Amounts, isotopic character, and ages of organic and ... · other half of exported riverine C [Meybeck, 1982, 1993; Cole et al., 2007]. Data from a limited number of estuarine sys-tems

GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 27, 1–16, doi:10.1002/gbc.20033, 2013

Amounts, isotopic character, and ages of organic and inorganiccarbon exported from rivers to ocean margins: 1. Estimates ofterrestrial losses and inputs to the Middle Atlantic BightKatie Hossler1 and James E. Bauer1

Received 8 July 2012; revised 29 November 2012; accepted 3 December 2012.

[1] Rivers transport carbon (C) from terrestrial ecosystems to the coastal ocean,providing significant heterotrophic support within both rivers and receiving coastalwaters. The amounts and ages of these terrestrial-river-coastal ocean C fluxes, however,are still poorly constrained. To address this uncertainty, a study of eight rivers dischargingto the Middle Atlantic Bight (MAB) was undertaken. The rivers were sampledperiodically over 2 years for concentrations and ı13C and �14C signatures of particulateorganic C (POC), dissolved organic C (DOC), and dissolved inorganic C (DIC). For thewatersheds draining to the MAB, it was estimated that �3800 Gg of terrestrial organic C(OC) and 700 Gg of terrestrial inorganic C was removed annually by fluvial transport. Ofthe terrestrial OC loss, �64% was contemporary C representing approximately 1% of theannual terrestrial net primary productivity. Net fluvial C inputs to the MAB shelf wereestimated to be �70 Gg�yr–1 of POC, 280 Gg�yr–1 of DOC, and 800 Gg�yr–1 of DIC.Terrestrial C, as opposed to in situ produced river C, comprised the majority of theriverine POC and DOC flux and around half of the total C flux. A smaller but significantfraction (<25%) of the river C flux was further composed of aged materials deriving fromfossil C and aged soil OC. The timing of fluvial OC inputs to the MAB, which appear tobe temporally offset from peak MAB primary production, could help support the netheterotrophy that has been observed there during periods of low productivity.Citation: Hossler, K., and J. E. Bauer (2013), Amounts, isotopic character, and ages of organic and inorganic carbon exportedfrom rivers to ocean margins: 1. Estimates of terrestrial losses and inputs to the Middle Atlantic Bight, Global Biogeochem.Cycles, 27, doi:10.1002/gbc.20033.

1. Introduction[2] Rivers play an integral role in the global carbon (C)

cycle by actively transporting different forms of organic andinorganic C (OC and IC, respectively) from terrestrial envi-rons to coastal ocean margins [Cole et al., 2007; Battinet al., 2009; Tranvik et al., 2009; Aufdenkampe et al., 2011;Bauer and Bianchi 2011]. On the terrestrial side, riversand other inland waters represent a significant fate of ter-restrial net primary productivity (NPP), with global NPPestimates being on the order of 56 Pg�yr–1 of C [Crameret al., 1999; Imhoff et al., 2004a; Ito, 2011], of which�3 Pg�yr–1 of C is exported to inland waters (i.e., �5%of terrestrial NPP) [Cole et al., 2007; Battin et al., 2009;Tranvik et al., 2009; Aufdenkampe et al. 2011]. Loadings of

All supporting information may be found in the online version of thisarticle.

1Department of Evolution, Ecology and Organismal Biology, The OhioState University, Columbus, Ohio, USA.

Corresponding author: Katie Hossler, Aquatic BiogeochemistryLaboratory, Department of Evolution, Ecology and Organismal Biology,The Ohio State University, 1314 Kinnear Road, Columbus, OH43212-1156, USA. ([email protected])

©2013. American Geophysical Union. All Rights Reserved.0886-6236/13/10.1002/gbc.20033

terrigenous, or allochthonous, organic matter (OM) supportthe predominant heterotrophy observed in inland aquaticecosystems [Cole and Caraco, 2001; Duarte and Prairie,2005; Bauer and Bianchi, 2011], resulting in a global netCO2 efflux from these waters on the order of 1 Pg�yr–1 ofC [Cole et al., 2007; Battin et al., 2009; Tranvik et al.,2009; Aufdenkampe et al. 2011]. Of the remaining C (includ-ing some in situ produced, or autochthonous, C), a portionis stored via sedimentation within freshwater systems andthe remainder—approximately 1 Pg�yr–1 of C globally—isexported to coastal ocean margins [Meybeck, 1982, 1993;Cole et al., 2007; Battin et al., 2009; Tranvik et al., 2009;Aufdenkampe et al., 2011].

