Factors controlling methane and nitrous-oxide variability in the southern British Columbia coastal upwelling system

    Factors controlling methane and nitrous-oxide variability in the southern British Columbia coastal upwelling system David W. Capelle, Philippe D. Tortell PII: DOI: Reference:

S0304-4203(16)30011-1 doi: 10.1016/j.marchem.2016.01.011 MARCHE 3348

To appear in:

Marine Chemistry

Received date: Revised date: Accepted date:

23 July 2015 29 January 2016 29 January 2016

Please cite this article as: Capelle, David W., Tortell, Philippe D., Factors controlling methane and nitrous-oxide variability in the southern British Columbia coastal upwelling system, Marine Chemistry (2016), doi: 10.1016/j.marchem.2016.01.011

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ACCEPTED MANUSCRIPT Factors controlling methane and nitrous-oxide variability in the southern British Columbia

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coastal upwelling system

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Department of Earth, Ocean, and Atmospheric Sciences, University of British Columbia

Department of Botany, University of British Columbia

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2

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Vancouver, British Columbia, Canada

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Vancouver, British Columbia, Canada

Corresponding Author:

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David W. Capelle1, Philippe D. Tortell1, 2

David W. Capelle

Dept. of Earth, Ocean, and Atmospheric Sciences, University of British Columbia 2207 Main Mall, Vancouver, British Columbia, Canada V6T 1Z3 [email protected]

ACCEPTED MANUSCRIPT Abstract Coastal upwelling systems are important marine sources of methane (CH4) and nitrous-oxide

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(N2O). Current understanding of the controls on CH4 and N2O distributions in these coastal

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waters is restricted by limited data availability. We present the first multi-year measurements of

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CH4 and N2O distributions from the seasonally upwelling shelf waters of British Columbia, Canada, a coastal end-member of the north Pacific oxygen minimum zone (OMZ). Our data

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show significant seasonal differences in CH4 and N2O distributions and fluxes driven

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predominantly by upwelling. Methane is supplied to the water column primarily from sediments (especially near methane seeps), and is transported to the surface mixed layer by upwelling. A

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positive correlation between CH4 concentrations and salinity indicates limited inputs from Fraser

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River estuary waters to the study site. Shelf waters receive N2O from a deep, off-shelf N2O maximum in the OMZ core, and from nitrification in the water column and possibly sediments.

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Both the physical transport of N2O and its apparent in situ production are enhanced under upwelling conditions. N2O yields from nitrification, estimated from changes in N2O and nitrate + nitrite (NO3-+NO2-) along isopycnals, ranged from 0.04 – 0.49%, with the highest values observed under low ambient O2 concentrations. Sea-air fluxes ranged from -4.5 – 21.9 µmol m-2 day-1 for N2O and 2.5 – 34.1 µmol m-2 day-1 for CH4, with the highest surface fluxes observed following summer upwelling over the broad continental shelf of southern Vancouver Island. Our results provide new insight into the factors driving spatial and inter-annual variability in marine CH4 and N2O in high productivity coastal upwelling regions. Continued time-series measurements will be invaluable in understanding the longer-term impacts of climate-driven variability on marine biogeochemical cycles in these dynamic near-shore waters.

ACCEPTED MANUSCRIPT Highlights Seasonal differences in CH4 and N2O distributions and fluxes are linked to upwelling

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Seeps appear be important sources of water column CH4

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Nitrification is dominant pathway of in situ N2O production

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Upwelling increases advective supply and in situ production of N2O in shelf waters

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N2O yields from nitrification increase under low O2 availability

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Keywords

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Methane; Nitrous Oxide; Upwelling; Continental Shelf; Nitrification; Seep

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Abbreviations

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WCVI – West Coast of Vancouver Island OMZ – Oxygen Minimum Zone

ACCEPTED MANUSCRIPT Introduction Methane (CH4) and nitrous-oxide (N2O) are the most important greenhouse gases after

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carbon-dioxide and water-vapour, accounting for ~ 17% and 6% of the global radiative forcing

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of all greenhouse gases, respectively (IPCC, 2013). These gases are actively cycled in low

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oxygen sub-surface ocean waters and sediments, where intensive microbial activity drives a diverse suite of metabolic pathways.

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The major processes driving marine N2O cycling are nitrification and denitrification. N2O

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is produced as a by-product of nitrification (step-wise oxidation of ammonium (NH4+) to nitrite (NO2-) and nitrate (NO3-)), which is carried out by a variety of chemo-autotrophic bacteria and

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archaea under oxic to nearly anoxic conditions (Casciotti and Buchwald, 2012; Freing et al.,

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2012). N2O yields from marine nitrification (i.e. mol N2O produced per mol NO2-+NO3-

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produced) are highly variable, ranging from 0.004 – 0.4 % (De Wilde and De Bie, 2000; Frame and Casciotti, 2010; Goreau et al., 1980; Punshon and Moore, 2004; Santoro et al., 2011; Stieglmeier et al., 2014), and have been shown to increase under low oxygen conditions (Frame and Casciotti, 2010; Goreau et al., 1980; Stieglmeier et al., 2014). The change in N2O yield may be due to the tendency of nitrifiers to preferentially reduce NO2- to N2O (nitrifier-denitrification) under O2-limitation (Frame and Casciotti, 2010). Denitrification (the step-wise reduction of NO3 to N2 via NO2-, nitric oxide (NO) and N2O) is typically confined to waters with <5 µM O2 (Codispoti et al., 2001), and is ultimately a sink of N2O under anoxic conditions. However, the enzyme N2O-reductase is more O2-sensitive than the other N-reductase enzymes in denitrification, resulting in N2O accumulation by partial/incomplete denitrification under very low (sub-micromolar) O2 concentrations (Betlach and Tiedje, 1981; Dalsgaard et al., 2014). Indeed, denitrification appears to be a dominant source of N2O in suboxic marine waters such as

ACCEPTED MANUSCRIPT the Arabian Sea and West-Indian continental shelf (Bange et al., 2001; Codispoti et al., 2001; Jayakumar et al., 2009; Naqvi et al., 2000), and at the peripheries of OMZs (Bange, 2008;

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Castro-González and Farías, 2004). The highest N2O production rates thus occur in low oxygen

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waters, where high N2O-yields from nitrification co-occur with net- N2O production from denitrification. Recent evidence suggests that N2O may be produced during dissimilatory

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reduction of nitrate to ammonium (Welsh et al., 2001), but this process appears to be confined to

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anoxic or very low O2 (< 10µM) waters (Lam et al., 2009).

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Oxygen levels also exert a significant control on the marine CH4 cycle. Until relatively recently, this gas was thought to be produced exclusively under anaerobic conditions during the

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biological or thermogenic breakdown of organic matter. Anaerobic CH4-producing environments

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are generally confined to organic-matter rich sediments or within the earth’s crust, although they can also be present inside sinking particles or digestive tracts of marine organisms (De Angelis

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and Lee, 1994; Holmes et al., 2000; Oremland, 1979; Sansone et al., 2001). Sediment-derived CH4 is often mostly consumed by methanotrophic organisms, thus limiting CH4 fluxes to the atmosphere, although the abundance of methanotrophs and their ability to consume CH4 from sediments can be highly variable (Reeburgh, 2007; Steinle et al., 2015). Methane from subsurface organic deposits may migrate upward through coarse grained sediments or tectonic faults and escape the water column in seep-derived bubbles, which can enhance CH4 transport into the mixed layer (Reeburgh, 2007; Rehder et al., 2009, 2002; Solomon et al., 2009). The release of CH4 from these seeps shows strong spatial and temporal variability over a range of time-scales (from hours to years; Boles, Clark, Leifer, & Washburn, 2001; Leifer & Boles, 2005; Tryon et al., 1999), and represents a potentially underestimated source of atmospheric CH4. Moreover, the potential destabilization of CH4-rich clathrate deposits under various ocean

ACCEPTED MANUSCRIPT warming scenarios has prompted significant research effort in recent years (Archer, 2007; Solomon et al., 2009; Sowers, 2006). There has also been increased interest in other in situ

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sources of CH4 in oxygenated marine surface waters, including the cleavage of methyl-groups

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from larger molecules, such as methylated sulfides (Damm et al., 2010; Florez-Leiva et al., 2013) and methylphosphonate (Cooke et al., 2012; Karl et al., 2008). Due to their proximity to

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the ocean-atmospheric interface, these surface water pathways of CH4 production may be

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important controls on sea-air CH4 fluxes.

