Chemosphere 119 (2015) 1281–1288
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Substantial nitrous oxide emissions from intertidal sediments and groundwater in anthropogenically-impacted West Falmouth Harbor, Massachusetts Serena Moseman-Valtierra a,b,⇑, Kevin D. Kroeger b, John Crusius b,c, Sandra Baldwin b, Adrian Green b, T. Wallace Brooks b, Emily Pugh b a
Department of Biological Sciences, University of Rhode Island, 120 Flagg Road, Kingston, RI 02881, United States US Geological Survey, Woods Hole Coastal and Marine Science Center, Woods Hole, MA 02543, United States c USGS at UW School of Oceanography, 1492 NE Boat Street, Box 355351, Seattle, WA 98195, United States b
h i g h l i g h t s Substantial summer N2O emissions were observed from intertidal sediments of a coastal estuary. Hotspot of intertidal N2O emissions was located in a known wastewater plume. Intertidal N2O emissions likely reﬂected local production and groundwater sources.
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Article history: Received 20 December 2013 Received in revised form 25 September 2014 Accepted 10 October 2014 Available online 6 November 2014 Handling Editor: X. Cao Keywords: Greenhouse gases Nitrogen Estuary Wastewater Denitriﬁcation Nitriﬁcation
a b s t r a c t Large N2O emissions were observed from intertidal sediments in a coastal estuary, West Falmouth Harbor, MA, USA. Average N2O emission rates from 41 chambers during summer 2008 were 10.7 mol N2O m2 h1 ± 4.43 lmol N2O m2 h1 (standard error). Emissions were highest from sediments within a known wastewater plume, where a maximum N2O emission rate was 155 lmol N2O m2 h1. Intertidal N2O ﬂuxes were positively related to porewater ammonium concentrations at 10 and 25 cm depths. In groundwater from 7 shoreline wells, dissolved N2O ranged from 488% of saturation (56 nM N2O) to more than 13 000% of saturation (1529 nM N2O) and was positively related to nitrate concentrations. Fresh and brackish porewater underlying 14 chambers was also supersaturated in N2O, ranging from 2 980% to 13 175% of saturation. These observations support a relationship between anthropogenic nutrient loading and N2O emissions in West Falmouth Harbor, with both groundwater sources and also local N2O production within nutrient-rich, intertidal sediments in the groundwater seepage face. N2O emissions from intertidal ‘‘hotspot’’ in this harbor, together with estimated surface water emissions, constituted 2.4% of the average overall rate of nitrogen export from the watershed to the estuary. This suggests that N2O emissions factors from coastal ecosystems may be underestimated. Since anthropogenic nutrient loading affects estuaries worldwide, quantiﬁcation of N2O dynamics is warranted in other anthropogenically-impacted coastal ecosystems. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Anthropogenic activities have substantially modiﬁed global nitrogen cycles (Galloway et al., 2008). Reactive nitrogen on land has roughly doubled, mainly through fertilizer production. Atmospheric nitrous oxide (N2O) concentrations have increased ⇑ Corresponding author at: Department of Biological Sciences, University of Rhode Island, 120 Flagg Road, Kingston, RI 02881, United States. Tel.: +1 401 874 7474. E-mail address: [email protected]
(S. Moseman-Valtierra). http://dx.doi.org/10.1016/j.chemosphere.2014.10.027 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
by approximately 20% since pre-industrial times (Galloway et al., 2008). N2O destroys stratospheric ozone and wields approximately 300 times the global warming potential of carbon dioxide (CO2) (Denman et al., 2007; Forster et al., 2007). N2O is an intermediate in denitriﬁcation (microbial reduction of nitrate to N2 gas). Complete denitriﬁcation typically terminates in reduction of N2O to inert N2, so this process is typically a sink for N2O. However, increasing nitrogen supply to soils can promote early termination of denitriﬁcation (Dalal et al., 2003; Liu and Greaver, 2009). N2O is also produced as a byproduct of nitriﬁcation (the oxidation of
S. Moseman-Valtierra et al. / Chemosphere 119 (2015) 1281–1288
