Marine Chemistry 160 (2014) 42–55

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Marine Chemistry

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Seasonal and spatial variability in the CO2 system on the Scotian Shelf(Northwest Atlantic)

Elizabeth H. Shadwick a,⁎, Helmuth Thomas b

a Antarctic Climate & Ecosystems Cooperative Research Centre, University of Tasmania, Hobart, Tasmania, Australiab Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada

⁎ Corresponding author.E-mail address: elizabeth.shadwick@utas.edu.au (E.H.

0304-4203/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.marchem.2014.01.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 October 2012Received in revised form 6 December 2013Accepted 14 January 2014Available online 22 January 2014

Keywords:Net community productionScotian shelfCarbon uptakeNitrate drawdownOcean Acidification

As part of the Atlantic Zone Monitoring Program, dissolved inorganic carbon (DIC), total alkalinity, and nitratemeasurements were made throughout the Scotian Shelf in 2007. A shelf-wide assessment of the spatio-temporal variability of the inorganic carbon system was made relying on observations in spring (April) andautumn (October). Over the 6-month period, a combination of biological production, surface dilution, and air–sea CO2 exchange resulted in seasonal decreases in surface DIC of up to 70 μmol kg−1 and subsurface(between 50 and 100m) increases of DIC on the order of 50 μmol kg−1 on the inner shelf. The regionalmean sur-facewater pHwas roughly 7.8 in spring and increased to greater than 8.0 in autumn; subsurface pHwas approx-imately 7.6 throughout the region and a seasonal decrease, attributed in part to the respiration of organic matterat depth, was observed. The surface aragonite saturation state increased from less than 2.0 to a maximum of 3.2between spring and autumn; the region as a whole exhibited relatively low saturation states, however valuesapproaching 1.0 were only observed in the Cabot Strait at depths below 100 m. Winter-to-spring and winter-to-autumn deficits in surface inorganic carbon and nitrate were used to estimate net community production(NCP) throughout the region. Thenitrate-based estimates ofNCPusing the autumnobservationswere significantlylower (0.1 to 0.3 mol Cm−2 month−1) than the carbon-based estimates (0.1 to 0.8 mol C m−2 month−1) at moststations. The cumulative autumn NCP based on nitrate (0.4 to 1.9 mol Cm−2 over 6 months) was up to 50% lowerthan the cumulative NCP based on inorganic carbon deficits (0.5 to 4.7mol Cm−2 over 6 months), suggesting thatcontinued biological production through the summer season occurs in nitrate-depleted waters.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Coastal oceans and continental shelves play an important role in thebiogeochemical cycling of carbon. Coastal regions are suppliedwith car-bon and nutrients from riverine and atmospheric inputs, upwelling andentrainment; these regions sustain disproportionately high biologicalactivity given their surface areas relative to the open ocean (e.g.Gattuso et al., 1998; Borges et al., 2005; Thomas et al., 2005; Shadwickand Thomas, 2011). The role of the global coastal ocean in the uptakeof atmospheric CO2 has long been debated (Smith and Mackenzie,1987; Smith and Hollibaugh, 1993; Cai et al., 2003, 2006; Borges et al.,2005). A recent synthesis of available observational data from numer-ous coastal regions suggests that at the global scale continental shelvesare sinks for atmospheric CO2 (Chen and Borges, 2009). Chen andBorges (2009) further suggest that temperate to subpolar continentalshelf systems are generally undersaturated with respect to atmosphericCO2 at the annual scale, and therefore act as CO2 sinks. This has been re-ported in the high latitude North Sea (Thomas et al., 2004), and in theAmundsen Gulf region of the Canadian Arctic Archipelago (Shadwicket al., 2011b). This situation has also been observed in the South (Jiang

Shadwick).

ghts reserved.

et al., 2008) and Middle Atlantic Bights (Boehme et al., 1998;DeGrandpre et al., 2002). In contrast to these systems, the nearby Gulfof Maine (Vandemark et al., 2011) and Scotian Shelf (Shadwick et al.,2010, 2011a), two temperate continental shelf systems, have recentlybeen characterised as sources of atmospheric CO2 at the annual scale.

Uptake of atmospheric CO2 on the Scotian Shelf occurs only brieflyduring and following the spring phytoplankton bloom. This bloom,which occurs at the annual minimum water temperature, is closelyfollowed by a period of surface water warming. Although biologicalproduction continues through the summer months, the biologically-mediated pCO2 decrease is out-weighed by the thermodynamicincrease due to warming (Shadwick et al., 2011a). In the autumn andwinter seasons, a combination of wind-driven and convective mixingas well as episodic upwelling events, deepen the mixed-layer and facil-itate the intrusion of carbon-rich subsurface waters to the surface layer.The region acts as a source of atmospheric CO2 until the onset of thespring bloom in the following productive season as a result of thiswater column destratification (Shadwick et al., 2011a).

The continental shelf pump mechanism (Tsunogai et al., 1999),which has the potential to transport carbon off the continental shelfinto the adjacent deep ocean, is not thought to operate on the ScotianShelf according to a recent modelling study (Fennel and Wilkin,2009). Fennel andWilkin (2009) suggest that vertical export of organic

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matter is insufficient for carbon export off the shelf since the respiratoryproducts of this material are re-exposed to the surface under conditionsof deep mixing that occur regularly in winter, and by onshore subsur-face transport of metabolic dissolved inorganic carbon (Burt et al.,2013). Accordingly, the majority of the outgassing on the Scotian Shelfoccurs during the autumn and winter seasons when the combinationof CO2 supersaturation and wind forcing enhance air–sea fluxes(Shadwick et al., 2010, 2011a).