[3] Of the exported riverine C, approximately half is OCin the forms of particulate and dissolved OC (POC andDOC, respectively) [Meybeck, 1982, 1993; Cole et al., 2007;Bauer and Bianchi, 2011]. The majority of the exportedOC may be remineralized in nearshore waters [Cai, 2011;Bauer and Bianchi, 2011; Bianchi and Bauer, 2011], sup-porting both the net heterotrophy observed in estuariesand inner continental shelves (i.e., nearshore) and the netautotrophy (e.g., through release of nutrients during rem-ineralization) reported for middle and outer continentalshelves, [Smith and Hollibaugh, 1993; Chen and Borges,2009; Borges and Abril, 2011; Cai, 2011]. Some fraction

1

HOSSLER AND BAUER: RIVERINE CARBON EXPORTS TO THE MAB

of refractory fluvial OC will be further exported to themiddle and outer shelves and still smaller amounts (<�2%of river exported OC) exported to the open ocean [e.g.,Meyers-Schulte and Hedges, 1986; Opsahl and Benner,1997; Hernes and Benner, 2006]. Riverine IC comprises theother half of exported riverine C [Meybeck, 1982, 1993; Coleet al., 2007]. Data from a limited number of estuarine sys-tems suggest that 0% to 90% of the initial riverine dissolvedIC (DIC) pool may be lost through degassing during tran-sit through nearshore waters [Cai and Wang, 1998; Abrilet al., 2000; Hunt et al., 2011]. However, for DIC as wellas POC and DOC, significant C additions may also occurduring transport through nearshore (e.g., estuarine) to shelfwaters [e.g., Fisher et al., 1998; Abril et al., 2002; Raymondand Hopkinson, 2003; Cai, 2011], resulting in net fluvialloads composing 10% to 60% of the total (i.e., net fluvial +nearshore) C exported to the coastal ocean margins [Kempet al., 1997; Cai and Wang, 1998; Raymond and Hopkinson,2003; Cai, 2011].

[4] Despite previous efforts to constrain components ofthe terrestrial-river-coastal C budget, there remains signifi-cant uncertainty in the amounts and characteristics of terres-trial C exported annually to rivers and the fluvial C loadsultimately reaching the coastal ocean. Also of growing inter-est are the relative contributions of aged (e.g., older than�60 years; note that following convention in radiocarbondating we define 1950 A.D. to be the dividing year betweenaged and modern [Stuiver and Polach, 1977]) versus modernforms of C involved in riverine C cycling and export. Whileit has been well established that fossil IC (e.g., carbonates)can comprise a significant fraction of riverine DIC and par-ticulate IC (PIC) pools [e.g., Meybeck, 1993; Telmer andVeizer, 1999; Raymond et al., 2004; Zeng et al., 2011], theimportance of fossil OC, as well as aged soil OC, to riverinePOC and DOC pools has only been relatively recently doc-umented [Spiker and Rubin, 1975; Meybeck, 1993; Kao andLiu, 1996; Cole and Caraco, 2001; Leithold and Blair, 2001;Blair et al., 2003; Masiello and Druffel, 2001; Raymondand Bauer, 2001a, b; Raymond et al., 2004; Longworth etal., 2007; Sickman et al., 2010]. Hence, rivers connect notonly contemporary, short-term stores of C between terres-trial and coastal ocean ecosystems, but also relic, long-termC stores. This latter process is potentially significant giventhe enormous sizes of fossil OC and IC reservoirs on land(�1500�104 Pg and >6000�104 Pg of C, respectively) rela-tive to terrestrial biomass (�2000 Pg of C) and atmosphericCO2 (�720 Pg of C) [Falkowski et al., 2000]. There is alsogrowing evidence that, once mobilized, even highly aged ter-restrial OM is not necessarily recalcitrant, but may be readilyremineralized in rivers, estuaries, and coastal waters [Coleand Caraco, 2001; McCallister et al., 2004; Schillawski andPetsch, 2008; Caraco et al., 2010; Griffith and Raymond,2011]. For these reasons, a better understanding of riverineC sources and characteristics is necessary for evaluating therole of aged OC and IC materials in terrestrial and oceanmargin C cycles.