NH+4 to NO 3 ) and other microbially-mediated nitrogen transformations (Smith and Zimmerman, 1981; Wrage et al., 2001). Coastal ecosystems, including estuaries and bays, can be intensely active sites of biogeochemical cycling, but they have largely been overlooked in global budgets of N2O (Bange, 2006; Naqvi et al., 2010). As recipients of nitrogen loading from agricultural and urban run-off, atmospheric deposition, and/or from wastewater inputs, coastal sediments are potential hot spots for nitrogen cycling (Bowen et al., 2007). Coastal ecosystems experiencing high anthropogenic nutrient inputs and hypoxia (Naqvi et al., 2010; Moseman-Valtierra et al., 2011) can release substantial amounts of N2O (Bange, 2006). In many coastal areas, groundwater is a major means by which nitrogen from wastewater (including septic tanks) and watersheds enters coastal bays and estuaries (LaMontagne et al., 2003; Kroeger et al., 2006a). Groundwater inputs may fuel microbial pathways that produce (or consume) N2O, and they can also be direct sources of N2O to coastal waters (Crusius et al., 2008). Nitrogen loading can stimulate both nitriﬁcation and denitriﬁcation, and these processes can be tightly coupled in coastal sediments (Jenkins and Kemp, 1984). The regular inundation and exposure of intertidal sediments creates alternating conditions in which N2O could be produced by both aerobic nitriﬁcation and anaerobic nitrate reduction (Blackwell et al., 2010). Dynamic changes in groundwater and porewater discharge and recharge occur with each tidal cycle in intertidal sediments (Robinson et al., 1998) resulting in delivery of nutrients and oxygen, and thereby potentially promoting N2O production and emissions. The magnitude and mechanisms behind N2O ﬂuxes in coastal ecosystems, including intertidal sediments, need to be known in order to predict impacts of anthropogenic nitrogen pollution. Net ﬂuxes of N2O are important to quantify in coastal bays and estuaries, particularly if they are increasing in response to global phenomena such as eutrophication and anoxic ‘‘dead’’ zones (Naqvi et al., 2010). Herein, we examine spatial patterns of N2O ﬂuxes from intertidal sediments in a shallow coastal embayment at West Falmouth Harbor on Cape Cod, MA. The objective was to quantify diffusive N2O ﬂuxes from intertidal sediments throughout the harbor during low tide, within and beyond the region of a wastewater plume in the harbor’s watershed. Forty-one ﬂux measurements were completed in the harbor throughout the summer of 2008 and a subset of those same locations were also measured in Fall 2007 and Fall 2008. Sites varied in proximity to a known discharge path for nitrogen-enriched groundwater, originating from a wastewater treatment plant. Porewater and shallow groundwater properties including nutrient concentrations and dissolved N2O concentrations were also determined to provide information regarding the effect of land use and terrestrial nitrogen sources on N2O ﬂuxes.
2. Materials and methods 2.1. Site description West Falmouth Harbor is a small estuary (76 hectares), in Buzzards Bay, MA, with a 1 m tidal range. Its watershed consists primarily of residential properties, but a wastewater treatment plant in the Snug Harbor subwatershed (Kroeger et al., 2006b) has been operating since 1986. For most of that time, secondary treated efﬂuent has been disposed of by placing it in sand ﬁlter beds and by spraying it on grass and pine and oak forests. The efﬂuent percolates through 30 m of vadose zone and is ultimately transported into West Falmouth Harbor through the groundwater (Kroeger et al., 2006b). The plant is estimated to contribute about 40% of the total land-derived nitrogen load to the harbor. Average
total dissolved nitrogen concentrations (TDN) in this plume are about 140 lM N (Kroeger et al., 2006b). For context, average TDN concentration in fresh, near shore groundwater in Cape Cod watersheds with low population density is typically between 30 and 50 lM, and average concentrations at moderate to high population density are typically in the range of 100–300 lM (Kroeger et al., 2006a). 2.2. Gas ﬂux measurements Single ﬂux measurements were made at forty-one locations using static ﬂux chambers, deployed over a distance of about 1.5 km in the sandy intertidal zone of West Falmouth Harbor between June and August 2008 (summer 2008). To more closely examine a ‘‘hotspot’’ of observed N2O ﬂuxes in the northeastern section of the harbor (Fig. 1), fourteen of the forty-one measurements were arranged in two 12-m long cross-shore transects (spaced 6 m apart) spanning from the low water line to the edge of the adjacent salt marsh. Each of the two transects consisted of 6 chambers, with two chambers between the transects (inset Fig. 1). N2O ﬂuxes were also measured at 4 sites during Fall 2007 and at 5 sites during Fall 2008 (Table 1) in northeastern portions of the harbor. To measure intertidal N2O ﬂuxes, transparent polycarbonate chambers (20 cm tall, 28 cm diameter) were gently placed onto moist sediment. A shallow layer (about 3 cm deep) of additional