Previous studies of the CO2 system on the Scotian Shelf have focusedlargely on the surface mixed-layer (Shadwick et al., 2010, 2011a). Herewe present observations of the inorganic carbon system collected aspart of the Atlantic Zone Monitoring Program (AZMP). In this study,the spatio-temporal variability of the inorganic carbon system parame-ters are evaluated based on full water-column sampling along four pri-mary transect lines throughout the Scotian Shelf region. Net communityproduction (NCP) is estimated at stations throughout the region fromwinter-to-spring, and winter-to-autumn deficits in surface inorganiccarbon and nutrients, and the resulting spatial distribution of NCP isdiscussed. The seasonal changes in pH and aragonite saturation stateas well as the spatial variability of the CO2 system parameters arepresented.

2. Oceanographic setting

The Scotian Shelf is located at the junction of the North Atlantic sub-polar and subtropical gyres and is downstream from the outflow of theGulf of St. Lawrence via the Cabot Strait (Loder et al., 1997) (Fig. 1). Theshelf-scale circulation is dominated by the Nova Scotia Current, whichflows to the southwest roughly parallel to the coast, and by an extensionof the Labrador Current which flows along the shelf edge in the samedirection. Transport by the Nova Scotia current varies seasonally instrength reaching peak values of 2 Sv in winter and spring, and mini-mum summer values of 0.4 Sv (Loder et al., 2003). Smaller scale circula-tion features are generated by topographic steering around submarinebanks and cross-shelf channels (Loder et al., 1988; Hannah et al.,2001). Tidal forcing and episodic wind-driven upwelling also influencethe region (Petrie et al., 1987; Loder et al., 1997; Greenan et al., 2004).Coastal upwelling and associated favourable wind conditions have

Fig. 1. Locations of station sampled along four primary transect lines in th

long been studied in the Scotian Shelf region (Hachey, 1935). Strongwinds (speed≥10m s−1) persisting for several days toward the north-east may force relatively cold and saline water towards the surface,displacing warmer, fresher water offshore (Petrie, 1983). Upwellingevents have been observed in nearshore regions of the Scotian Shelfand these events have been reproduced by modelling studies (Petrie,1983; Donohue, 2000). It has been suggested that these events mayplay a role in the initiation of the spring phytoplankton bloom by bring-ing nutrient-rich subsurfacewaters to the surface (Greenan et al., 2004).

Biological production in the Scotian Shelf region is dominated by thespring phytoplankton bloom which accounts for more than a thirdof the total annual gross primary production, estimates of whichrange from 60 to 130 g C m−2 yr−1 (Fournier et al., 1977; Mills andFournier, 1979). This bloom, which is dominated by large phytoplank-ton classes (Mousseau et al., 1996), rapidly exhausts surface nutrients.The spring bloom represents the major control on phytoplankton bio-mass although production in summer and a smaller autumn bloomalso play a role (Craig et al., 2013). Satellite based estimates of particu-late organic carbon (POC), using the Giovani online data system(Acker and Leptoukh, 2007) indicate that POC concentrations in Aprilon the central Scotian Shelf (42°N to 45.5°N, and 59°W to 66°W) aremore than double those observed in the following months, despite therelatively deep mixed layer in April (~40 m) compared to the summermonths (~10 m). Chlorophyll-a concentrations of greater than12 mg Chl-a m−3 have been observed at station Halifax Line 2 (HL2,see Fig. 1) in April, compared to values of 2 to 3 mg Chl-a m−3 at thesame location during the smaller autumn phytoplankton bloom(Shadwick et al., 2010).

3. Methods

3.1. Discrete sampling

Discrete bottle sampleswere collected on spring and autumn cruisesas part of the Atlantic Zone Monitoring Program at stations distributedalong four transects throughout the Scotian Shelf (Fig. 1). Cruises tookplace from 6 to 22 of April, and 5 to 18 of October, 2007. Approximately350 water samples were collected on each cruise from throughout the

e Scotian Shelf region on AZMP cruises in spring and autumn 2007.

Fig. 2. DIC and TA as a function of salinity for all stations occupied in spring (blue +) andautumn (red o). (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

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water column with higher vertical (10 m) resolution within the eupho-tic zone at all stations shown in Fig. 1. Dissolved inorganic carbon (DIC)and total alkalinity (TA) samples were tapped from 12-L Niskin bottlesmounted on a General Oceanic 24-bottle rosette fitted with a SeaBirdCTD such that all chemical data are associated with high precision in-situ temperature, and salinity data. Following water collection, DICand TA sampleswere poisonedwith a solution ofHgCl2 to halt biologicalactivity and stored in the dark at 4 °C to await analysis. DIC and TAwereanalysed on-board by coulometric and potentiometric titration, respec-tively, using a VINDTA3C (Versatile Instrument for theDetermination ofTitration Alkalinity by Marianda) and following standard procedures(Dickson et al., 2007). Regular analysis of Certified Reference Materials(provided by A. G. Dickson, Scripps Institution of Oceanography) en-sured that the uncertainty of the DIC and TA measurements was lessthan 2 to 3 μmol kg−1, respectively.

Following the determination of DIC and TA, discrete values of pH (onthe total scale) and aragonite saturation state (Ω) were computed byusing the standard set of carbonate system equations, with the CO2Sysprogram of Lewis and Wallace (1998). We used the equilibrium con-stants of Mehrbach et al. (1973) refit by Dickson and Millero (1987).The calcium (Ca2+) concentration was assumed to be conservative andcalculated from salinity. Nitrate (hereafter NO3

−) samples were collectedin parallel with samples for DIC and TA (Harrison et al., 2003) and weremeasured by colorimetric determination using a standard auto-analysertechnique (Grasshoff, 1999). Analytical detection limit for NO3

− was0.05 μmol kg−1.