[5] Here, we present findings from a study designed tocharacterize the amounts, sources, and ages of POC, DOC,and DIC for eight rivers of the northeastern U.S. The riversdischarge to a common, well-studied ocean margin, theMiddle Atlantic Bight (MAB) [e.g., Walsh et al., 1988;Biscaye et al., 1994; Verity et al., 2002] and represent�80%

of the water discharging to the MAB annually. Estimatesof total, allochthonous, and aged POC, DOC, and DICexports (see also Hossler and Bauer [2012]) provide newconstraints for terrestrial C losses and coastal ocean terrige-nous imports for the MAB drainage and other ocean marginregions. Natural and anthropogenic controls on these pro-cesses are addressed in our companion paper [Hossler andBauer, 2013].

2. Materials and Methods2.1. Study Area

[6] The eight rivers included in this study were the Con-necticut, Hudson, Delaware, Schuylkill (a tributary of theDelaware), Susquehanna, Potomac, Pamunkey and Roanoke(Figure 1). The rivers covered a range of hydrogeomor-phologies, underlying lithologies, land uses, and otheranthropogenic impacts (see Tables 1–3 in Hossler and Bauer[2013]). Across the study region, mean annual tempera-tures ranged from 6.4ıC to 13.5ıC and annual precipitationranged from 979 mm to 1160 mm (Table 1 and section S3 inHossler and Bauer [2013]).

2.2. Sample Collection and Processing[7] Details of sample collection and processing can be

found in Hossler and Bauer [2012]. Briefly, each river wassampled at approximately 3–4 months intervals between2005 and 2007 at the point farthest downstream that wasaccessible by small boat and above the reach of tidal influ-ence. Surface water samples (�0.1 m depth) were collectednear the mid-point of each river and filtered through pre-baked Whatman quartz fiber filters (QFF; 0.8�m). (Notethat one caveat to our sampling strategy is that by col-lecting surface water samples (although common practice)as opposed to width- or depth-integrated samples we havelikely underestimated the C pools, particularly the POC frac-tion [Curtis et al., 1979; Martin et al., 1992; Raymond et al.,2007]. Another potential source of sampling bias is the smallnumber of sampling events per river (i.e., 7); however, wedid capture a representative range of hydrologic conditions(Figure S3 in Text S1 of the auxiliary material), includingseveral high discharge events that are estimated to contributeto the majority of riverine C export [see e.g., Raymond andSaiers, 2010]. In a trade-off with finer scale sampling withina single river, this study focused on sampling a spectrum ofrivers within a single region.)

[8] POC was considered to be the fraction collecteddirectly on the QFF filters, and DOC was consideredthe fraction passing through the QFF filters. Samples forDIC were collected directly from the rivers using gas-tight syringes, then injected immediately into gas-tight glassserum bottles and fixed with saturated HgCl2 (0.2% v/v).(Note that the DIC fraction is a measure of the total DICspecies: i.e., dissolved CO2, carbonic acid, bicarbonate, andcarbonate. At the pH range (5.67 to 8.76) of the rivers in thisstudy, the predominant form of DIC is bicarbonate.)

[9] Acidified POC and DOC samples from which inor-ganic carbonates had been removed were oxidized to CO2(by sealed tube combustion and high-energy UV irradia-tion, respectively), then purified on a vacuum extraction line[Sofer, 1980; Bauer et al., 1992; Druffel et al., 1992; seealso Hossler and Bauer, 2012]. Acidified DIC samples were

2


Midd

le Atla

ntic

Bight

100 km

ConnecticutHudson

DelawareSchuylkill

Susquehanna

Potomac

Pamunkey

Roanoke

ConnecticutHudson

DelawareSchuylkill

Susquehanna

Potomac

Pamunkey

Roanoke36°N

40°N

44°N

70°W74°W78°W

Figure 1. Map of the U.S. east coast indicating the sampled watersheds (sample locations are markedwith yellow squares). The subregional watersheds (see section S3 in Hossler and Bauer [2013]) areoutlined in light blue. The red outline indicates the approximate region draining to the MAB. The basemap coloring depicts land relief and bathymetry with blue coloration indicating lower elevations, greencoloration indicating intermediate elevations, and yellow coloration indicating higher elevations. Thesmaller inset map shows the region of study with respect to North America. (See also section S1 fordetails on how the map was generated.)

sparged to extract the CO2 gas which was then purified ona vacuum extraction line [Sofer, 1980; Druffel et al., 1992;see also Hossler and Bauer, 2012]. Aliquots of CO2 werecollected in sealed Pyrex tubes and analyzed for ı13C and�14C at the National Science Foundation-Arizona Acceler-ator Mass Spectrometry (AMS) Facility. Concentrations ofPOC and DIC were determined by quantification of the CO2yield directly on the vacuum extraction line. DOC concen-trations were analyzed independently by high temperaturecatalytic oxidation.