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Coastal upwelling regions are sites of active CH4 and N2O cycling, and disproportional contributors to the global marine emissions of these gases to the atmosphere (Bange, 2008;

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Nevison et al., 2004; Rehder et al., 2002; Sansone et al., 2001). High CH4 and N2O fluxes have

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previously been demonstrated in a number of upwelling regions (Bange et al., 1996, 1994; Pierotti and Rasmussen, 1980; Sansone et al., 2001), and recent research efforts have explicitly

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examined the effects of upwelling on coastal CH4 and N2O distributions in the waters of coastal Peru (Kock et al., 2015), Chile (Cornejo and Farías, 2012; Farías et al., 2015), Mauritania in NW Africa (Kock et al., 2008; Wittke et al., 2010), California (Cynar and Yayanos, 1992; Lueker et al., 2003; Nevison et al., 2004), Oregon (Rehder et al., 2002), and the Arabian Sea (Bange et al., 2001). In these upwelling systems, high surface productivity results in significant fluxes of organic carbon to sub-surface waters, fuelling microbial O2-demand and driving redox gradients that favor N2O and CH4 production at relatively shallow depths (Naqvi et al., 2010; Sansone and Popp, 2001). Upwelling can also act to transport CH4 and N2O-rich sub-surface water into the mixed layer (Bange et al., 2001; Cornejo and Farías, 2012; Lueker et al., 2003; Naqvi et al., 2010; Nevison et al., 2004; Rehder et al., 2002). Short-term variability in upwelling over periods of hours to months has been shown to influence CH4 and N2O fluxes from coastal upwelling

ACCEPTED MANUSCRIPT systems (Bange et al., 2001; Cornejo and Farías, 2012; Lueker et al., 2003; Rehder et al., 2002; Wittke et al., 2010). Upwelling of O2-depleted water has been linked to high N2O production

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rates and sea-air fluxes in a number of coastal systems, including the equatorial Pacific

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upwelling zone of Chile and Peru (Cornejo and Farías, 2012; Farías et al., 2009), and the Arabian Sea (Bange et al., 2001). The ongoing expansion and intensification of OMZs (Falkowski et al.,

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2011; Keeling et al., 2010; Stramma et al., 2010; Whitney et al., 2007) and the intensification of

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coastal upwelling due to stronger land-sea atmospheric pressure gradients (Bakun, 1990; Bylhouwer et al., 2013; Wang et al., 2015) – both of which are predicted effects of climate

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change – may thus lead to increased CH4 and N2O fluxes from coastal upwelling systems (Codispoti, 2010; Codispoti et al., 2001; Naqvi et al., 2010; Rehder et al., 2002). In addition to

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upwelling, other factors including eutrophication, sedimentary diffusion, freshwater inputs, and local bathymetry also appear to influence coastal N2O and CH4 distributions, resulting in high

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spatial and temporal variability in surface water concentrations and sea-air fluxes. This variability, combined with a scarcity of data, limits our ability to quantify CH4 and N2O emissions in coastal upwelling systems, and our understanding of longer-term (e.g. inter-annual) responses to environmental forcing. In this article, we present new field data documenting the seasonal and inter-annual variability in CH4 and N2O concentrations and sea-air fluxes along the west coast of Vancouver Island (WCVI), British Columbia (BC). This coastal region is characterized by high seasonal productivity, resulting from wind-driven summer time upwelling. Our study site lies in close proximity to the large oxygen-minimum zone (OMZ) of the subarctic North Pacific, which supplies O2-depleted water to the shelf during upwelling (Crawford and Peña, 2013). The persistently low O2 levels in these waters have been declining in recent decades (Crawford and

ACCEPTED MANUSCRIPT Peña, 2013; Whitney et al., 2007), and this has increased the likelihood of periodic upwelling of hypoxic water onto the shelf (Roegner et al., 2011), potentially enhancing the sea-air flux of CH4

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and N2O. Our study site also contains a number of sedimentary bubble plumes (seeps), which

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have been identified as important sources of CH4 to the water column along the Oregon Coast (Grant and Whiticar, 2002; Heeschen et al., 2005; Suess et al., 1999). The combination of

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upwelling, intensifying shelf hypoxia, and presence of seeps make the WCVI a potentially

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significant site for CH4 and N2O production and high sea-air fluxes. To date, CH4 and N2O

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distributions in this region have not been examined systematically. Based on data obtained from five spring and summer cruises, we present detailed

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observations of the spatial and temporal variability of N2O and CH4 concentrations and sea-air

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fluxes, and use these observations to examine the processes affecting the distributions of these gases in the water column. In particular, we examine the influence of upwelling and fresh water

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(Fraser River) fluxes on CH4 and N2O distributions along the BC continental shelf, the potential contribution of sedimentary bubble-plumes to water column CH4 budgets, and the oxygendependent changes in the N2O yields from nitrification. This work represents a starting point for future time-series observations of CH4 and N2O dynamics in coastal BC waters. Methods Study site The WCVI region is located at the northern end of the eastern, North Pacific upwelling region (Figure 1). The upwelling season typically runs from June to September each year, while downwelling occurs between October and May (Bylhouwer et al., 2013). The onset, duration and intensity of upwelling is variable on an inter-annual basis, and this variability has been

ACCEPTED MANUSCRIPT associated with the Pacific Decadal Oscillation (PDO) and El-Niño Southern Oscillation (ENSO) (Bylhouwer et al., 2013). There is a permanent oxygen minimum zone (OMZ; defined as O2

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concentrations less than 20 µM) located between 800-1200 m depth in the water directly

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adjacent to the continental shelf (Figure 1, inset). The study area is influenced by several local water masses. The Vancouver Island Coastal Current (VICC) is a buoyancy-driven freshwater

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current that runs in a northerly direction along the coast of Vancouver Island (shore-ward of the

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150m depth contour), fed by the Fraser river freshet (i.e. snow-melt runoff) during the spring and summer, and by coastal mountain rain runoff from Vancouver Island during the fall and winter

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(Foreman, 2000; Masson and Cummins, 1999). The VICC can extend to the bottom of the water column (Masson and Cummins, 1999). Over the outer continental shelf region, surface currents

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are driven by seasonal winds, flowing predominantly northward during winter and south during summer. Sub-surface currents in this outer shelf region are dominated by the northward-flowing

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warm, saline and nutrient-rich California Undercurrent (CUC), with a core depth ranging from 125 – 250 m (Foreman, 2000; Thomson and Krassovski, 2010). A persistent cyclonic eddy (Juan de Fuca Eddy) is found in the southeastern region of our study area (near the coastal LB and LC Line stations, Figure 1) during summer and fall, associated with the Juan de Fuca and Tully canyons. Here, local currents and bottom topography enhance the upward transport of low O2, nutrient-rich deep-water on to the shelf (Crawford and Peña, 2013). Continental shelf sediments along the WCVI are primarily sands and gravels with low organic matter content (<1% by weight), though fine-grained silts and clays with ~3% organic matter content are present in some near-shore locations (Carter, 1973). Large CH4-hydrate deposits have been mapped at depth (>1200 m) off the Vancouver Island coast (Riedel et al.,

ACCEPTED MANUSCRIPT 2002), and CH4 has been observed leaking into seawater along the Cascadia tectonic margin off the west coast of Oregon (Suess et al., 1999).

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Field sampling and gas analysis

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Sample collection took place during 5 coastal research cruises on the CCGS John P.

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Tully in June and September between 2012 and 2014 (cruises 2012-25, 2012-59, 2013-38, 201358, and 2014-21). During each cruise, we collected depth profile samples from 11 stations along

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two perpendicular transects; one along-shore coastal transect, and one cross-shelf transect

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(Figure 1). Additional surface (5m) samples were collected during the June 2014 cruise (Figure 1) in order to derive more spatially resolved sea-air flux estimates. Discrete water column

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samples for dissolved CH4 and N2O analysis were collected using a rosette equipped with 12 L

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Niskin bottles. Water from the Niskin bottle was transferred to 60 mL glass vials (two replicates

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for each sample depth) using a flexible silicon tube in a manner that eliminated bubbles, with vials overfilled three volumes to prevent air contamination. Each vial was immediately poisoned with 100 µL saturated HgCl2 solution, crimp-sealed with rubber butyl stoppers, and stored at 4°C until analysis by automated purge and trap gas-chromatography-mass-spectrometry (PT-GCMS). Our method, described in detail by Capelle et al. (2015), provides an average precision of 3%, and detection limits of 0.4 nM for both CH4 and N2O when purging 5 mL of sample water. The median difference between all duplicate concentration measurements in this study was less than 5%. The rosette was equipped with a CTD (SBE-911plus) and oxygen sensor (SBE 43) to measure salinity, temperature, and oxygen. Discrete measurements of oxygen and NO2-+NO3were also made at each depth following protocols employed the Institute of Ocean Sciences (Barwell-Clarke and Whitney, 1996). Upwelling Intensity

ACCEPTED MANUSCRIPT Upwelling indices for 48.0°N, 125.0°W were obtained with 6-hourly resolution from the Pacific Fisheries Environmental Laboratory (http://www.pfeg.noaa.gov/products/). The

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upwelling index provides a measure of the strength of upwelling or downwelling-favourable

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winds. It is derived from the atmospheric pressure gradients along the coastal ocean margin and Ekman transport calculations. Upwelling (downwelling) occurs when winds are blowing parallel

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to the coastline in a southerly (northerly) direction. To determine a characteristic time-scale over

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which upwelling influenced shelf water properties, we computed the mean values of salinity, temperature, oxygen, and NO2-+NO3- in sub-surface (below 50 m) waters on the continental shelf

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(maximum depth 200m), and correlated these properties with the mean upwelling intensity derived over a range of time intervals, from 1 to 150 days prior to each cruise (in 10 day

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intervals). From this analysis, we observed the highest correlation coefficients when using an averaging period between 80 and 110 days. This time-scale agrees well with the estimates of

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shelf residence time of ~ three months determined by Ianson et al. (2009). We thus used an averaging window of 90 days to determine the mean upwelling state prior to each cruise. The onset of the upwelling season was calculated according to the method of Bylhouwer et al. (2013), based on the date where the cumulative annual upwelling value equals 10% of the total annual upwelling. River discharge The primary source of freshwater to the WCVI between spring and fall is the Fraser River (Masson and Cummins, 1999). Mean daily discharge values were obtained from the hydrometric gauge station at Hope, BC (Station 08MF005, Environment Canada, 2015). Sea-air fluxes

ACCEPTED MANUSCRIPT Sea-air fluxes of CH4 and N2O (µmolm-2 day-1) were calculated as the product of the air-

