sampling point, typically <50 cm below the sediment water interface, to estimate N 2 O fluxes.

Statistical Analysis
Multivariate and correlation analysis was performed using JMP 10.Two factor ANOVA analyses were used to test for differences in N 2 O concentrations between site (factor 1) and date (factor 2).Samples were pooled according to site and sampling period (May and October).Sample collection required a period of 4 consecutive days for each individual site and date.Therefore, tide stage was not considered in the ANOVA analysis.Sample variability, as determined from field duplicates, was less than 8% for the statistical parameters examined by ANOVA and PCA.Principal component analysis (PCA) was used to compare primary variables (salinity, dissolved oxygen, NO 3 , N 2 O, DOC, and N 2 denitrification , and distance offshore) in these two coastal aquifers to assess which may relate to N 2 O production.

Results
Seepage rates were consistent across each sampling time (May 2011, October 2011 and August 2012) in each of the different sites.Seepage rates ranged from 76 cm•day −1 to 149 cm•day −1 [31] in Port Jefferson Harbor and from 26 to 102 cm•day −1 in Stony Brook Harbor.

Spatial Distribution of Solutes and N 2 O Concentrations
In PJH the coastal aquifer was largely fresh in both May and October (Figure 1) with salinity below ~2 for all samples except for a shallow sample with a salinity of 16.3 from the high tide rack line at a depth of 66 cm (Figure 1).The elevated salinity suggests saltwater is not flushed as rapidly in this zone.SBH exhibits an upper saline zone in which the salinity ranges from 3.9 to 17.8.Below the upper salinity zone, the salinity is <2.In May saline water in the upper saline zone (USZ) penetrated to a depth of ~3 m but in October it only penetrated to a depth of ~2 m.N 2 O concentrations differed between the two harbors and sampling dates (Figure 2).3e).A principal component analysis (PCA) was performed individually for PJH and SBH.At each site data from both sampling time periods was merged to provide a data set sufficiently large for the analyses.At PJH the first component explains 44% of the variation and the second component explains 20.3% of the variation (Figure 4a).At PJH N2O and NO3 − concentrations plot closely together and are at 180° from N2 denitrification but are ~90 from DO% and DOC concentrations.The SBH PCA found the first component explains 45% of the variation and the second component explains 17% of the variation (Figure 4b).Variables N2O concentration, NO3 − concentration and DO% plot close together but are at 180° from salinity and DOC concentration.

Flux of N2O to Surface Waters
Multiplying

N2O in Fresh and Saline Coastal Aquifers
Salinity distributions of coastal aquifers depend on the balance between inland hydraulic head and elevation of adjacent coastal water bodies [41,42].This balance affects salinity distribution at SBH and PJH.An upper saline zone (USZ) forms at SBH because of the tides and waves [42][43][44], but the USZ is missing at PJH SGD rates are greater there than SBH because of higher inland elevations (Figure 1) [27].DOC concentrations are greater than in Long Island Sound surface water than in the pore water in the upper Glacial aquifer [28].Therefore, the correlation between porewater salinity and DOC concentration (Figure A1) suggests much DOC originates from mixing of surface estuarine water into the USZ.Elevated DOC concentrations of Long Island Sound reflect primary production in the photic zone and DOC remineralization in the pore water by elevated oxygen concentrations [45].Circulation of saline surface water into shallow sediments at SBH thus provides a source of electron donors for reduction of inorganic nitrogen.
Although positive correlations have been found between N2O fluxes and groundwater NO3 − concentration in West Falmouth Harbor, Massachusetts, USA [11], in our study only porewater at