3.2. Estimates of net community production

The spring bloom on the Scotian Shelf occurs when the water tem-perature is still close to the annual minimum (Greenan et al., 2008;Shadwick et al., 2011a) and the mixed-layer may be quite deep; inApril the physical situation on the Scotian Shelf is thus representativeof the winter or early spring season. In October, by contrast, the wateris near the annual temperature maximum (salinity minimum) and thephysical system is representative of the summer or early autumn season(Shadwick et al., 2011a). We computed the seasonal change in salinity-normalised (S=35)DIC andNO3

− (nDIC andnNO3−), relative to inferred

winter conditions, in spring (April) and autumn (October). The depth-integrated nDIC deficit (ΔnDIC) was computed via:

Δ nDICð Þ ¼Z z¼40

z¼0nDICwinter−nDICobs

� �dz: ð1Þ

where nDICwinter is the inferred winter concentration estimated fromthe (50 m) subsurface value in April at each station, nDICobs is observedconcentration in either April or October, and z is depth (in metres). Wedo not havewinter observations ofDIC that can beused to assess the ap-propriateness of the April subsurface value for a winter concentration.However, a January DIC concentration computed from pCO2 observa-tions at station HL2, and salinity-derived TA (see Fig. 13a in Shadwicket al., 2011a), is consistentwith the April subsurface value assumed rep-resentative of winter conditions used here. DIC-based net communityproduction (NCPC) was estimated as:

NCPC ¼ ΔnDIC; ð2Þ

and the NO3−-based estimate was calculated by scaling ΔnNO3

− by theRedfield ratio of C:N of 106:16 (Redfield et al., 1963), for comparisonwith the DIC-based estimates:

NCPN ¼ 6:625ΔnNO−3 : ð3Þ

A climatology of nitrate in the central Scotian Shelf region (see Fig. 3 inShadwick et al., 2011a), indicates that the winter concentrations of sur-faceNO3

− (ranging from5 to 10 μM)are found at 50mdepth in the sum-mer months, and thus we consider the 50 m NO3

− concentrations in

April to be representative of the winter conditions. Although themixed-layer depth at the time of sampling was often shallower than40 m, particularly in October, we integrated over the upper 40 m(which represents the climatological mixed-layer depth) to accountfor deeper mixing, which may bring carbon- (and nutrient-) rich sub-surface waters into the surface layer. By integrating over the upper40 m, our computed ΔnDIC and ΔnNO3

− should not be affected by en-richment from deep water. Furthermore, since the mixed-layer depthmay change on timescales of days or shorter, and we are computingestimates of biological production on seasonal timescales, the use of aclimatological, rather than in-situ,mixed-layer depth for the integrationis appropriate. The NCP estimates represent cumulative productionbetween ‘winter’ and the time of observation. The spring estimates areassumed valid over 1 month; the autumn estimates are valid over a pe-riod of 6months, and have been divided by 6 so that all estimates of NCPare reported in units of mol C m−2 month−1.

By using salinity normalised DIC and NO3−, we assume that we have

accounted for seasonal hydrographic changes, and by integrating overthe upper 40 m, we assume that we have accounted for vertical mixingin computing our seasonal deficits. The conservative relationship be-tween TA and salinity (Fig. 2) supports the assumption that calcificationdoes not represent an important seasonal control on the CO2 system inthis region, and will not be considered further here. However, estimat-ing NPCC based on seasonal changes in nDIC neglects the contributionfrom air–sea CO2 exchange. Based on the findings of Shadwick et al.(2010), who used an algorithm based on remotely sensed sea-surfacetemperature, chlorophyll-a, and wind speed, to estimate the air–seaflux of CO2 in the Scotian Shelf region, the cumulative fluxes ranged be-tween outgassing of 0.2 mol C m−2 and uptake of 0.6 mol C m−2 de-pending on the location (and discussed in more detail in Section 5).We assume that the maximum contribution from air–sea exchange(0.6 mol C m−2 month−1) represents the uncertainty in our computa-tions of NCP, though this should be considered a lower limit.

The conventional salinity-normalisation used here does not takeinto account the non-zero concentration of DIC (and NO3

−) of thezero-salinity end-member (e.g. Friis et al., 2003). Shadwick et al.(2011a), report zero-salinity CO2 system end-members of DICS=0 =546 μmol kg−1 and TAS=0 = 805 μmol kg−1, based on a linear regres-sion of DIC and TA data with salinity for observations collected in theScotian Shelf region. Although seasonal variability in these endmemberproperties is likely (Shadwick and Thomas, 2011), these values are

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consistent with observations reported for the neighbouring Gulf ofMaine (Cai et al., 2010). To asses the impact of this choice, we comparednDIC values at various salinities using the Friis et al. (2003) normalisa-tion:

nDICF ¼ DICobs−DICS¼0

Sobs35þ DICS¼0; ð4Þ

where nDICF is the resulting salinity-normalised concentration, thesubscript ‘obs’ refers to the observed or in-situ value, and DICS = 0 isthe riverine endmember value, with those computed using the conven-tional normalisation used here (see Table 1). While the difference innDIC based on the choice of normalisation is certainly not negligible,and particularly large at low salinity, the priority in this applicationwas to directly compare the DIC- and NO3

−-based estimates, and thus,the choice appears justified.