(1)

Flux = kw * ΔC = kw * (Cobs - Ceq)

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sea disequilibrium (ΔC) and a gas exchange coefficient (i.e. piston velocity, kw)

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The excess (or deficit) of CH4 or N2O in surface waters relative to saturation values, was

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calculated from the difference between the mean observed (Cobs ) and air-equilibrium (Ceq) concentrations in the mixed layer (or upper 15 m of the water column where the mixed layer was

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not sampled). Air equilibrium CH4 and N2O concentrations were calculated using the equations

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of Weiss & Price (1980) and Wiesenburg & Guinasso (1979) for N2O and CH4 solubility, respectively, using mean monthly atmospheric CH4 and N2O concentrations from 2012, 2013

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and 2014 measured at Barrow, Alaska (Data provided by NOAA ESRL Global Monitoring

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Division, Boulder, Colorado, USA; http://esrl.noaa.gov/gmd/). Piston velocities were computed

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as a function of the Schmidt number for each gas and wind speeds during the period two weeks prior to sampling. Daily wind speeds were derived from the mean of measurements from two moored buoys in our study area (46206 and 46132, data provided by Fisheries and Oceans Canada http://www.meds-sdmm.dfo-mpo.gc.ca/isdm-gdsi/waves-vagues/index-eng.htm) and the NCEP/NCAR daily wind speeds from four locations in our study area (47.5N, 125.0W; 47.5N, 127.5W;50.0N, 127.5W; and 50.0N, 130.0W) (http://www.esrl.noaa.gov/psd/data/, Kalnay et al., 1996). We found reasonably good agreement between these different sources of wind speed data, with an overall standard deviation between the daily buoy and NCEP/NCAR wind speeds of 1.8m/s. We followed the approach of Reuer et al. (2007) to derive a weighted piston velocity over the two weeks prior to our measurements. This approach takes into account recent wind speed history to obtain a weighting function based on the fraction of the mixed layer depth ventilated on any given day (see Reuer et al., 2007 for full details). The weighted piston velocity

ACCEPTED MANUSCRIPT provides a less biased estimate of gas exchange coefficients over the residence time of gases in the surface mixed layer.

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Calculation of in situ N2O production

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To estimate the amount of N2O in shelf waters derived from in situ production, we

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calculated differences in N2O concentrations across isopycnals between an off-shelf (LC11) and on-shelf (LC04) station along the LC transect (see Figure 1 for station locations, and Figure 6 for

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positions of upper and lower isopycnals used for this calculation from each cruise). For this

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analysis, discrete N2O measurements from each of these stations were interpolated to 0.01 kg m-3 density intervals, and the difference between on-shelf and off-shelf N2O concentration (dN2O)

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across each of these density surfaces was computed. The density surfaces, ranging from 25.3 to

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26.8 σθ, represented depths between 50 m depth and the bottom of the shelf station (see Figure 6

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for specific isopycnal ranges used for each cruise). Using this same approach, we computed the change in NO2-+NO3- concentrations (dNO2-+NO3-) along these density surfaces over the crossshelf transect. N2O yields of nitrification on each isopycnal were then derived by dividing the dN2O by dNO2-+NO3-, and computing the average N2O-yield for each cruise. This method also enabled us to assess the influence of O2 availability on N2O-yields from nitrification, by examining the relationship between derived N2O-yields and the corresponding O2-concentrations at the on-shelf sampling station. The calculations described above rely on some key assumptions. First, we assume that N2O concentrations along isopycnals should be constant in the absence of biological N2O cycling, so that changes in N2O along an isopycnal can be ascribed to in situ production. Moreover, we assume that no appreciable denitrification occurs in the water column of our study region. As discussed in the results section, this assumption is supported by an examination of

ACCEPTED MANUSCRIPT N2O, NO2-+NO3- and O2 data. Also, since primary producers (which consume NO3-) are restricted to the euphotic zone, nitrification is assumed to be the primary biological factor

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affecting both N2O and NO3-in sub-surface waters. Sedimentary nitrification could supply NO3and N2O to the water column near the sediment water interface, but our analysis does not

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distinguish between N2O and NO2-+NO3- derived from sediments vs. the water column.

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Similarly, our analysis does not account for alongshore transport or diapycnal mixing, which also

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likely contribute to the dissolved N2O, O2, and NO3- gradients in our study area.

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Results

Hydrographic conditions - Upwelling intensity, riverine inputs, and O2 concentrations

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Upwelling along the continental shelf of our study area was highly variable over short

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timescales (Figure 2a), but the 14-day running mean showed distinct seasonal patterns over the

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2.5 years of our time-series. In general, upwelling occurred over much of the summer between May and September. Mean upwelling values were consistently positive during the 90-days before the two September cruises (between ~ 15 and 20 m3 s-1 100 m coastline-1), and negative (net downwelling) prior to the June 2012 and 2014 cruises (~ -20 to -30 m3 s-1 100 m coastline-1). In contrast to June cruises in 2012 and 2014, weak positive upwelling was observed in June 2013. Moreover, the onset of the upwelling season was roughly one month earlier in 2013 than in 2012 or 2014 (data not shown). Daily Fraser River discharge values between 2012 and 2014 are shown in Figure 2b. Peak discharge values typically occurred during May or June, fed by melting snowpack in the coastal mountains of SW British Columbia. Discharge values were lower during September cruises, and of similar magnitude in 2012 and 2013. The earliest peak discharge occurred during

ACCEPTED MANUSCRIPT 2013, nearly one month earlier than the 2014 peak discharge, and almost two months earlier than the 2012 peak discharge. However, there were small inter-annual differences between the date at

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which the cumulative annual discharge reached 50% of the total annual discharge, and the

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cumulative annual discharge on June 1 of each year.

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Oxygen concentrations in our study area ranged from 5 µM to 450 µM. The lowest oxygen concentrations were observed in deep waters near the shelf sediments, and in the off-

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shelf OMZ waters between 800-1000 m, while the highest O2 concentrations were found near the

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surface (Figures 4 and 6). The minimum O2 in shelf waters (depth < 200 m) was 60 µM. Surface water concentrations and sea-air CH4 and N2O Fluxes

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Measurements conducted on the June 2014 cruise provide a broad spatial overview of

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surface CH4 and N2O concentrations and sea-air fluxes in the waters adjacent to Vancouver

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Island. We observed CH4 supersaturation in the mixed layer of all stations, and N2O supersaturation for most stations during that cruise (Figure 3). Super-saturations were strongest in the southeastern portion of the study area, overlying the broad continental shelf in the vicinity of the Juan de Fuca Eddy (Figure 3). We also observed a significant decrease in CH4 and N2O supersaturation along the cross-shelf gradient, with generally higher values observed in nearshore waters, decreasing beyond the shelf break (~1000 m isobath). To examine seasonal and inter-annual variability in regional CH4 and N2O sea-air fluxes and surface supersaturation, we used surface gas measurements collected from a more limited set of stations (Figure 1, black circles). Mean fluxes, surface supersaturation and other parameters used to calculate fluxes are listed in Table 1. Methane fluxes and saturation values were always positive (net flux to atmosphere), and September fluxes (3.4 to 34.1 µmol m-2 d-1) were

ACCEPTED MANUSCRIPT significantly higher than June fluxes (2.5 – 20.7 µmol m-2 d-1; Wilcoxon rank sum, p<0.05). Nitrous-oxide fluxes were also significantly higher during September (0.5 – 21.9 µmol m-2 d-1)

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than June (-4.5 – 9.9 µmol m-2 d-1), and N2O fluxes were significantly higher following periods

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of upwelling than downwelling.

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Depth-dependent N2O and CH4 concentrations

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Along shelf variability

In the along-shore transect (Figure 4), CH4 concentrations ranged from 5.1 – 35.9 nM, and N2O

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concentrations ranged from 9.0 – 33.0 nM. Depth profile measurements along the coastal transect (see Figure 1) enabled us to examine the influence of freshwater inputs on gas

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concentrations. As shown in Figure 4, there was a clear signature of low salinity water along the

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southern portion of the transect, derived from the Fraser River. This salinity-gradient sets up the

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buoyancy-driven Vancouver Island Coastal Current (VICC), which flows northward along the coast, extending down to the bottom of the water column within a few km from the coast. In general, the fresh water signature was most apparent during the June cruises (closest to the peak Fraser River discharge), but there was significant variability in the intensity and spatial extent of this signal. For example, in June 2013 (the year with the earliest peak river discharge), low salinity extended beyond our northernmost sampling station (~ 50 °N). In contrast, the freshwater signatures in the northern region of the transect were more limited during June 2012 and, particularly, in 2014. In general, maximum N2O concentrations (~ 30 nM) were observed in the deep saline and O2-depleted waters below ~100 m depth, with increasing concentrations towards the north where the influence of Fraser River waters was diminished (Figure 4a-e). For most cruises, the

ACCEPTED MANUSCRIPT low salinity surface waters were associated with relatively low N2O concentrations (minimum values ~8 nM). During June 2012, subsurface N2O concentrations were much lower than any

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other cruise. A shallow (~ 20 – 40 m depth), subsurface N2O maximum (~22 nM) was also

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observed in the southernmost stations between September 2012 and September 2013.