[10] Note that PIC (another form of riverine C) was notquantified in this study. However, based on available C datafrom U.S. Geological Survey (USGS) water monitoring sta-tions for the Connecticut, Hudson, Delaware, Schuylkill, andPamunkey rivers from 2001 through 2012 [U.S. GeologicalSurvey, 2012], the PIC fraction appears to be only a minorconstituent of the total C load (�0.1%) for these northeasternU.S. rivers. Hence, by quantifying only the POC, DOC, andDIC pools, we still account for the vast majority of riverineC transported by the eight study rivers.

2.3. Data Analyses[11] All calculations, estimations, and statistical analy-

ses were performed in R 2.9.1 [R Development Core Team,2009]. In addition to the following analytical methodologies(subsections 2.3.1–2.3.3), section S2 details the calculationsfor extrapolating to the entire MAB region which will bepresented in section 3.4.

2.3.1. Weighted Mean Concentrations andIsotopic Signatures

[12] For each river, discharge-weighted mean concentra-tions were calculated from the two-year sample set as

NCs,p =

X

t(Cs,p(t)Fs(t))

X

tFs(t)

(1)

where NCs,p is the discharge-weighted concentration forsite s (i.e., Connecticut, Hudson, Delaware, Schuylkill,Susquehanna, Potomac, Pamunkey, or Roanoke) and carbonpool p (i.e., POC, DOC, or DIC); Cs,p(t) is the concentrationfor site s and carbon pool p measured on sampling date t; andFs(t) is the mean daily discharge for site s on sampling datet. (Mean daily discharge data for each site and sampling date(i.e., Fs(t)) were obtained from the publicly available USGSwater data [U.S. Geological Survey, 2011]; see also sectionS3 in Hossler and Bauer [2013].)

[13] Mean �14C signatures were calculated in a similarmanner, but using flux-weighted data

NIs,p =

X

t(Is,p(t)Cs,p(t)Fs(t))

X

t(Cs,p(t)Fs(t))

(2)

where NIs,p is the flux-weighted isotopic signature (i.e.,�14C)for site s and carbon pool p; Is,p(t) is the isotopic signature

3


for site s and carbon pool p measured on sampling datet; Cs,p(t) is the concentration for site s and carbon pool pmeasured on sampling date t; and Fs(t) is the mean dailydischarge for site s on sampling date t.2.3.2. Total C Export Estimates

[14] Total (i.e., sum of allochthonous, autochthonous,aged, and modern) POC, DOC, and DIC exports were calcu-lated for each watershed from the discharge-weighted meanconcentrations (equation (1)) and mean annual discharge

NXs,p = NCs,p NFs (3)

where NXs,p is the mean annual export for site s and carbonpool p; NCs,p is the discharge-weighted concentration for sites and carbon pool p (equation (1)); and NFs is the mean annualdischarge for site s. (Mean daily discharge data for theentire 2003–2010 period were also acquired from the pub-licly available USGS water data [U.S. Geological Survey,2011] to determine the mean annual discharge for each site(i.e., NFs); see also section S3 in Hossler and Bauer [2013].)Estimation of confidence intervals for total C exports isdescribed in section S3.2.3.3. Allochthonous and Aged CContribution Estimates

[15] Allochthonous C was considered to be any OC or ICmaterial originating external to the river (i.e., terrestrially-derived), as opposed to autochthonous materials producedwithin the river (e.g., photosynthetically fixed CO2). AgedC was defined to be any OC or IC material older than�60 years (i.e., not “modern”; note that following con-vention in radiocarbon dating, we define 1950 A.D. to bethe dividing year between aged and modern [Stuiver andPolach, 1977]).