sediment was gently placed around the exterior rim of each chamber to help prevent gas exchange between the outside and inside of the chamber. Gases were circulated continuously in a closed loop of Tygon tubing using a peristaltic pump and a battery powered fan. For pressure equilibration, a 55 cm length of 0.8 mm inner diameter steel tubing on each chamber was open to the atmosphere. Discrete gas samples were drawn from chambers using 60-ml nylon syringes with stopcock valves (Cole Parmer) at initial time points and every 2 min for 8 min, as well as after 13, 18, 23, and 28 min. Samples were kept cool and dark in the nylon syringes and analyzed within 24 h. Prior laboratory tests conﬁrmed that less than 5% loss of N2O occurred over 24 h with this storage method. Temperatures were measured inside of each chamber using Hobo pendant (Onset Inc.) data loggers. Fluxes were calculated from chamber height and linear periods of change in gas concentrations using the ideal gas law (Healy et al., 1996). Qualitative observations were recorded of sediment type (gravel, rocky, sandy, muddy) and presence or absence of surface macro- and microalgae within each chamber. 2.3. Nutrient sampling Nutrient samples were collected from surface pools within 21 chambers throughout West Falmouth Harbor between June 30 and August 4, 2008 (hereafter referred to as ‘‘surface’’ samples at 0 cm depth in the sediment). Additional porewater samples were collected at depths of 5 cm, 10 cm, and 25 cm for 14 chambers at the N2O ‘‘hot spot’’ (inset of Fig. 1), in 15 ml volumes, and porewater salinities were determined in the laboratory using a YSI 650 multiprobe. All porewater samples were collected using a stainless steel sipper that is designed for the sandy, permeable sediments commonly present in the intertidal zone of such estuaries (Berg and McGlathery, 2001). Water samples were immediately ﬁltered (0.2 lm polyethersulfone), stored on ice, and then frozen until analyzed for phosphate, nitrate, nitrite, and ammonium concentrations. All nutrient analyses were conducted by the Woods Hole Oceanographic Institution Nutrient Analytical Facility, using a Lachat autoanalyzer. N2O concentrations in porewater were sampled from the same 14 chambers. Water was gently drawn, as described above, at 5, 10,
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Fig. 1. Intertidal N2O ﬂuxes in West Falmouth Harbor, MA, during summer 2008. Each circle represents one chamber deployment. Colors indicate ﬂux sizes. Shaded region marks a previously described wastewater plume. Numbers indicate positions of groundwater wells sampled in this study. Star symbols indicate approximate locations of Fall chamber deployments Inset: Intertidal N2O ﬂuxes measured along a cross-shore transect located in Snug Harbor. Individual chambers are numbered along this transect.
Table 1 Porewater N2O and nutrient concentrations in West Falmouth Harbor during summer 2008 (– means data not available). Chamber ID
4 25 26 27 28 29 30 31 32 33 34 35 36 37
Porewater nitrate concentrations (lM)
Porewater ammonium concentrations (lM)
Porewater N2O (% saturation)
– 59.69 262.98 158.25 89.5 221 – 147.33 252.38 265.91 67.02 – 242.72 95.03
84.55 104.4 248.8 191.64 235.53 238.03 – 262.6 218.91 173.69 191.85 67.4 175.67 89.05
86.24 161.3 80.19 181.44 188.21 230.11 278.43 329.36 241.13 210.89 322.86 223.73 161.58 132.15
59.99 193.78 129.74 269.1 251.03 310.99 213.89 341.45 218.66 – 335.7 161.84 291.32 187.55
– 1.02 0.07 2.82 2.51 1.21 – 0.74 5.95 0.7 12.34 – 3.36 34.06
0.46 1.64 13.75 5.42 1.12 24.38 – 1.23 7.45 1.92 3.47 3.25 12.98 4.18
0.49 1.31 3.51 2.95 11.83 1.85 1.28 19.71 1.97 1.18 18.14 7.39 4.49 4.22
0.74 0.54 0 0.29 0.11 0.62 3.52 0 0.06 – 0.36 2.42 0.24 7.37
3652 5264 5279 3021 3300 – 3652 4464 7790 5778 11 463 – 4743 3035
3100 5267 4785 1384 3290 3074 3100 6996 8114 5929 8953 – 4633 2572
3452 5614 5517 2807 2980 3283 3452 6498 13 175 – – – 5793.5 4055
Salinity of water on sediment surface (psu) – 0.2 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 1.6 – 0.8
and 25 cm depths below the sediment surface into 60 ml sterile polypropylene syringes with attached Luer-Lok tips attached to stopcocks and stored in cool water. Upon return to the laboratory (within a few hours), the samples were equilibrated with helium gas (ultra-high purity) at a 1:1 ratio of helium to water by shaking vigorously for 1 min. The equilibrated headspace was injected directly into the GC and N2O concentrations were calculated as described below. Salinity and temperature of these water samples were recorded. Salinity for all porewater samples was