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In contrast to the distribution of N2O, the lowest CH4 concentrations (~ 6 nM) were observed in the deep saline, O2-depleted waters in the northern section of Vancouver Island. The

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highest CH4 concentrations (~30 nM) were observed near the sediments in the southern portion

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of the transect during the two Sept. cruises, indicating the importance of sedimentary CH4 sources in the wide, southern portion of the shelf. A shallow (~ 10 – 80 m depth) subsurface CH4

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maximum (~15 nM) was apparent in the northern portion of the transect for all cruises. This

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feature was strongest during Sept. 2012 and June 2013, and weakest following a period of downwelling in June 2012. The location of this feature was not associated with any appreciable

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subsurface turbidity maxima. In surface waters (<15 m depth), we observed a positive relationship between CH4 concentrations and salinity along the coastal transect (Figure 5, R2 = 0.373, p=0.003).No such correlation with salinity was apparent for N2O. Cross-shelf variability

Cross-shelf gradients in hydrography and gas concentrations reflected changes in the transport of deep water masses onto the continental shelf via upwelling. The influence of upwelling on N2O concentrations can be clearly seen in our across-shelf transect (Figure 6). For all cruises, maximum N2O concentrations (> 40 nM) were found in the deep, off-shore waters of the OMZ core (Figure 6, b-f). During periods of upwelling, these deep, N2O-rich, and O2depleted waters appear to be transported onto the continental shelf, resulting in elevated N2O levels in the mid-shelf waters, particularly near the location of the Tully canyon and the Juan de

ACCEPTED MANUSCRIPT Fuca Eddy (Figure 6). Indeed, mid-shelf N2O concentrations were highest (>30nM) during the three cruises that followed periods of net upwelling (i.e. Sep-2012, Jun-2013, and Sep-2013), and

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we observed a strong positive correlation between mean upwelling intensity and N2O

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concentrations in near surface waters (shallower than 50 m) over the shelf (Figure 7a). As discussed below, in situ production may also account for the elevated N2O concentrations over

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the continental shelf.

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Methane concentrations along the cross-shelf transect ranged from 1.5 – 104 nM. Unlike

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N2O, minimum CH4 concentrations were observed in off-shelf deep waters, while the highest concentrations were observed on the outer shelf region, in the vicinity of known bubble seeps

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(indicated by the horizontal black bars in Figure 6, g-k; see also Figure 1). Even though off-

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shore waters were low in CH4, we did observe a positive relationship between mean upwelling

7b). Discussion

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intensity and near-surface water (<50 m) CH4 concentrations over the continental shelf (Figure

Our field data provide new measurements of water column N2O and CH4 distributions and sea-air fluxes along the WCVI shelf, an important, yet under-sampled region. Our work is the first multi-year study from the WCVI upwelling region that includes both surface and depthresolved water-column measurements of CH4 and N2O. Such depth-resolved data are needed to link the distributions and fluxes of N2O and CH4 to local sources and transportation processes (e.g. sedimentary diffusion, water column production, upwelling, and freshwater inputs). Our results can thus contribute new insight into how variability in oxygen-availability, upwelling intensity, sedimentary processes and fresh water inputs influence N2O and CH4 cycles in the

ACCEPTED MANUSCRIPT coastal waters of southern British Columbia. In the discussion below, we examine the processes driving the distributions and fluxes of CH4 and N2O along the WCVI.

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Sources of CH4

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The absence of a negative correlation between salinity and CH4 in our study area

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indicates that CH4 was not supplied from freshwater or estuarine sources (Figure 5). Considering the nearly 200km distance between the Fraser River (the primary spring / summer freshwater

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source) and the WCVI, it is likely that much, if not all, of the CH4 in the river water would have

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been ventilated to the atmosphere before reaching our study area (e.g. Sansone et al., 1999). Similarly, the lack of correlation between turbidity (beam transmissivity) and CH4 indicates that

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high CH4 concentrations are not associated with high particle loads or re-suspended sediments.

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Although we cannot exclude or confirm water column production of CH4, the high

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concentrations of CH4 near sediments, particularly in regions with abundant seeps, suggests that CH4 is supplied primarily from seeps and other sedimentary sources to the water column. To date there have been few direct measurements of dissolved CH4 concentrations in the immediate vicinity of known seeps. We aim to obtain such measurements in future work. Sources of N2O

Across all of the station depth profiles we examined, N2O exhibited strong correlations with O2 and NO2-+NO3- (Figure 8), indicative of nitrification. In contrast, the absence of any concomitant loss of N2O and NO2-+NO3- at low O2 concentrations indicates that denitrification and dissimilatory nitrate reduction to ammonia are not likely significant processes in our study region. This is not surprising given that the lowest O2 concentrations in our study area are above the nominal O2 threshold for denitrification (Codispoti et al., 2001). However, shelf O2

ACCEPTED MANUSCRIPT concentrations along the WCVI have been shown to fall as low as 30 µM at some times (Crawford and Peña, 2013), and the continued decline of shelf O2 could allow denitrification to

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become an important process in the future.

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Our results suggest that much of the N2O in shelf waters is supplied directly from off-

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shelf N2O maximum in the OMZ. The observed changes in N2O concentrations along isopycnal surfaces between off-shelf and on-shelf waters of the LC transect support this idea. Assuming

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that the total N2O on the shelf is the sum of N2O produced over the shelf and N2O supplied by

(2)

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advection, i.e.;

N2Oon-shelf = N2Oproduced + N2Oadvected

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we can determine how much N2O was supplied by advection vs. on-shelf production. Since we

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assume that N2O is transported along isopycnal surfaces, we would expect on-shelf N2O

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concentrations at a given isopycnal to be equal to the off-shelf N2O concentrations at the same isopycnal in the absence of on-shelf sources, such that: (3)

N2Oadvected = N2Ooff-shelf

We can rearrange equation (2) to solve for the amount of N2O produced over the shelf, and substitute in N2Ooff-shelf (which we measured), yielding: (4)

N2Oproduced = N2Oon-shelf – N2Oadvected = N2Oon-shelf – N2Ooff-shelf

We applied this approach to the concentration vs. density data from our on-shelf (LC04) and offshelf (LC11) stations (Figure 9, supplementary figures) to estimate that 70-75% of N2O on the shelf was supplied by advection, and the remaining 25-30% was produced in the water column and/or supplied by shelf sediments or mixing. We observed a corresponding increase in NO2-

ACCEPTED MANUSCRIPT +NO3-and decrease in O2 across the same isopycnals for all cruises except June 2012 (Figure 9, b and c, and supplementary figures), strongly suggesting that nitrification was the process

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responsible for the excess N2O in shelf waters. The excess N2O in the shelf water was greater

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during post-upwelling cruises (dN2O ~10 nM), with the greatest N2O production observed during September 2013. These results suggest an enhancement of nitrification during periods of

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upwelling.

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Biological production of N2O

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Based on the differences in N2O and NO2-+NO3-along isopycnals, we determined the N2O-yield during nitrification (moles of N2O per mole NO2-+NO3-). Our computed N2O yields

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ranged from 0.04 – 0.45%, and were highest under low ambient O2 conditions (Figure 10). This

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(discussed below).

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range of values does not include one negative N2O yield derived from the June 2012 cruise

The N2O yield of nitrification we observed was largely within the range (0.004 – 0.4%) observed in other marine environments (De Wilde and De Bie, 2000; Punshon and Moore, 2004), and the 0.25 – 0.31% yields observed in pure cultures of ammonia-oxidizing bacteria by Goreau (1980) under similar O2 concentrations (~50 µM). However, the N2O yields we observed were higher than those observed in pure culture of ammonia-oxidizing archaea (0.004 – 0.11%; Santoro et al., 2011; Stieglmeier et al., 2014). This may suggest that N2O was produced mostly by bacterial ammonia oxidizers rather than archaeal ammonia oxidizers in our study area, although we lack data to directly support this idea. Our inferred N2O yields were also higher than the range of N2O yields (0.028-0.04%) previously measured at station LC11 (station P4 on the Line P transect), at the western limit of our study area (Grundle et al., 2012). This discrepancy could reflect a signature of sedimentary nitrification over the continental shelf,

ACCEPTED MANUSCRIPT which would also supply a high ratio of N2O: NO2-+NO3-to the water column due to the very low O2 concentrations in sediments.

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During the June 2012, cruise, we observed negative N2O yields, resulting from a small

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negative dN2O (~ -2 nM) and positive dNO2-+NO3 (~ +5 µM) between the on-shelf and off-shelf

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stations (Supplementary Figure 1). N2O loss associated with sea-air exchange could provide one possible explanation for this apparent N2O consumption. For example, if upwelled waters (rich

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in NO2-+NO3-and N2O) are returned to the sub-surface after a brief surface residence time, gas

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exchange could ventilate N2O to the atmosphere more rapidly than phytoplankton could consume the dissolved NO3, resulting in an apparent N2O deficit. Alternatively, the rapid downwelling

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during 2012 could have reduced the residence-time of shelf water, effectively flushing the N2O

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from the shelf more quickly than it was produced. The strong downwelling observed prior to the June 2012 cruise is consistent with both of these mechanisms, but we lack definitive data to

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firmly establish the cause for the apparent negative N2O yields during this cruise. Dominant transport mechanisms for N2O and CH4. Figure 11 presents a simplified schematic diagram illustrating the primary mechanisms likely influencing cross-shelf variability in CH4 and N2O distributions. Upwelling and downwelling are the dominant transport mechanism for CH4 and N2O across the continental shelf. During upwelling favourable conditions, N2O and CH4 are transported along isopycnals towards the coast, though in reality, this transport is not uniform or unidirectional due to shortterm variability in tides and upwelling. Moreover, enhanced upwelling in the vicinity of the Juan de Fuca and Tully Canyons appears to increase local supply of N2O and CH4 from deep waters. As water is transported towards the coast, nitrification acts to increase N2O concentrations, while CH4 concentrations decrease due to aerobic CH4-oxidation and mixing. Excess N2O and CH4 in

ACCEPTED MANUSCRIPT near surface waters can be ventilated to the atmosphere via sea-air flux, resulting in elevated fluxes in near shore regions. In contrast, under downwelling conditions, air-equilibrated surface

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waters low in dissolved CH4 and N2O are transported into the sub-surface and advected off the

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shelf. Water column and sedimentary nitrification still supply N2O to the sub-surface water column, but likely at lower rates than under upwelling conditions due to reduced supply of

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organic matter during downwelling. Low CH4 concentrations remain throughout the water

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column, except near the sediments and seeps where CH4 diffuses into the bottom water before

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being consumed by water-column methanotrophs.