N 2 O in Fresh and Saline Coastal Aquifers
Salinity distributions of coastal aquifers depend on the balance between inland hydraulic head and elevation of adjacent coastal water bodies [41,42].This balance affects salinity distribution at SBH and PJH.An upper saline zone (USZ) forms at SBH because of the tides and waves [42][43][44], but the USZ is missing at PJH SGD rates are greater there than SBH because of higher inland elevations (Figure 1) [27].DOC concentrations are greater than in Long Island Sound surface water than in the pore water in the upper Glacial aquifer [28].Therefore, the correlation between porewater salinity and DOC concentration (Figure A1) suggests much DOC originates from mixing of surface estuarine water into the USZ.Elevated DOC concentrations of Long Island Sound reflect primary production in the photic zone and DOC remineralization in the pore water by elevated oxygen concentrations [45].Circulation of saline surface water into shallow sediments at SBH thus provides a source of electron donors for reduction of inorganic nitrogen.
Although positive correlations have been found between N 2 O fluxes and groundwater NO 3 − concentration in West Falmouth Harbor, Massachusetts, USA [11], in our study only porewater at SBH in May exhibits a positive correlation between NO 3 − concentrations and N 2 O concentrations (Figure A2).Therefore, at these sites, groundwater NO  [11,[47][48][49] it is evident that much of the groundwater NO 3 − is not denitrified during transit through the coastal aquifer.Previous investigations in coastal aquifers do not consider DOC as a driver of N 2 O reduction of an influence on N 2 O flux to surface water.However, DOC is considered in determining N 2 O concentrations of other settings, such as river and estuarine sediments.In a study of river sediments, N 2 O production resulted from only 0.01% of the nitrate reduction when sediments were amended with carbon [50], and 6% of NO 3 − reduced formed NO 2 − with the balance of NO 3 − reduction continuing to form N 2 or NH 4 + .Similar to these river settings, we find N 2 O production to be negatively correlated with DOC concentration at SBH, where DOC is entrained in the aquifer in the USZ (Figure 4).Concentrations of N 2 denitrification were statistically different between PJH and SBH, with SBH having a higher mean N 2 denitrification concentration (Figure 3e).Therefore, the elevated DOC concentrations in the USZ of SBH appear to drive the reduction of N 2 O to completion, resulting in an accumulation of N 2 and lower N 2 O concentrations.
The recognition of a DOC-rich USZ as a sink for N 2 O is critical in accurately predicting N 2 O production in sandy coastal sediments.Benthic sediments consume N 2 O, particularly in the presence of benthic algae cover [51].In coastal sediments a lack of an USZ can occur due to a lack of tides or waves [52] or high freshwater hydraulic head.Where an USZ delivers carbon to the subterranean estuary, its remineralization will deplete pore water in DO and cause complete denitrification, removing N 2 O from porewater.Understanding the salinity configuration, and thus the DOC concentrations, of sandy coastal aquifers will be important to estimating global N 2 O concentrations and atmospheric fluxes from coastal sediments.
The cause of these shifts in location of elevated N 2 O concentrations through time likely relates to hydrological and biogeochemical reactions.Temporal variations in coastal aquifer heads can drive the saltwater-freshwater interface landward during winter months and seaward during the summer months (46), with additional freshwater recharge to catchment areas of subterranean estuaries due to recharge as snowmelt (49) or precipitation (50) increases water table height (48).Consequently, variations in freshwater discharge, with its elevated NO 3 concentrations, alter the overall composition of SGD (48) as the fresh fraction of SGD increases during spring and summer.Elevated NO 3 concentrations would limit complete denitrification and increase porewater N 2 O concentrations, generating greater N 2 O flux in the spring.This variation may help explain the subtidal maximum N 2 O values observed in PJH (Figure 2).Causes are less clear for temporal variation in N 2 O distribution in SBH.The absence of N 2 O in shallow (<3 m) porewater in May is clearly linked to the presence of a DOC rich USZ, but the USZ does not reach depths greater than 2 m in October, possibly due to changes in wave or wind action, which have temporal patterns.

N 2 O Flux Estimates and EF5
A review of 56 studies of N 2 O fluxes from estuarine sediments found fluxes to range from −2.9 to 25.2 µmol•m −2 •h −1 from intertidal sediments, with an estimated global intertidal N 2 O flux estimate of 0.007 to 0.05 Tg N 2 O-N•year −1 [13].These values are low compared to global estimates of N 2 O flux from estuarine open water, but are similar to estimates from mangroves and salt marshes.We estimate average N 2 O fluxes to range from 0.06 to 0.07 µmol/m shoreline/h at SBH, which are the low end of fluxes observed from intertidal sediments (Figure 5).Our estimated fluxes are around two orders of magnitude greater at PJH than SBH, with average values ranging between 2.7 and m (Figure 5).In addition to the different DOC concentrations of the two aquifers and its control on the N 2 O concentrations, the difference in fluxes between our two sites results from the large range in SGD rates from the two sites.The large range in flux values at two nearby sites points out the difficulty in establishing global N 2 O fluxes from coastal aquifers and the strong need to distinguish the range in types of coastal aquifers and their control on SGD, which is poorly constrained (e.g., [55]).
The average EF5 values for all porewater sampled in PJH and SBH were 0.002 and 0.0007 respectively.The average value for PJH is similar to the value of 0.0025 used by the IPCC for groundwater, estuaries and rivers [3], but the EF5 for SBH is about 70% lower.The flux of N 2 O from groundwater to the atmosphere is thought to be altered by transformations that occur in the overlying soil, such as nitrification [56].Direct diffusive flux from aquifers to the atmosphere appears to be highly variable and depend on aquifer composition [57].Our calculated EF5 values are an average of all porewater N 2 O concentrations at each site and therefore differences in N 2 O flux, driven by DOC inputs at SBH, are masked by the high N 2 O concentrations in the freshwater portion of the aquifer.In addition, mean EF5 values at PJH are like those measured for groundwater and thus it is possible that the mixing of oxygenated seawater with oxygenated groundwater does not set up a sufficient geochemical gradient to alter the N 2 O concentrations in the aquifer, which are derived from the groundwater endmember.