4. Results and discussion

4.1. The carbonate system

The relationships between DIC and salinity, and TA and salinity inthe region in spring and autumn are shown in Fig. 2. Salinity on theshelf extends from roughly 30 to greater than 35, with surface andnear-surface values ranging from 30 to 33; the concentration of DICranges between 1950 μmol kg−1 and 2250 μmol kg−1, with lowestvalues corresponding to lower salinity surface values andDIC increasingwith depth and with distance from shore (discussed in more detail inSection 4.2). Themaximum salinities (35.5) are found in the deeperwa-ters of stations located offshore, particularly in the southern region ofthe Browns Bank Line (see Fig. 1); themaximumDIC and TA concentra-tions, on the order of 2250 μmol kg−1 and 2350 μmol kg−1 are alsofound offshore. There is in general more variability in the relationshipbetween DIC and salinity in autumn than in spring (Fig. 2).

Hourly observations of surface water pCO2 (made by a CARIOCAbuoy) at station HL2 (see Fig. 1 for location) indicate that on the 2ndof April surface pCO2 was roughly 200 μatm (Shadwick et al., 2011a).This is 50 μatm lower than the minimum pCO2 observed at the heightof the 2008 spring bloom, which was only 14 days in duration(Shadwick et al., 2010, 2011a) and indicates that the 2007 springbloom was underway before the sampling began. The drawdown ofDIC in the surface layer had therefore already begun at the time of ob-servation. The higher degree of variability in the relationship betweenDIC and salinity in autumn relative to spring is due in part to the impactof biological processes on the inorganic carbon concentration during theproductive season, the majority of which may be reflected in the au-tumn concentrations. This is also illustrated by the significantly lowerconcentration of DIC observed throughout the full range of salinity,from 1950 to 2050 μmol kg−1 in autumn compared to values betweenroughly 2000 and 2150 μmol kg−1 in spring. The distribution of surfacealkalinity closely resembles that of surface salinity (Figs. 3 to 6), sug-gesting a conservative relationship between the two (Fig. 2), and atwo end-member system in the region, with a less saline water massfrom the Gulf of St. Lawrencemixingwith amore saline, Gulf Stream in-fluenced, water mass from the open ocean further offshore. The

Table 1Salinity-normalised DIC using the Friis et al. (2003) method (nDICF, see Eq. (4)), is com-pared to the conventional normalisation employed here (nDIC) for a range of salinityand DIC concentrations (units are μmol kg−1, see Fig. 2b). The nDICF values are lowerthan the nDIC values, with the larger differences at lower salinities.

S DIC nDICF nDIC

31 2000 2188 225832.5 2050 2166 220834 2150 2197 2213

conservative behaviour of TA with respect to salinity supports the as-sumption that calcification (and carbonate dissolution), which influ-ences both DIC and TA, can be neglected. Biological processes alsoinfluence TA due to the uptake and release of nitrate during photosyn-thesis and respiration (e.g. Zeebe and Wolf-Gladrow, 2001), thoughthis change is smaller than the corresponding change in DIC, and the re-gional distribution of TA closely follows that of salinity with respect toboth seasonal and spatial variations, reflecting water mass mixing andtransport (Figs. 3–6 panels d and h). Like DIC, both pHand aragonite sat-uration state (Ω) are strongly influenced by biological processes whichwill be discussed in more detail below. In general the saturation state isrelatively low in this region, however, seasonal profiles do not indicatewidespread undersaturation.

4.1.1. The Browns Bank Line sectionIn spring, the surface DIC concentrations along the Browns Bank Line

section had amean surface value of 2002 μmol kg−1, with theminimumconcentrations (roughly 1985 μmol kg−1) observed at stations BBL4,BBL5 and BBL6 (Fig. Fig. 3c, see also Fig. 2 for station locations). It ap-pears that biological activity had already begun, and surface NO3

− con-centrations along this section were less than 0.5 μmol kg−1 at the timeof sampling in spring. In the subsurface, DIC concentrations rangedfrom minimum values of 2015 μmol kg−1 at station BBL1 closest tothe coast, to 100mvalues on the order of 2100 μmolDIC kg−1 at stationsBBL4 and BBL5 and values approaching 2170 μmol DIC kg−1 at depthsgreater than 100 m at the offshore stations BBL6 and BBL7 (Fig. 3c).

In autumn, at all stations along the BrownsBank Line, surfaceDICwasreduced relative to spring conditions discussed above (compare Fig. 3cand g). This DIC decrease was most pronounced at the nearshore stationBBL1 and the offshore station BBL7 (Fig. 3g). Autumn surface concentra-tions ranged from 1945 μmol kg−1 (BBL1) to 1998 μmol kg−1 (BBL5). Inthe subsurface (from 50 to 100 m) there was an increase in DIC in au-tumn relative to spring, particularly evident at stations BBL1, BBL3,BBL4 and BBL7, on the order of 30 to 50 μmol DIC kg−1. Increases in sur-face pH, from values of roughly 7.9 at all stations along the Browns BankLine, to values of up to 8.0 at stations BBL5 and BBL7 were observed be-tween spring and autumn (Fig. 7a). The pH profiles shown are not at in-situ temperature, but normalised to 25 °C to remove the effects ofchanging temperature. This pH increase can thus be attributed in partto the biological uptake of carbon, decreasing the surface pCO2 and en-hancing the aragonite saturation state (Ω, Fig. 8a) in autumn. Surfacevalues of Ω along the Browns Bank Line increased from roughly 2.0 inspring to values greater than 2.0 and approaching 3.0 at the offshore sta-tion BBL7.