In the along-shelf transect, the dominant transport mechanism is the VICC, which carries

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freshwater northward, gradually mixing it with the more saline waters beneath. Here, sediments

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appear to be a source of CH4 (seeps are less conspicuous along this transect), while the Fraser River likely supplies only a small amount of CH4 (if any) based on the positive correlation

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between CH4 and salinity (Figure 5). N2O is supplied to the water column by nitrification and the dominant advective source is from deep, N2O-rich marine water. Upwelling appears to bring CH4 and N2O rich waters from sediments closer to the surface, as well as N2O from hypoxic deep waters. This can be seen in June 2013, where the early onset of upwelling coincided with high subsurface CH4 and N2O concentrations near the southern end of the transect, relative to June 2012 and June 2014 (Figure 4), and by the positive correlations between upwelling and mean CH4 and N2O concentrations in the upper 50 m of shelf water (Figure 7). In contrast, strong downwelling appears to transport air-equilibrated CH4 and N2O depleted water into the sub-surface, while simultaneously preventing the shore-ward transport of CH4 and N2O-rich waters. This was observed during June 2012, when anomalously low sub-surface CH4 and N2O (and high O2) concentrations were observed in the central and northern parts of the coastal

ACCEPTED MANUSCRIPT transect (Figure 4). Taken together, our results thus provide evidence for a critical role of upwelling on sub-surface distributions of N2O and CH4 along the WCVI continental shelf. As

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discussed below, variable upwelling intensity also influences sea-air fluxes of these gases.

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Air-sea fluxes

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Sea-air fluxes in our study region (Table 1) were much higher than open ocean values (< 1 µmol m-2 day-1 for both CH4 and N2O) (Naqvi et al., 2010). Our maximum N2O flux values

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(21.9 µmol m-2 day-1) were significantly lower than the fluxes observed in shelf waters off the

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coast of Peru (up to 1,800 µmol m-2 day-1; Arévalo-Martínez et al., 2015) and in the Arabian Sea (up to 3200 µmol m-2 day-1; Naqvi et al., 2010), which experience rapid N2O production from

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both nitrification and denitrification in suboxic (O2< 5µM) near-surface waters (Arévalo-

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Martínez et al., 2015; Naqvi et al., 2010). In contrast, surface N2O measurements along the

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WCVI (concentrations and sea-air fluxes) were similar to values observed in the surface waters off southern California (~2.3 – 7.4 nM N2O excess; Pierotti & Rasmussen, 1980), in the Benguela upwelling system (-1.8 – 43.4 µmol N2O m-2 d-1; Frame et al., 2014), and higher than the fluxes (0 -2.2 µmol N2O m-2 d-1) observed near Mauritania in NW Africa (Wittke et al., 2010), where O2 minima are relatively less intense and further from the surface. Surface CH4 surface supersaturation along the WCVI was higher than previous measurements off the Oregon coast (2 – 7 nM excess CH4; Rehder et al., 2002), but lower than the surface concentrations and fluxes (~800 nM CH4 and 8.6 – 1300 µmol CH4 m-2 d-1) reported from coastal California waters (Coal Oil Point; Mau et al., 2007). These differences (i.e. Coal Oil Point > WCVI > Oregon Coast) may be due to the different depths of CH4 seeps in these different region. Along the WCVI, seeps are located mainly between 50 - 200 m (Figure 1), as compared to 600-800m along the Oregon coast and <70 m in the Coal Oil Point region. Regions

ACCEPTED MANUSCRIPT with the shallowest seeps (Coil Oil Point) thus appear to have the highest CH4 sea-air fluxes, possibly due to the limited time available for water column CH4 oxidation. Of course, this is

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likely an oversimplification since many additional factors can affect bubble-mediated CH4

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transport to the surface, such as microbial activity, films, water column stratification, and the

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partial pressure of CH4 in gas bubbles (Mau et al., 2007; Schmale et al., 2011). Previous field studies have demonstrated that upwelling can exert a strong control on sea-

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air N2O and CH4 fluxes (Lueker et al., 2003; Nevison et al., 2004; Rehder et al., 2002). For

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example, a continuous record of atmospheric N2O at Trinidad Head, CA shows strong negative correlations between N2O fluxes and sea surface temperatures (SSTs), with high atmospheric

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N2O mixing ratios and low SST occurring during periods of strong upwelling (Lueker et al.,

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2003). Similarly, Rehder et al. (2002) found strong correlations between SST, upwelling favorable winds, and surface ocean CH4 concentrations off the coast of Oregon, and argued that

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this supports the link between upwelling and increased CH4 flux. We have shown that upwelling is positively correlated with N2O and CH4 concentrations in the upper 50m of the water column (Figure 7), suggesting that upwelling increases the potential for high sea-air fluxes of these gases in our study area. This was corroborated by the significantly higher N2O fluxes following upwelling periods (mean 6.0 ± 4.9 µmol m-2 day-1) relative to post-downwelling sampling (mean 1.5 ± 3.1 µmol m-2 day-1; Wilcoxon rank sum, p<0.05). Although CH4 also showed a tendency towards higher fluxes following upwelling (12.6 ± 8.1 vs. 10.5 ± 5.6 µmol m-2 day-1, respectively), the difference was not statistically significant. However, sea-air fluxes were significantly higher in September than June for both CH4 (15.2 ± 8.9 µmol m-2 day-1 and 9.5 ± 4.8 µmol m-2 day-1, respectively) and N2O (8.6 ± 5.2 µmol m-2 day-1 and 5.9 ± 4.0 µmol m-2 day-

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, respectively), highlighting the importance of capturing seasonal variability in WCVI regional

flux estimates.

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5. Conclusion

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Our results show that seasonally variable upwelling exerts an important control on CH4

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and N2O distributions and sea-air fluxes in the WCVI. Sea-air fluxes and the surface supersaturation of these gases were within the ranges reported for other coastal upwelling systems,

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with highest values near the coast in the vicinity of the Juan de Fuca Eddy, and strong spatial

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variability across the continental shelf. Seeps are a potentially significant (albeit localized) source of water column CH4, which are unevenly distributed throughout the region. We

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determined that upwelling leads to increased subsurface N2O concentrations due to advection of

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N2O rich water, and through the enhancement of nitrification in the water column and potentially

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sediments. The N2O-yield from nitrification along the coast of Vancouver Island appears to be consistent with other studies, and increases under O2-limitation, as expected. The continued decline of O2 (Crawford and Peña, 2013) and intensification of summertime upwelling in the WCVI and other coastal upwelling systems (Bakun, 1990; Bylhouwer et al., 2013; Wang et al., 2015) may thus lead to higher CH4 and N2O fluxes, which would act as a positive feedback on climate change. Our study represents the start of a coastal upwelling time series of N2O and CH4 measurements that may provide valuable insights into longer term changes in the concentrations of these gases and their response to ecosystem changes. The continuation of these time-series observations will provide an important source of information on inter-annual variability in coastal CH4 and N2O cycling. This information is needed to describe the effects of hypoxic

ACCEPTED MANUSCRIPT upwelling events and long-term changes in upwelling intensity on the distribution and cycling of

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these gases in the region.

ACCEPTED MANUSCRIPT Acknowledgements We would like to acknowledge the efforts of the captain and crew of the CCGS JP Tully, and the

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research scientists at the Institute of Ocean Sciences, particularly Doug Yelland and Marie

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Robert for significant assistance at sea, and for providing ancillary hydrographic and nutrient

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data. We also wish to thank Dr. Vaughn J. Barrie and Dr. Frank Whitney for providing us with the positions of CH4-seeps derived from underway acoustic data. Funding for this research was

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provided by the National Scientific and Engineering Research Council of Canada and the Peter

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Wall Institute for Advanced Studies at the University of British Columbia.