[16] To determine the allochthonous and aged C contri-butions to POC, DOC, and DIC exports, we first estimatedfractional contributions from six potential sources for POCand DOC (i.e., C3 plant material (C3 OC), C4 plant material(C4 OC), algal material (algal OC), slow-turnover soil OC(slow SOC; turnover time 25 years), passive-turnover soilOC (passive SOC; turnover time 5000 years), and fossil OC)and four potential sources for DIC (i.e., atmospheric CO2exchange, carbonate dissolution, POC remineralization, andDOC remineralization) using a time-varying isotope mix-ing model (see Hossler and Bauer [2012] (e.g., Figures1 and S2 and Tables S6–S8) for details). Allochthonouscontributions to POC and DOC were then the combinedproportions of C3 OC, C4 OC, slow SOC, passive SOC,and fossil OC to the respective POC and DOC pools.Allochthonous contributions to DIC were the combinedproportions of IC deriving from carbonate dissolution, rem-ineralization of allochthonous POC, and remineralizationof allochthonous DOC. Note that for POC and DOC, weassumed that all C3 and C4 plant materials were terrestri-ally derived, although some portion of these contributingmaterials were likely from emergent aquatic vegetationand therefore autochthonous. For DIC, we assumed thatthe allochthonous proportions of remineralized POC andremineralized DOC were identical to the allochthonous pro-portions of the initial POC and DOC pools (i.e., no preferen-tial remineralization of allochthonous versus autochthonousOC; note that this assumption should be a reasonablefirst approximation given the much greater proportion of

allochthonous OC versus autochthonous OC despite the pre-sumably greater lability of the latter [Kritzberg et al., 2005,2006]).

[17] Aged C sources included passive SOC (�14C = �–540�; see section S8 in Hossler and Bauer [2012]) andfossil OC (�14C = –1000�) for POC and DOC, and carbon-ate dissolution (note that this IC source was modeled to bean even mixture of carbonate rock (with �14C = –1000�)and the weathering agent carbonic acid (with an estimatedaged proportion equivalent to that of the DOC pool)) andaged fractions of remineralized POC and DOC for DIC.Note that as for allochthonous DIC, we assumed that theaged proportions of POC and DOC were exactly representedin the contributions of remineralized POC and remineral-ized DOC to the DIC pool. For POC, DOC, and DIC, wealso assumed that the “modern” sources were comprisedentirely of modern C, when in fact because of the cyclingof C, these modern sources would also contain some pro-portion of aged C. Estimation of confidence intervals forboth allochthonous and aged C contributions is described insection S3.

3. Results and Discussion3.1. River C Concentrations and Isotopic Character

[18] Of the total riverine C (i.e., sum of POC, DOC, andDIC; see also section 2.2) in the rivers of the present study,DIC comprised the majority of the total C, with discharge-weighted mean percentages ranging from 63% to 87% inseven of the eight rivers (Table 1). DOC comprised the sec-ond largest fraction (11% to 29%), and POC comprised thesmallest fraction (2% to 9%) of the total C. For the eightrivers combined, the majority of the total riverine C was inthe form of DIC (�75%), followed by DOC (�19%), thenPOC (�6%; Table 1). Considering the OC separately, thedominant component was DOC which comprised �76% ofthe total OC in the eight rivers combined, and ranged from61% in the Potomac River to 85% in the Delaware andSchuylkill rivers.

[19] The river OC concentrations of the present study(Table 1) were comparable to previous observations fromthe Hudson, Delaware, and Susquehanna rivers and fromthe geographically proximate Parker, York, and Neuse riversof 0.2 mg�L–1 to 9.1 mg�L–1 POC (�26% of the totalOC) and 1.4 mg�L–1 to 12.0 mg�L–1 DOC (�74% of thetotal OC) [Malcolm and Durum, 1976; Findlay et al.,1996; Fisher et al., 1998; Mannino and Harvey, 1999;Raymond and Bauer, 2000; Raymond and Hopkinson, 2003;Raymond et al., 2004; Longworth et al., 2007]. The riverOC concentrations from the present study also coincidedwith reported estimates of 1.0 mg�L–1 to 10.0 mg�L–1 POCand 0.6 mg�L–1 to 18.1 mg�L–1 DOC for North America[Meybeck, 1982; Ludwig et al., 1996; Dai et al., 2012;Lauerwald et al., 2012] and of � 4.7 mg�L–1 POCand 1.0 mg�L–1 to 20.0 mg�L–1 DOC for temperate andtemperate-wet bioclimates [Meybeck, 1982; Ludwig et al,1996].