4.1.2. The Halifax Line sectionThe seasonal distribution of DIC along theHalifax Line section closely

resembles that observed along the Browns Bank line described above(Fig. 4). In spring the mean surface concentration along the HalifaxLine was 20 μmol DIC kg−1 higher than on the Browns Bank Line witha value of 2022 μmol kg−1. The minimum surface DIC (1975 μmol kg−1)was observed at station HL2; the neighbouring station HL3 had surfaceconcentration of 1983 μmol DIC kg−1. Themaximum surface concentra-tions (on the order of 2060 μmol DIC kg−1) were observed further off-shore at stations HL6 and HL7 (see also Fig. 4). As in the case of theBrowns Bank Line, subsurface concentrations increased with distanceoffshore and had values of ~2100 μmol DIC kg−1 at nearshore stations,increasing to values approaching 2200 μmol DIC kg−1 below depths of100 m at stations HL6 and HL7.

In autumn surface DIC concentrations were reduced relative tospring at all stations along the Halifax Line section (Fig. 3g). Values ofless than 1932 μmol kg−1 were observed at stations HL1, HL2 andHL3, and at all stations the surface concentrations were less than1960 μmol DIC kg−1, representing a seasonal decrease of 60 μmol DICkg−1 along this section. There was a corresponding increase in subsur-face DIC in autumn, particularly at stations HL4, HL6 and HL7. Seasonal

Fig. 3. Sections of: (a and e) temperature; (b and f) salinity; (c and g) DIC; and (d and h) TA along the Browns Bank Line section (see Fig. 1) in spring (top panels) and autumn (bottom panels).

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Fig. 4. Sections of: (a and e) temperature; (b and f) salinity; (c and g) DIC; and (d and h) TA along the Halifax Line section (see Fig. 1) in spring (top panels) and autumn (bottom panels).

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Fig. 5. Sections of: (a and e) temperature; (b and f) salinity; (c and g) DIC; and (d and h) TA along the Louisbourg Line section (see Fig. 1) in spring (top panels) and autumn (bottom panels).

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Fig. 6. Sections of: (a and e) temperature; (b and f) salinity; (c and g) DIC; and (d and h) TA along the Cabot Strait Line section (see Fig. 1) in April (top panels) and September (bottom panels).

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Fig. 7. Seasonal profiles of pH (at 25 °C along the (a) Browns Bank, (b) Halifax, (c) Louisbourg, and (d) the Cabot Strait Line transects (see Fig. 1). Spring values are given in blue (+) andautumn in red (o). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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changes in pH (Fig. 7b) indicate surface increases from 7.8 in spring tovalues approaching 8.0 at stations HL3 through HL6, and greater than8.1 at the offshore station HL7 in autumn. There is a corresponding sea-sonal increase in Ω from 1.9 to values ranging from 2.1 to 3.1 (at HL7)with the smallest seasonal changes observed inshore at station HL1and HL2 and the largest increases observed offshore (Fig. 8b).

4.1.3. The Louisbourg Line sectionAs in the case of the Browns Bank and Halifax Line sections, the

Louisbourg Line observations (Fig. 5) indicate strong seasonal changesin both surface, and subsurface, DIC between spring and autumn(Fig. 5c and g). In spring the mean surface DIC concentration along theLouisbourg section was 2035 μmol kg−1, 30 μmol DIC kg−1 higherthan the mean surface concentration along the Browns Bank Line sec-tion and 10 μmol DIC kg−1 higher than that observed along the HalifaxLine section. Theminimum surface DIC value (2010 μmol kg−1) was ob-served at the inshore station LL2; the maximum surface value in spring(2067 μmol kg−1) was observed at station LL8, the furthest from shore.This is consistent with observations along the Browns Bank and HalifaxLine sections which indicate increasing surface DIC concentrations withdistance from the coast. Subsurface concentrations in spring are similarto the Browns Bank and Halifax Line values, ranging from roughly2100 μmol DIC kg−1 to 2190 μmol DIC kg−1 at depths greater than100 m.

In autumn the surface DIC concentrations are reduced by up to70 μmol kg−1 (at station LL2), with the surface DIC decrease particularlypronounced at stations LL2, LL4 and LL8 (Fig. 3g). There is also an in-crease in subsurface DIC between spring and autumn, with the largestincreases, on the order of 50 μmol kg−1, at stations LL4, LL5 and LL7. Sur-face pH andΩ indicate seasonal increases, likely attributed to biologicaluptake of inorganic carbon, as in the case of the Browns Bank and Hali-fax Line sections, between spring and autumn (Fig. 7c). Along theLouisbourg Line section surface pH increases from roughly 7.8 to 7.9(LL4 and LL5) and greater than 8.0 (LL8); at station LL7 there is amodestchange in pH only, reflecting the rather smaller seasonal decrease in DICobserved at this location. The seasonal changes in surface Ω mirrorthose in surface pH, with the largest increases observed at stationsLL4, LL5 and LL8 (Fig. 8c). Between spring and autumn there is a de-crease in subsurface pH which corresponds to the subsurface increasein DIC, particularly evident at stations LL4, LL5 and LL7, and likely attrib-utable to respiration or remineralisation or organic matter produced inthe surface and exported to depth. This respiration produces inorganiccarbon, increasing DIC and decreasing pH (and Ω).