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Jun-2012

Sep-2012

Jun-2013

Mean ± Std. Dev. (µmol m-2 day-1)

-0.3 ± 3.3

7.6 ± 4.7

4.7 ± 2.9

Range (µmol m-2 day-1)

-4.5 - 3.9

1.5 - 14.2

N2O (nM)

0.0 ± 2.4

3.7 ± 2.5

Mean ± Std. Dev. (µmol m-2 day-1)

10.8 ± 6.1

Flux (µmol m-2 day-1)

4.4 - 20.7

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Cruise

CH4 (nM)

6.7 ± 2.8

Sep-2013

Jun-2014

3.0 ± 2.3

0.9 - 9.9

0.5 - 21.9

0.5 - 7.9

3.8 ± 2.6

4.3 ± 4.2

1.4 ± 1.1

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6.0 ± 6.3

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CH4

19.7 ± 10.0

8.5 ± 3.7

11.2 ± 5.8

10.3 ± 5.6

4.5 - 34.1

2.5 - 16.6

3.4 - 19.9

4.3 - 19.3

9.5 ± 5.1

6.8 ± 3.0

8.0 ± 4.9

5.0 ± 2.5

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Other Parameters

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N2O

Mixed Layer Depth (m)

7.5 ± 0.6

7.0 ± 0.6

7.0 ± 0.8

9.6 ± 0.8

8.6 ± 1.4

kw (m d-1)

1.6 ± 0.4

2.1 ± 0.2

1.3 ± 0.2

1.5 ± 0.4

2.1 ± 0.3

n (stations with data) 8 10 11 10 10 Table 1: Mean N2O and CH4 fluxes, excess concentrations above atmospheric equilibrium (N2O and CH4), mixed layer depths, and time-weighted piston velocities (kw) for each cruise. The number of stations used to calculate fluxes for each cruise is denoted by n. See methods for details of weighted piston velocity calculations.

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Figure 1: Map of the study area along the west coast of Vancouver Island (WCVI). Depthresolved samples were collected from profile stations (black circles) during five cruises (Jun2012, Sep-2012, Jun-2013, Sep-2013, and Jun-2014) along the Coastal Transect and the crossshelf LC Line Transect. During June 2014, 5 m samples were collected from a number of additional stations (black triangles). The locations of CH4-seeps (located using 12 kHz echo sounder data) are shown by black x’s (Vaughn J. Barrie, pers. comm.). The O2 concentrations at 800m from the World Ocean Atlas climatology (Garcia et al., 2009) are shown in the inset, with the 20µ MO2 contour line shown, highlighting the subarctic North Pacific OMZ.

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Figure 2: Upwelling Index values from the WCVI study region (48 °N, 125 °W, panel a) and

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Fraser River discharge values (panel b) between January 2012 and November 2014. Upwelling

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values are plotted with 1-day (grey line) and 14-day (black line) running means. White bars (panel a) indicate mean upwelling index values during the 90-day period before each cruise.

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Vertical dashed lines indicate approximate sample collection dates during the 5 cruises.

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Figure 3: Excess CH4 (panel a) and N2O (panel b) above equilibrium concentrations measured at 5 m depth during June, 2014.

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Figure 4: Distributions of salinity (panels a-e), N2O (panels f-j), CH4 (panels k-o), and O2 (panels

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black dots.

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p-t) from each cruise along coastal transect. The locations of discrete samples are shown by

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Figure 5: Relationship between mean salinity and mean CH4 concentrations in shallow waters (less than 15 m depth) along the coastal transect (R2 = 0.373; P = 0.003; n= 22). Error bars denote ±1 standard deviation from the mean of all available measurements between 0 – 15 m depth at each station.

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Figure 6: Mean pre-cruise upwelling index (90 day average; shown in panel a) and depth

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sections of N2O, CH4, and O2 along the cross-shelf LC Line Transect. N2O (panels b-f), CH4

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(panels g-k), and O2 (panels l-p). Region with abundant CH4 seeps indicated by horizontal black

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lines in panels g-k (see Figure 1 for seep locations). The upper and lower isopycnals used to calculate along-isopycnal changes in N2O, NO3-, O2, and N2O-yields are indicated by black lines in panels b-p.

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Figure 7: Correlation between mean pre-cruise upwelling indices (90 day average) and average CH4 (a) and N2O (b) concentrations in shelf waters shallower than 50 m. Average gas concentrations were derived from samples collected within the top 50 m at all on-shelf stations (bottom depth less than 200 m), and error bars indicate ± 1 standard deviation. Both correlations are statistically significant (R2> 0.92; P < 0.01).

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Figure 8: Relationship between O2, N2O, and NO2-+NO3-across all samples for the 5 cruises. The negative correlation indicates nitrification is the dominant source of N2O in our study region. The absence of decreasing NO2-+NO3-or N2O under low O2 suggests that denitrification is not occurring at appreciable levels in the water column.

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Figure 9: Comparison of density-dependent profiles of CH4 and N2O at an on-shelf (LC04, black lines) and off-shelf (LC11, grey lines) station during September, 2012. The changes in O2, NO2+NO3- and N2O along isopycnals are ascribed to in situ nitrification during the transit of water masses onto the shelf.

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Figure 10: Relationship between mean O2 concentrations at LC04 (on shelf station) and N2O yields from nitrification. N2O yields were derived from an analysis of N2O and NO2-+NO3changes along isopycnals (see Figure 9 and methods for details). The negative relationship implies increased N2O yields under low O2 concentrations in our study area. Grey triangles represent mean values derived for each cruise (average of all points interpolated to 0.01 kg m-3 density intervals), with error bars representing ± 1 standard deviation. Small black diamonds represent the individual calculated points for each cruise.

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Figure 11: Schematic diagram showing CH4 and N2O sources, sinks, and physical transport processes along the WVCI under upwelling (panel a) and downwelling (panel b) conditions.

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Thick blue arrows indicate water circulation, dashed blue lines represent isopycnals and wavy, black lines indicate diffusion gradients. Upwelling transports N2O-rich waters from the deep N2O max off the shelf, and CH4 from seeps near the shelf break along isopycnals towards the coast. During transport, water column nitrification contributes additional N2O and NO2-+NO3-, while CH4-oxidation mitigates on-shelf CH4 increases. Sedimentary fluxes also increase the water column inventory of CH4 and N2O in shelf waters. The higher on-shelf CH4 and N2O concentrations lead to enhanced sea-air flux of these gases. Under downwelling conditions (panel b), surface waters near air-equilibrium concentrations in O2, CH4 and N2O are carried below the surface near the coast. Low surface primary productivity (due to limited nutrient supply) results in relatively low rates of water column N2O production from nitrification.

ACCEPTED MANUSCRIPT Subsurface concentrations gradually increase as water flows away from the coast due to supply

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from CH4 seeps and the N2O maximum near the shelf-break.

ACCEPTED MANUSCRIPT References Archer, D., 2007. Methane hydrate stability and anthropogenic climate change. Biogeosciences

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Discuss. 4, 993–1057. doi:10.5194/bgd-4-993-2007

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Arévalo-Martínez, D.L., Kock, A., Löscher, C.R., Schmitz, R. a., Bange, H.W., 2015. Massive

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nitrous oxide emissions from the tropical South Pacific Ocean. Nat. Geosci. 8, 530–533.

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doi:10.1038/ngeo2469

Bakun, a, 1990. Global climate change and intensification of coastal ocean upwelling. Science

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247, 198–201. doi:10.1126/science.247.4939.198

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Biogeochem. … 10, 197–207.

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Bange, H., Rapsomanikis, S., Andraea, M.O., 1996. Nitrous oxide in coastal waters. Glob.

Bange, H.W., 2008. Gaseous nitrogen compounds (NO, N2O, N2, NH3) in the ocean. Elsevier.

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doi:10.1016/B978-0-12-372522-6.00002-5 Bange, H.W., Andreae, M.O., Lal, S., Law, C.S., Naqvi, S.W.A., Patra, P.K., Rixen, T., UpstillGoddard, R.C., 2001. Nitrous oxide emissions from the Arabian Sea: A synthesis. Atmos. Chem. Phys. Discuss. 1, 167–192. doi:10.5194/acpd-1-167-2001 Bange, H.W., Bartell, U.H., Rapsomanikis, S., Andreae, M.O., 1994. Methane in the Baltic and North Seas and a reassessment of the marine emissions of methane. Global Biogeochem. Cycles 8, 465–480. Barwell-Clarke, J., Whitney, F. a, 1996. Institute of Ocean Sciences nutrient methods and analysis. Can. Tech. Rep. Hydrogr. Ocean Sci. 49. Betlach, M.R., Tiedje, J.M., 1981. Kinetic explanation for accumulation of nitrite, nitric oxide,

ACCEPTED MANUSCRIPT and nitrous oxide during bacterial denitrification. Appl. Environ. Microbiol. 42, 1074–1084. doi:Article

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Boles, J.R., Clark, J.F., Leifer, I., Washburn, L., 2001. Temporal variation in natural methane

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seep rate due to tides, Coal Oil Point area, California. J. Geophys. Res.

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doi:10.1029/2000JC000774

Bylhouwer, B., Ianson, D., Kohfeld, K., 2013. Changes in the onset and intensity of wind-driven

NU

upwelling and downwelling along the North American Pacific coast. J. Geophys. Res.

MA

Ocean. 118, 2565–2580. doi:10.1002/jgrc.20194

Capelle, D.W., Dacey, J.W., Tortell, P.D., 2015. An automated, high through-put method for

D

accurate and precise measurements of dissolved nitrous-oxide and methane concentrations

TE

in natural waters. Limnol. Oceanogr. Methods n/a–n/a. doi:10.1002/lom3.10029

AC CE P

Carter, L., 1973. Surficial Sediments of Barkley Sound and the Adjacent Continental Shelf, West Coast Vancouver Island. Can. J. Earth Sci. 10, 441–459. Casciotti, K.L., Buchwald, C., 2012. Insights on the marine microbial nitrogen cycle from isotopic approaches to nitrification. Front. Microbiol. 3, 1–14. doi:10.3389/fmicb.2012.00356 Castro-González, M., Farías, L., 2004. N2O cycling at the core of the oxygen minimum zone off northern Chile. Mar. Ecol. Ser. 280, 1–11. Codispoti, L.A., 2010. Interesting times for marine N2O. Science (80-. ). 327, 1339–40. doi:10.1126/science.1184945 Codispoti, L.A., Brandes, J., Christensen, J.P., Devol, A.H., 2001. The oceanic fixed nitrogen

ACCEPTED MANUSCRIPT and nitrous oxide budgets: Moving targets as we enter the anthropocene? Sci. Mar. 65, 85– 105.