Conclusions
Production of N 2 O in the coastal aquifers of PJH and SBH was regulated by high concentrations of NO 3 − , but N 2 O concentrations in shallow porewater were altered in the presence or absence of a DOC rich USZ.At SBH, the USZ was present due to tidal circulation of saline surface water.Saline surface water carries DOC which fuels denitrification resulting in a larger portion of NO 3 − reduced to excess N 2 as opposed to N 2 O.At PJH a large hydraulic gradient between groundwater and surface water prevented the formation of an upper saline zone, which limited the introduction of DOC and elevated the N 2 O concentrations in shallow porewater.Average EF5 values for intertidal sediments of the coastal aquifers studied here are similar to those reported in the IPCC for estuaries, groundwater and rivers.An increase in anthropogenic nitrogen loading to coastal aquifers is likely to increase the total N 2 O production in the groundwater endmember of coastal aquifers, ostensibly increasing N 2 O flux to the atmosphere.Our data suggest processes such as sea level rise, which shallow the hydraulic gradient and result in the formation of upper saline zones in coastal aquifers, may offset some N 2 O flux to the atmosphere as DOC inputs drive denitrification processes to completion, thereby removing N 2 O from porewater prior to SGD.

Figure 1 .
Figure 1.Site map of Port Jefferson Harbor coastal aquifer (▪) and Stony Brook Harbor (SBH) coastal aquifer (•) located on embayment's of Long Island Sound (left).Cross section of salinity at Port Jefferson Harbor during (a) May 2011 and (b) October 2011 and Stony Brook Harbor in (c) May 2011 and (d) October 2011.Sampling points (•) are shown for porewater samples at both sites (a-d).

Figure 1 .
Figure 1.Site map of Port Jefferson Harbor coastal aquifer ( ) and Stony Brook Harbor (SBH) coastal aquifer ( ) located on embayment's of Long Island Sound (left).Cross section of salinity at Port Jefferson Harbor during (a) May 2011 and (b) October 2011 and Stony Brook Harbor in (c) May 2011 and (d) October 2011.Sampling points ( ) are shown for porewater samples at both sites (a-d).

Figure 2 .
Figure 2. Cross section of Nitrous oxide (N2O) concentrations (nmol•L −1 ) for Port Jefferson Harbor in (a) May and (b) October and Stony Brook Harbor in (c) May and (d) October.Black dots (•) indicate individual porewater sampling points.

Figure 2 .
Figure 2. Cross section of Nitrous oxide (N 2 O) concentrations (nmol•L −1 ) for Port Jefferson Harbor in (a) May and (b) October and Stony Brook Harbor in (c) May and (d) October.Black dots ( ) indicate individual porewater sampling points.

Figure 3 .
Figure 3. Box-and-whisker plots of (a) N 2 O concentrations; (b) dissolved organic carbon concentrations; (c) NO 3 − concentrations; (d) dissolved oxygen % saturation and (e) N 2 denitrification concentrations for SBH and PJH porewater in May and October.

Figure 4 .
Figure 4. Principal component analysis (PCA) of the major nitrogen cycling geochemical solutes in porewater from (a) Port Jefferson Harbor and (b) Stony Brook Harbor.
the measured SGD rates by the subtidal shallow N2O concentration yields an average N2O flux from PJH of 448 µ mol/m shoreline/d and 246 µ mol/m shoreline/d for May and October sampling respectively (Figure 5).At the discharge point shoreward of the upper saline zone at SBH N2O flux averaged 26.5 µ mol/m shoreline/d and 26.1 µ mol/m shoreline/d for May and October respectively (Figure 5).Errors were calculated based on the range of measured SGD rates recorded at the two sites during sampling times in May and October of 2011.Error bars encompass the standard deviation for SGD measurements for each site and thus give an overall range of N2O fluxes calculated for each site and sampling date.Only the shallowest N2O concentration was used for each site and sampling date.

Figure 4 .
Figure 4. Principal component analysis (PCA) of the major nitrogen cycling geochemical solutes in porewater from (a) Port Jefferson Harbor and (b) Stony Brook Harbor.