4.1.4. The Cabot Strait sectionAlong the Cabot Strait section there are also decreases in surface DIC

in autumn relative to spring, as described above (Fig. 6). However, someof the stations along this section indicate different seasonal changes

Fig. 8. Seasonal profiles ofΩ along the (a) Browns Bank, (b) Halifax, (c) Louisbourg, and (d) the Cabot Strait Line transects (see Fig. 1). Spring values are given in blue (+) and autumn inred (o). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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than those observed along the sections in the central Scotian Shelf. Inspring, the mean surface DIC concentration was 2030 μmol kg−1, withminimum concentrations (1995 μmol DIC kg−1) at station CSL1, clos-est to the Nova Scotia side of the Strait (Fig. 6c), and a maximum sur-face concentration of 2067 μmol DIC kg−1 at station CSL6 at theopposite (Newfoundland) side of the Strait. In autumn the mean sur-face concentration was only 5 μmol kg−1 lower with a value of2026 μmol DIC kg−1.

The minimum autumn surface DIC concentration (1949 μmol kg−1)was observed at the same location as the spring minimum at stationCSL1. The maximum autumn surface concentrations (2030 μmol kg−1)were observed at both station CSL6 and station CSL4. At stations CSL1,CSL2 and CSL5, similar seasonal changes in DIC (decrease in the surface,increase in the subsurface) described above for the central Scotian Shelfwere observed (see also Fig. 6). At stations CSL4 and CSL6 however, dif-ferent seasonal changes were observed. At station CSL4 there was in-crease in DIC between spring and autumn throughout the watercolumn, with corresponding decrease in both pH and Ω (Figs. 7d and8d). The opposite situation was observed at station CSL6; there is de-crease in DIC throughout the water column and a corresponding in-crease in pH and Ω extending well below the mixed-layer to a depthof 250 m.

4.2. Net community production

Estimates of NCP, based on winter-to-spring and winter-to-autumnnDIC and nNO3

− deficits (see Section 3), at all stations are given inTable 2. As discussed above, the productive season on the ScotianShelf begins in early April. In addition to this spring production, domi-nated by the spring phytoplankton bloom, there is a significant reduc-tion in surface DIC between spring and autumn (up to 70 μmol kg−1).Biological production occurring after the spring phytoplankton bloom,and after the depletion of surface nutrients has been shown to makean important contribution to the annual biological uptake of carbon inthe Scotian Shelf based on sustained observations made at station HL2(Shadwick et al., 2011a; Thomas et al., 2012).

A comparison of estimates of NCP made based on seasonal changesin nNO3

− with those made based on seasonal changes in nDIC confirmsthe importance of post-bloom production throughout the Scotian Shelfregion. Seasonal profiles of nNO3

− are given in Fig. 10; these profilesindicate surface NO3

− depletion in spring, with autumn concentrationsnot distinguishable from zero at the majority of stations throughoutthe region. The resulting estimates of NCPN are fairly similar in springand autumn (Table 2). By contrast, the ongoing surface depletion innDIC in autumn, relative to spring, at several stations, yield cumulative

Table 2Estimates of NCP based on DIC (NCPC) in spring and autumn with NO3

−-based estimates(NCPN) in parentheses. The first two columns are in units of mol C m−2 month−1. In thelast column the cumulative (Cum) autumn NCP is reported in units of mol C m−2 over6 months (these values are divided by 6 to give the per month estimates in column 2).

Station Apr Sep Cum

Browns Bank LineBBL1 1.3 (0.8) 0.3 (0.3) 1.8 (1.5)BBL2 1.8 (1.5)BBL3 1.1 (1.6) 0.3 (0.3) 1.8 (1.7)BBL4 1.1 (1.6) 0.3 (0.3) 1.8 (1.6)BBL5 1.2 (1.6) 0.7 (0.3) 4.0 (1.7)BBL6 0.1 (1.7) − (0.3) − (1.7)BBL7 0.0 (1.6) 0.6 (0.3) 3.5 (1.7)BB mean 0.9 (1.6) 0.4 (0.3) 0.6 (1.7)

Halifax LineHL1 1.4 (1.4) 0.3 (0.3) 1.8 (1.7)HL2 1.1 (1.6) 0.3 (0.3) 1.5 (1.6)HL3 0.9 (1.7) 0.3 (0.3) 2.0 (1.7)HL4 1.6 (1.4) 0.4 (0.2) 2.1 (1.4)HL5 0.2 (0.2) 1.0 (1.4)HL6 1.1 (1.7) 0.3 (0.3) 1.6 (1.8)HL7 −0.6 (1.8) 0.8 (0.3) 4.7 (1.9)HL mean 0.9 (1.6) 0.4 (0.3) 0.1 (1.7)

Louisbourg LineLL1 0.6 (0.2) 3.7 (1.3)LL2 −0.1 (1.2) 0.2 (0.2) 1.4 (1.3)LL4 1.3 (1.4) 0.4 (0.3) 2.5 (1.5)LL5 1.2 (1.6) 0.4 (0.3) 2.5 (1.7)LL6 − (0.8) 0.2 (0.2) 1.2 (1.0)LL7 1.4 (1.4) 0.3 (0.3) 2.0 (1.6)LL8 0.0 (1.4) 0.6 (0.3) 3.4 (1.7)LL mean 0.8 (1.4) 0.4 (0.3) 0.2 (1.5)

Cabot Strait LineCSL1 0.0 (1.3) 0.1 (0.3) 0.5 (1.5)CSL2 −0.3 (0.9) 0.2 (0.2) 2.5 (1.0)CSL4 0.0 (0.7) −0.2 (0.1) −1.4 (0.4)CSL5 −0.2 (0.6) 0.4 (0.1) 2.3 (0.8)CSL6 0.0 (0.2) 0.4 (0.1) 2.1 (0.8)CSL mean 0.0 (1.5) 0.3 (0.3) 0.6 (1.7)

52 E.H. Shadwick, H. Thomas / Marine Chemistry 160 (2014) 42–55

estimates of NCPC in autumn, that are significantly larger than both thespring values and the NO3

−-based estimates for the same stations. Atsome locations as much as 60% of the estimated NCP takes place be-tween observations in spring and autumn (Table 2).