PT

Cornejo, M., Farías, L., 2012. Meridional variability of the vertical structure and air–sea fluxes

RI

of N2O off central Chile (30–40°S). Prog. Oceanogr. 92-95, 33–42.

SC

doi:10.1016/j.pocean.2011.07.016

Crawford, W.R., Peña, M.A., 2013. Declining Oxygen on the British Columbia Continental

NU

Shelf. Atmosphere-Ocean 51, 88–103. doi:10.1080/07055900.2012.753028

MA

Cynar, F.J., Yayanos, A.A., 1992. The distribution of methane in the upper water of the Southern California Bight. J. Geophys. Res. 97, 11,211–269,285.

TE

D

Dalsgaard, T., Stewart, F.J., Thamdrup, B., De Brabandere, L., Revsbech, N.P., Ulloa, O., Canfield, D.E., DeLong, E.F., 2014. Oxygen at Nanomolar Levels Reversibly Suppresses

AC CE P

Process Rates and Gene Expression in Anammox and Denitrification in the Oxygen. MBio 5, 1–14. doi:10.1128/mBio.01966-14.Editor Damm, E., Helmke, E., Thoms, S., Schauer, U., 2010. Methane production in aerobic oligotrophic surface water in the central Arctic Ocean. Biogeosciences 7, 1099–1108. De Angelis, M. a., Lee, C., 1994. Methane production during zooplankton grazing on marine phytoplankton. Limnol. Oceanogr. 39, 1298–1308. De Wilde, H.P.J., De Bie, M.J.M., 2000. Nitrous oxide in the Schelde estuary: Production by nitrification and emission to the atmosphere. Mar. Chem. 69, 203–216. doi:10.1016/S03044203(99)00106-1 Falkowski, P., Algeo, T., Codispoti, L., 2011. Ocean Deoxygenation: Past, Present, and Future.

ACCEPTED MANUSCRIPT Eos (Washington. DC). 92, 409–420. Farías, L., Besoain, V., García-Loyola, S., 2015. Presence of nitrous oxide hotspots in the coastal

PT

upwelling area off central Chile: an analysis of temporal variability based on ten years of a

RI

biogeochemical time series. Environ. Res. Lett. 10, 044017. doi:10.1088/1748-

SC

9326/10/4/044017

Farías, L., Castro-González, M., Cornejo, M., Charpentier, J., Faúndez, J., Boontanon, N.,

NU

Yoshida, N., 2009. Denitrification and nitrous oxide cycling within the upper oxycline of

MA

the eastern tropical South Pacific oxygen minimum zone. Limnol. Oceanogr. 54, 132–144. doi:10.4319/lo.2009.54.1.0132

D

Florez-Leiva, L., Damm, E., Farías, L., 2013. Methane production induced by dimethylsulfide in

TE

surface water of an upwelling ecosystem. Prog. Oceanogr. 112-113, 38–48.

AC CE P

doi:10.1016/j.pocean.2013.03.005

Foreman, M., 2000. Seasonal current simulations for the western continental margin of Vancouver Island. J. Geophys. … 105, 19665–19698. Frame, C., Deal, E., Nevison, C.D., Casciotti, K.L., 2014. N2O production in the eastern South Atlantic: Analysis of N2O stable isotopic and concentration data. Global Biogeochem. Cycles 28, 1262–1278. doi:10.1002/2013GB004790.Received Frame, C.H., Casciotti, K.L., 2010. Biogeochemical controls and isotopic signatures of nitrous oxide production by a marine ammonia-oxidizing bacterium. Biogeosciences 7, 2695–2709. doi:10.5194/bg-7-2695-2010 Freing, A., Bange, H.W., Wallace, D.W.R., 2012. Global oceanic production of nitrous oxide. Philos. Trans. R. Soc. B Biol. Sci. 367, 1245–1255. doi:10.1098/rstb.2011.0360

ACCEPTED MANUSCRIPT Garcia, H.E., Locarnini, R.A., Boyer, T.P., Antonov, J.I., Baranova, O.K., Zweng, M.M., Johnson, D.R., 2009. World Ocean Atlas 2009, Volume 3: Dissolved Oxygen, Apparent

PT

Oxygen Utilization, and Oxygen Saturation, S. Levitus, Eds. NOAA Atlas NESDIS 70. U.S.

RI

Government Printing Office, Washington, D.C.

SC

Goreau, T.J., Kaplan, W.A., Wofsy, S.C., McElroy, M.B., Valois, F.W., Watson, S.W., 1980.

Appl. Environ. Microbiol. 40, 526–532.

NU

Production of NO2 and N2O by nitrifying bacteria at reduced concentrations of oxygen.

MA

Grant, N.J., Whiticar, M.J., 2002. Stable carbon isotopic evidence for methane oxidation in plumes above Hydrate Ridge, Cascadia Oregon Margin. Global Biogeochem. Cycles 16,

D

1124. doi:10.1029/2001GB001851

TE

Grundle, D.S., Maranger, R., Juniper, S.K., 2012. Upper Water Column Nitrous Oxide

AC CE P

Distributions in the Northeast Subarctic Pacific Ocean. Atmosphere-Ocean 50, 475–486. doi:10.1080/07055900.2012.727779 Heeschen, K.U., Collier, R.W., de Angelis, M.A., Suess, E., Rehder, G., Linke, P., Klinkhammer, G.P., 2005. Methane sources, distributions, and fluxes from cold vent sites at Hydrate Ridge, Cascadia Margin. Global Biogeochem. Cycles 19, GB2016. doi:10.1029/2004GB002266 Holmes, E.M., Sansone, F.J., Rust, T.M., Popp, B.N., 2000. Methane production , consumption , and air-sea exchange in the open ocean : An evaluation based on carbon isotopic ratios and Brian via the upper mixed layer at a rate value of ap-. Global Biogeochem. Cycles 14, 1–10. Ianson, D., Feely, R.A., Sabine, C.L., Juranek, L.W., 2009. Features of coastal upwelling regions that determine net air-sea CO2 flux. J. Oceanogr. 65, 677–687. doi:10.1007/s10872-009-

ACCEPTED MANUSCRIPT 0059-z IPCC, 2013. Technical Summary, in: Allen, S.K., Boschung, J., Nauels, A., Xia, Y., Bex, V.,

PT

Midgley, P.M. (Eds.), Climate Change 2013: The Physical Science Basis. Contribution of

RI

Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate

SC

Change. doi:10.1017/CBO9781107415324

Jayakumar, A., O’Mullan, G.D., Naqvi, S.W.A., Ward, B.B., 2009. Denitrifying bacterial

NU

community composition changes associated with stages of denitrification in oxygen

MA

minimum zones. Microb. Ecol. 58, 350–362. doi:10.1007/s00248-009-9487-y Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, S., Saha, S.,

D

White, G., Zhu, Y., Leetmaa, A., Reynolds, R., Chelliah, M., Ebisuzaki, W., Higgins, W.,

TE

Janowiak, J., Mo, K.C., Ropelewski, C., Wang, J., Jenne, R., Joseph, D., Iredell, M., Saha,

AC CE P

S., White, G., Woollen, J., Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K.C., Ropelewski, C., Wang, J., Leetmaa, A., Reynolds, R., Jenne, R., Joseph, D., 1996. The NCEP/NCAR 40-Year Reanalysis Project. Bull. Am. Meteorol. Soc. doi:http://dx.doi.org/10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2 Karl, D.M., Beversdorf, L., Björkman, K.M., Church, M.J., Martinez, A., Delong, E.F., 2008. Aerobic production of methane in the sea. Nat. Geosci. 1, 473–478. doi:10.1038/ngeo234 Keeling, R.E., Körtzinger, A., Gruber, N., 2010. Ocean deoxygenation in a warming world. Ann. Rev. Mar. Sci. 2, 199–229. doi:10.1146/annurev.marine.010908.163855 Kock, A., Arévalo-Martínez, D.L., Löscher, C.R., Bange, H.W., 2015. Differences between coastal and open ocean distributions of N2O in the oxygen minimum zone off Peru. Biogeosciences Discuss. 12, 10167–10193. doi:10.5194/bgd-12-10167-2015

ACCEPTED MANUSCRIPT Kock, A., Gebhardt, S., Bange, H.W., 2008. Methane emissions from the upwelling area off Mauritania (NW Africa). Biogeosciences 5, 1119–1125. doi:10.5194/bg-5-1119-2008

PT

Lam, P., Lavik, G., Jensen, M.M., van de Vossenberg, J., Schmid, M., Woebken, D., Gutiérrez,

RI

D., Amann, R., Jetten, M.S.M., Kuypers, M.M.M., 2009. Revising the nitrogen cycle in the

SC

Peruvian oxygen minimum zone. Proc. Natl. Acad. Sci. U. S. A. 106, 4752–4757. Leifer, I., Boles, J., 2005. Measurement of marine hydrocarbon seep flow through fractured rock

MA

doi:10.1016/j.marpetgeo.2004.10.026

NU

and unconsolidated sediment. Mar. Pet. Geol. 22, 551–568.

Lueker, T.J., Walker, S.J., Vollmer, M.K., Keeling, R.F., Nevison, C.D., Weiss, R.F., 2003.