3. 2 .
Flux of N 2 O to Surface Waters Multiplying the measured SGD rates by the subtidal shallow N 2 O concentration yields an average N 2 O flux from PJH of 448 µmol/m shoreline/d and 246 µmol/m shoreline/d for May and October sampling respectively (Figure 5).At the discharge point shoreward of the upper saline zone at SBH N 2 O flux averaged 26.5 µmol/m shoreline/d and 26.1 µmol/m shoreline/d for May and October respectively (Figure 5).Errors were calculated based on the range of measured SGD rates recorded at the two sites during sampling times in May and October of 2011.Error bars encompass the standard deviation for SGD measurements for each site and thus give an overall range of N 2 O fluxes calculated for each site and sampling date.Only the shallowest N 2 O concentration was used for each site and sampling date.

Figure 5 .
Figure 5. N2O flux from a fresh coastal aquifer (PJH) and a coastal aquifer containing an USZ (SBH).Error bars are calculated from the range of submarine groundwater discharge (SGD) flux measurements taken for each site during the respective time periods.

Figure 5 .
Figure 5. N 2 O flux from a fresh coastal aquifer (PJH) and a coastal aquifer containing an USZ (SBH).Error bars are calculated from the range of submarine groundwater discharge (SGD) flux measurements taken for each site during the respective time periods.
) is the concentration of dissolved N 2 O and N 2 denitrification is the amount of N 2 attributable to denitrification in the porewater sample.Here we used initial nitrate concentration to calculate EF for two coastal aquifers (EF5): − -N final (µmol•L −1 ) is the concentration of nitrate in the porewater sample, N 2 O (µmol•L −1 Dissolved oxygen saturation at PJH averaged 52.8% in May and 58.4% in October and was similar, but with a wider range, at SBH with values of 70.8% in May and 56.6% in October (Figure3d).Excess N 2 was not detected in all samples, with average concentrations at PJH of 42 ± 37 µmol•L −1 in May and 58.4 ± 8.4 in October, with similar values at SBH with average concentrations of 58.3 ± 57 µmol•L −1 in May and 30.8 ± 57 µmol•L −1 in October (Figure − concentrations at PJH averaged 281.6 ± 94.1 µmol•L −1 and 168.1 ± 73.4 µmol•L −1 in May andOctober respectively, and were similar at SBH, with average concentrations of 187.2 ± 113.2 µmol•L −1 in May and 315.6 ± 24.6 µmol•L −1 in October (Figure3c).

Table 1 .
Emission factors (EF5).Mean and median calculated from all porewater samples at each coastal aquifer.

Table 1 .
Emission factors (EF5).Mean and median calculated from all porewater samples at each coastal aquifer.
[11]concentration does not necessarily predict N 2 O concentration, unlike West Falmouth Harbor[11].The relationship between all the variables can be gleaned from PCA results.At PJH, PCA results indicate that NO 3 − and N 2 O are positively correlated and are both inversely correlated with N 2 denitrification .Salinity, DOC, DO% and distance offshore (below mean high tide) are positively correlated with each other but are orthogonal to N 2 O, NO 3 − and N 2 denitrification , indicating little correlation between these factors.The close relation between salinity, DOC, distance offshore and DO% reflects a saline water source of DOC and that saltwater intrusion depends on the distance offshore below mean high tide at PJH. NO 3 − and N 2 O reduction produces N 2 denitrification , (Equation (2)) resulting in their inverse correlation (Figure 4a).At SBH, salinity and DOC are closely linked because of the USZ, which is missing at PJH (Figure 1).A strong inverse relationship between DOC and N 2 O indicates DOC drives the reduction of N 2 O.The reduction of N 2 O should result in the formation of N 2 denitrification , although this is not evident in the PCA analysis as N 2 O and N 2 denitrification are orthogonal to each other.In both PJH and SBH the third principal component explains 12%-14% of the variability and depends primarily on NO 3 − concentrations which suggests the porewater NO 3 − is independent of the other coastal aquifer parameters.Anthropogenic NO 3 − comprises the bulk of DIN inputs to the coastal aquifer system [46], therefore NO 3 − concentrations are dictated more by inputs to the upper Glacial aquifer than processes in the coastal zone.Given the overall low concentrations of N 2 denitrification and N 2 O, when compared to other groundwater investigations in temperate sediments , and single time point sampling can lead to inaccuracies in mean annual N 2 O flux to the atmosphere.At SBH and PJH average N 2 O concentrations were greater in May than in October, despite no statistical difference in NO 3 − concentrations.The location of the maximum concentration changed between sample times; at PJH, maximum N 2 O concentrations occurred in the shoreward-most porewater at depths less than 2 m in May while maximum N 2 O concentrations occurred near high tide at depths greater than 2 m in October (Figure 2a,b).At SBH, maximum N 2 O concentrations occurred in the landward portion of the freshwater zone, at depths greater than 3 m in May, but in October maximum N 2 O concentrations occurred at shoreward depths greater than 3 m and at shallow depths less than 1 m (Figure