This method of computing NCPC does not include the contributionfrom the air–sea exchange of CO2, which may account for some of thedifference between the DIC- and NO3

−-based estimates. Based on thecumulative air–sea CO2 fluxes over the 6-month period from spring toautumn (April to October) computed by Shadwick et al. (2010), theair–sea exchange of CO2 represents a concentration change of lessthan 3 μmol kg−1 (corresponding to 0.1 mol C m−2) along the BrownsBank Line and at stations HL1, andHL2 due to the uptake of atmosphericCO2. Similarly, at the nearshore end of the Louisbourg Line section, andat stations CSL1 to CSL3 in Cabot Strait, air–sea CO2 fluxes represent asomewhat larger underestimate of NCPC, (equivalent to a 19 μmol kg−1

in DIC and 0.6 mol C m−2) due to enhanced CO2 uptake. At the offshoreend of the Louisbourg Line section the fluxes represent an overestimate(less than 8 μmol kg−1 change in DIC representing a 0.2 mol C m−2)due to outgassing. A similar overestimate due to outgassing on theNewfoundland side of the Cabot Strait, (representing a concentrationchange of less than 7 μmol kg−1, or 0.2 mol C m−2) is computed. Thus,with some regional variations, maximum over/underestimate of NCPCdue to air–sea exchange is smaller than the discrepancy between NCPCand the NO3

−-based estimate (Table 2).In addition to the contribution from the air–sea flux, there are other

factors thatmay contribute to the discrepancy between the carbon- andNO3

−-based estimates of NCP. The method used to compute NCPC andNCPN is based on a simple salinity normalisation that does not account

for seasonality in the freshwater end members, or hydrographic chang-es that bring water with a different relationship between DIC and salin-ity (and/or between NO3

− and salinity) into the region. Finally, thevertical integration of the ΔnDIC and ΔnNO3

− which we assume ac-counts for the upward intrusion of carbon- or nutrient-rich subsurfacewaters (e.g., Burt et al., 2013), does not allow vertical mixing to be ex-plicitly assessed.

Along the Browns Bank Line, the two estimates of NCP are generallyin good agreement in spring, with the exception of the two offshore sta-tions, BBL6 and BBL7. At these locations, the DIC-based estimates inspring yielded very low values of NCP, suggesting that biological draw-down of DIC in the surface waters had not yet begun. By contrast, thenNO3

−-based estimate in spring is the same as the autumn estimate,which suggests that the drawdown of NO3

− was complete at the timeof the spring sampling. A comparison of the seasonal profiles of nDICand nNO3

− at station BBL7 confirms that the two tracers of biologicalactivity are not consistent (see Figs. 9a and 10a). In autumn, the nDIC-based estimates of NCP at the nearshore stations are in good agreementwith the nNO3

−-based estimates, however further offshore the nDIC-based estimates are more than twice as large as the nNO3

−-based esti-mates. The situation is similar along theHalifax Line, with general agree-ment between the two estimates of NCP in spring, and significantlyhigher values computed with the nDIC deficit in autumn. This is partic-ularly true for stations HL3, HL4 andHL7 (see Table 2). Similar to stationBBL7 described above, there is a decrease in nDIC at station HL7throughout the water column that is not associated with a parallel in-crease in nNO3

− (compare Figs. 9b and 10b).Along the Louisbourg Line the carbon and nitrate-based NCP esti-

mates are in general agreement, with the exception of stations LL2and LL8. At both these locations, the nDIC deficit in spring suggeststhat biological productivity has not yet begun, (similar to the situationobserved at stations BBL6 and BBL7), while the nNO3

− deficit is similarin magnitude to that computed in autumn. At all stations on theLouisbourg line, the DIC-based NCP in autumn exceeds that computedon the basis of the nNO3

− deficit, with particularly large values of NCPCat the nearshore (LL1) and offshore stations (LL8). At the Cabot Straitstations in spring, the nDIC deficit suggests that springtime biologicalproduction has not yet begun, though as observed at the LouisbourgLine stations, the nNO3

− deficit suggests nutrient drawdown in springthat is only slightly less than that observed in autumn. By contrast, thenDIC deficit in autumn is much larger than the nNO3

− deficit for thesame period, and results in NCPC more than twice the magnitude ofthose computed via the spring nDIC deficit (Table 2).

At station CSL4 the NCPC is negative in autumn, suggesting an accu-mulation of DIC due to respiration, or remineralisation, of organic mat-ter in the surface waters. The estimate of NCPN is lower than the springvalue, but is still positive, due to the depleted NO3

− in the very near sur-face waters (see Fig. 10d). At station CSL4 the profile of nDIC in autumnindicates that concentrations are elevated relative to spring (Fig. 9d), afeature that is mirrored in the profiles of nNO3

− (with the exception ofvery shallow observations). It is possible that at this location, midwayacross the Cabot Strait, the delivery of organic material from the Gulfof St. Lawrence fuels the apparent respiration signal seen throughoutthe water column. By contrast, at station CSL6, the autumn profile ofnDIC reveals concentrations with even greater DIC depletion than wasseen in the profiles of in-situ DIC, despite the fact that there is a negligi-ble seasonal change in salinity at this station. Station CSL6 also indicatesreduced nNO3

− concentrations in autumn, extending well below thedepth of the summer mixed layer. If biological production reducesDIC, it may take place at the northeastern side of the Strait, or the de-crease may be the result of the input of lower DIC (and NO3

−) watersof similar salinity, originating elsewhere. Given that the signal ismixed throughout the water column, well below the euphotic zone,the latter is more likely.