D

Coastal upwelling air-sea fluxes revealed in atmospheric observations of O2, N2, CO2 and

TE

N2O. Geophys. Res. Lett. 30, 1–4. doi:10.1029/2002GL016615

AC CE P

Masson, D., Cummins, P.F., 1999. Numerical Simulations of a Buoyancy-Driven Coastal Countercurrent off Vancouver Island. J. Phys. Oceanogr. doi:10.1175/15200485(1999)029<0418:NSOABD>2.0.CO;2 Mau, S., Valentine, D.L., Clark, J.F., Reed, J., Camilli, R., Washburn, L., 2007. Dissolved methane distributions and air-sea flux in the plume of a massive seep field, Coal Oil Point, California. Geophys. Res. Lett. 34, 2–6. doi:10.1029/2007GL031344 Metcalf, W.W., Griffin, B.M., Cicchillo, R.M., Gao, J., Janga, S.C., Cooke, H.A., Circello, B.T., Evans, B.S., Martens-Habbena, W., Stahl, D.A., van der Donk, W.A., 2012. Synthesis of Methylphosphonic Acid by Marine Microbes: A Source for Methane in the Aerobic Ocean. Science (80-. ). 337, 1104–1107. doi:10.1126/science.1219875 Naqvi, S.W.A., Bange, H.W., Farías, L., Monteiro, P.M.S., Scranton, M.I., Zhang, J., 2010.

ACCEPTED MANUSCRIPT Marine hypoxia/anoxia as a source of CH4 and N2O. Biogeosciences 7, 2159–2190. doi:10.5194/bg-7-2159-2010

PT

Naqvi, S.W.A., Jayakumar, D.A., Narvekar, P. V., Naik, H., Sarma, V.V.S.S., D’Souza, W.,

RI

Joseph, S., George, M.D., 2000. Increased marine production of N2O due to intensifying

SC

anoxia on the Indian continental shelf. Nature 408, 346–349.

Nevison, C.D., Lueker, T.J., Weiss, R.F., 2004. Quantifying the nitrous oxide source from

NU

coastal upwelling. Global Biogeochem. Cycles 18, n/a–n/a. doi:10.1029/2003GB002110

MA

Oremland, R.S., 1979. Methanogenic activity in plankton samples and fish intestines: A mechanism for in situ methanogenesis in oceanic surface waters. Limnol. Oceanogr. 24,

TE

D

1136–1141.

Pierotti, B.D., Rasmussen, R.A., 1980. Nitrous oxide measurements in the eastern tropical

AC CE P

Pacific Ocean. Tellus 32, 56–72.

Punshon, S., Moore, R.M., 2004. Nitrous oxide production and consumption in a eutrophic coastal embayment. Mar. Chem. 91, 37–51. doi:10.1016/j.marchem.2004.04.003 Reeburgh, W.S., 2007. Oceanic Methane Biogeochemistry. Chem. Rev. 107, 486–513. Rehder, G., Collier, R.U., Heeschen, K.U., Kosro, P.M., Barth, J.A., Suess, E., 2002. Enhanced marine CH4 emissions to the atmosphere off Oregon caused by coastal upwelling. Global Biogeochem. Cycles 16. doi:10.1029/2000GB001391 Rehder, G., Leifer, I., Brewer, P.G., Friederich, G., Peltzer, E.T., 2009. Controls on methane bubble dissolution inside and outside the hydrate stability field from open ocean field experiments and numerical modeling. Mar. Chem. 114, 19–30.

ACCEPTED MANUSCRIPT doi:10.1016/j.marchem.2009.03.004 Reuer, M.K., Barnett, B. a., Bender, M.L., Falkowski, P.G., Hendricks, M.B., 2007. New

PT

estimates of Southern Ocean biological production rates from O2/Ar ratios and the triple

RI

isotope composition of O2. Deep Sea Res. Part I Oceanogr. Res. Pap. 54, 951–974.

SC

doi:10.1016/j.dsr.2007.02.007

Riedel, M., Spence, G.D., Chapman, N.R., Hyndman, R.D., 2002. Seismic investigations of a

NU

vent field associated with gas hydrates, offshore Vancouver Island. J. Geophys. Res. 107,

MA

1–16. doi:10.1029/2001JB000269

Roegner, G.C., Needoba, J. a., Baptista, A.M., 2011. Coastal upwelling supplies oxygen-depleted

TE

D

water to the columbia river estuary. PLoS One 6. doi:10.1371/journal.pone.0018672 Sansone, F., Popp, B., Gasc, A., Graham, A.W., Rust, T.M., 2001. Highly elevated methane in

AC CE P

the eastern tropical North Pacific and associated isotopically enriched fluxes to the atmosphere. Geophys. Res. Lett. 28, 4567–4570. Sansone, F.J., Holmes, M.E., Popp, B.N., 1999. Methane stable isotopic ratios and concentrations as indocators of methane dynamics in estuaries. Global Biogeochem. Cycles 13, 463–474. doi:10.1029/1999GB900012 Santoro, A.E., Buchwald, C., McIlvin, M.R., Casciotti, K.L., 2011. Isotopic signature of N2O produced by marine ammonia-oxidizing archaea. Science 333, 1282–5. doi:10.1126/science.1208239 Schmale, O., Haeckel, M., McGinnis, D.F., 2011. Response of the Black Sea methane budget to massive short-term submarine inputs of methane. Biogeosciences 8, 911–918. doi:10.5194/bg-8-911-2011

ACCEPTED MANUSCRIPT Solomon, E.A., Kastner, M., MacDonald, I.R., Leifer, I., 2009. Considerable methane fluxes to the atmosphere from hydrocarbon seeps in the Gulf of Mexico. Nat. Geosci 2, 561–565.

PT

Sowers, T., 2006. Late-Quaternary atmospheric CH4 isotopre record suggests marine clathrates

RI

are stable. Science (80-. ). 311, 838–840.

SC

Steinle, L., Graves, C. a., Treude, T., Ferré, B., Biastoch, A., Bussmann, I., Berndt, C., Krastel, S., James, R.H., Behrens, E., Böning, C.W., Greinert, J., Sapart, C.-J., Scheinert, M.,

NU

Sommer, S., Lehmann, M.F., Niemann, H., 2015. Water column methanotrophy controlled

MA

by a rapid oceanographic switch. Nat. Geosci. 8, 1–6. doi:10.1038/ngeo2420 Stieglmeier, M., Mooshammer, M., Kitzler, B., Wanek, W., Zechmeister-Boltenstern, S.,

D

Richter, A., Schleper, C., 2014. Aerobic nitrous oxide production through N-nitrosating

TE

hybrid formation in ammonia-oxidizing archaea. ISME J. 8, 1135–46.

AC CE P

doi:10.1038/ismej.2013.220

Stramma, L., Schmidtko, S., Levin, L.A., Johnson, G.C., 2010. Ocean oxygen minima expansions and their biological impacts. Deep Sea Res. Part I Oceanogr. Res. Pap. 57, 587– 595. doi:10.1016/j.dsr.2010.01.005 Suess, E., Torres, M.E., Bohrmann, G., Collier, R.W., Greinert, J., Linke, P., Rehder, G., Trehu, a., Wallmann, K., Winckler, G., Zuleger, E., 1999. Gas hydrate destabilization: Enhanced dewatering, benthic material turnover and large methane plumes at the Cascadia convergent margin. Earth Planet. Sci. Lett. 170, 1–15. doi:10.1016/S0012-821X(99)00092-8 Thomson, R.E., Krassovski, M. V., 2010. Poleward reach of the California Undercurrent extension. J. Geophys. Res. Ocean. 115, 1–9. doi:10.1029/2010JC006280 Tryon, M.D., Brown, K.M., Torres, M.E., Tréhu, A.M., McManus, J., Collier, R.W., 1999.

ACCEPTED MANUSCRIPT Measurements of transience and downward fluid flow near episodic methane gas vents, Hydrate Ridge, Cascadia. Geology 27, 1075–1078. doi:10.1130/0091-

PT

7613(1999)027<1075:MOTADF>2.3.CO;2

RI

Wang, D., Gouhier, T.C., Menge, B.A., Ganguly, A.R., 2015. Intensification and spatial

SC

homogenization of coastal upwelling under climate change. Nature 518, 390–394. doi:10.1038/nature14235

NU

Weiss, R., Price, B., 1980. Nitrous Oxide Solubility in Water and Seawater. Mar. Chem. 8, 347–

MA

359.

Welsh, D., Castadelli, G., Bartoli, M., Poli, D., Careri, M., de Wit, R., Viaroli, P., 2001.

D

Denitrification in an intertidal seagrass meadow, a comparison of 15 N-isotope and

TE

acetylene-block techniques: dissimilatory nitrate reduction to ammonia as a source of N 2

AC CE P

O? Mar. Biol. 139, 1029–1036. doi:10.1007/s002270100672 Whitney, F.A., Freeland, H.J., Robert, M., 2007. Persistently declining oxygen levels in the interior waters of the eastern subarctic Pacific. Prog. Oceanogr. 75, 179–199. doi:10.1016/j.pocean.2007.08.007 Wiesenburg, D.A., Guinasso, N.L., 1979. Equilibrium solubilities of methane, carbon monoxide, and hydrogen in water and sea water. J. Chem. Eng. Data 24, 356–360. doi:10.1021/je60083a006 Wittke, F., Kock, A., Bange, H.W., 2010. Nitrous oxide emissions from the upwelling area off Mauritania (NW Africa). Geophys. Res. Lett. 37. doi:10.1029/2010GL042442