In the neighbouring Gulf of Maine it has been suggested that organicmatter produced is consumed, or buried within the system (Charette

Fig. 9. Seasonal profiles of DIC normalised to a salinity of 35, nDIC, along the (a) Browns Bank, (b) Halifax, (c) Louisbourg, and (d) the Cabot Strait Line transects (see Fig. 1). Spring valuesare given in blue (+) and autumn in red (o). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

53E.H. Shadwick, H. Thomas / Marine Chemistry 160 (2014) 42–55

et al., 2001). It has also been suggested, based on amodelling study, thatthe continental shelf pump mechanism, which has the potential totransport dissolved carbon off the continental shelf does not operatein the Scotian Shelf region (Fennel and Wilkin, 2009). Vertical exportof organic matter is thought insufficient for carbon export from theshelf since the remineralised products of this material are brought tothe surface by deep winter mixing and/or upwelling in the region(Fennel and Wilkin, 2009). Our observations indicate that biologicalproduction is spatially variable throughout the region and suggestthat production occurring well after the height of the spring bloommakes an important contribution to the annual uptake of carbon inthe region.

At the northwestern side of the Cabot Strait aswell as at the offshoreend of the Browns Bank, Halifax and Louisbourg Line sections, the ad-vection of water with higher salinity and enhanced DIC concentrations(though not associatedwith an enhancement in subsurface NO3

−) in au-tumn is observed, with the largest increases in salinity (of 2.4 units) ob-served at depths between 50 m and 100 m. By contrast, at the inshorestations along the Halifax and Louisbourg Line sections, surface DIC con-centrations are influenced by the input of low-salinity, low-DIC, water

from the Gulf of St. Lawrence, while there is an increase in subsurface(at roughly 50m and below) salinity, which contributes to the seasonalincrease in subsurface DIC. Thus, the circulation in the region plays arole in the seasonal changes in inorganic carbon system, though biolog-ical processes, inferred here from seasonal surface deficits that do notallow physical contributions to be explicitly accounted for, dominateat the surface. Furthermore, this is consistentwith recentwork in the re-gion which, using radium isotopes and a 2-D diffusion model, showedan onshore flux of subsurface waters with excess DIC (relative to TA),which fuels the observed CO2 outgassing on the Scotian Shelf (Burtet al., 2013).

The largest values of NCP were observed at the offshore ends of allthree sections in the central Scotian Shelf, with high values also ob-served at the inshore end of the Louisbourg Line and at stationsmidwayalong the Halifax Line; these observations are broadly consistent withthe spatial distribution of chlorophyll-a reported by Shadwick et al.(2010) for the year 2007. The regional mean NCP in the central ScotianShelf (computed on the basis of nDIC changes) is 2.4 mol C m−2, equiv-alent to roughly 30 g C m−2 yr−1 if we assume that the biological pro-duction observed in the spring through autumn seasons is equal to the

Fig. 10. Seasonal profiles of nNO3 along the (a) Browns Bank, (b) Halifax, (c) Louisbourg, and (d) the Cabot Strait Line transects (see Fig. 1). Spring values are given in blue (+) and autumnin red (o). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

54 E.H. Shadwick, H. Thomas / Marine Chemistry 160 (2014) 42–55

annual production, which represents 23% to 50% of annual primary pro-duction based on the range PP=60 to 130 g Cm−2 yr−1 (Fournier et al.,1977; Mills and Fournier, 1979). This is consistent with the results ofCharette et al. (2001), who report that between 11% and 25% of the pri-mary production in the Gulf of Maine is exported out of themixed-layeras POC.

5. Conclusion

TheScotian Shelf is a biologically productive region of the northwest-ern Atlantic Ocean. This system is characterised by an autotrophic sur-face layer and acts as a source of atmospheric CO2 on the annual scale.Seasonal production varies spatially throughout the region with thelargest biological CO2 drawdown found in the southeastern BrownsBank region. Estimates of seasonal production based on nitrate amountto 50% of the inorganic carbon-based estimates due to the ongoing bio-logical production through the summermonths despite the rapid deple-tion of surface nutrients after the onset of the openwater phytoplanktonbloom in early spring. Seasonal changes in the inorganic carbon systemin this region are dominated by biology in the surfacewaters with large-scale circulation features influencingboth surface and subsurface carbon

distributions. At offshore stations in the central Scotian shelf and at theNewfoundland side of the Cabot Strait advection of high-salinity,water with enhanced inorganic carbon concentration is observed inthe summer and early autumn in the subsurface. The quantification ofthe advective supply or removal of carbon from the region goes beyondthe limits of the methods presented here and should be the focus of fu-ture work in the region.

Acknowledgements

Weare grateful to the captains, officers and crew of the CCGS Hudsonand to E. Head, E. Horne, J. Spry, and F. Prowe, for their co-operation inthe collection of field data. We thank the Bedford Institute of Oceanog-raphy, (Fisheries and Oceans Canada), for making the nutrient and hy-drographic data available. This project was funded by the CanadianNatural Sciences and Engineering Research Council (NSERC), MetOceanData Systems, and by theAustralianGovernments Cooperative ResearchCentres Program, through the Antarctic Climate and EcosystemsCooperative Research Centre (ACE CRC). This work contributes toIGBP/LOICZ.

55E.H. Shadwick, H. Thomas / Marine Chemistry 160 (2014) 42–55

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