[20] The DIC concentrations observed in the present study(Table 1) were typically over twice the 2.7 mg�L–1 to7.7 mg�L–1 DIC range reported from an earlier study thatincluded the Hudson, Delaware, and Susquehanna rivers[Raymond et al., 2004]. However, the samples from the

4


Table 1. Discharge-Weighted Concentrations and Percentages of POC, DOC, and DIC in the Eight Study Riversa

Concentration (mg�L–1) Percent of Total (%)

POC DOC DIC POC DOC DIC

Connecticut RiverMean 0.7 2.9 6.6 7 28 64Range (0.2, 1.2) (2.0, 3.9) (5.5, 8.8) (2, 14) (20, 35) (60, 78)

Hudson RiverMean 0.7 3.1 13.6 4 18 78range (0.5, 1.4) (2.4, 4.5) (11.8, 15.6) (3, 7) (13, 23) (70, 84)

Delaware RiverMean 0.5 2.2 10.8 3 17 80range (0.3, 1.2) (1.5, 3.0) (8.7, 15.0) (2, 8) (10, 25) (73, 87)

Schuylkill RiverMean 0.4 2.4 19.1 2 11 87range (0.2, 0.5) (1.6, 3.6) (18.1, 26.0) (1, 2) (8, 16) (82, 90)

Susquehanna RiverMean 0.7 2.1 12.8 5 14 82range (0.2, 1.5) (1.4, 2.8) (9.9, 19.6) (1, 9) (7, 20) (76, 92)

Potomac RiverMean 2.3 3.6 19.6 9 14 77range (0.4, 3.2) (1.6, 4.1) (13.3, 27.8) (1, 11) (7, 22) (70, 91)

Pamunkey RiverMean 2.7 7.0 4.5 17 48 36range (0.2, 3.8) (2.7, 8.8) (3.9, 10.5) (2, 23) (31, 54) (24, 65)

Roanoke RiverMean 1.1 3.8 8.1 8 29 63range (0.3, 2.8) (2.5, 5.8) (6.3, 9.2) (2, 17) (20, 36) (47, 74)

All Eight Rivers CombinedMean 0.9 3.0 12.5 6 19 75range (0.6, 1.4) (2.2, 3.9) (11.3, 15.7) (3, 7) (14, 25) (68, 82)

a The percent contribution to total was determined by dividing the concentration of each of POC, DOC, and DIC by their sum concentration. The rangeof measurements over the seven sampling dates is also indicated for each mean (“range” row, with values in parentheses). The final two rows providedischarge-weighted means and ranges for all eight rivers combined.

earlier study had been collected much farther upstream(40 km to 200 km) than the samples from the present study,which may account for the discrepancy. On the other hand,the DIC concentrations in this study compared well withobservations from other temperate-wet rivers (e.g., the St.Lawrence River and major tributaries in Canada and theOuseburn, Tern, and Tyne rivers in Great Britain), withreported DIC concentrations ranging from 6.3 mg�L–1 to64.0 mg�L–1 [Hélie and Hillaire-Marcel, 2006; Baker et al.,2008]. (Note also, for the rivers in the present study, the DICpool was typically composed of �20% free dissolved CO2and carbonic acid and �80% bicarbonate.)

[21] The �14C signatures of the three major C poolsvaried considerably across the eight rivers. For POC, theflux-weighted mean�14C signatures ranged from –163� to89�; �14C-DOC ranged from –55� to 509�; and �14C-DIC ranged from –142� to 405� (Figure 2 and Table S1).The extreme variation in �14C was due almost exclusivelyto just two of the rivers: the Schuylkill and Pamunkey. Thesetwo rivers had exceptionally enriched �14C signatures inall three C fractions (Figure 2 and Table S1). The mostlikely explanation was some form of radiocarbon contami-nation from upstream nuclear power plants (see section S5in Hossler and Bauer [2013]).

[22] For the eight rivers combined, flux-weighted mean�14C signatures were –92�, 17�, and –86� for POC,

DOC, and DIC, respectively (Figure 2 and Table S1). (Notethat because the two radiocarbon contaminated rivers (i.e.,Schuylkill and Pamunkey) were much smaller, the other sixrivers, excluding the two outlying rivers, changed the com-bined flux-weighted mean �14C signatures only slightly:–98�, 7�, and –90� for POC, DOC, and DIC, respec-tively; see also Figure 2.) Both the POC and DIC fractionscontained a substantial portion of aged C based on the equiv-alent mean 14C ages of �800 yr BP (conventional radiocar-bon years before 1950 A.D.) and �700 yr BP, respectively(and �800 yr BP for both POC and DIC excluding theSchuylkill and Pamunkey rivers; Figures 2a and 2c). In con-trast, the DOC fraction was primarily modern in character(Figure 2b). Across diverse river systems, �14C signatureshave been shown to range from highly aged to fully moderndepending on such factors as hydrogeomorphology, lithol-ogy, temperature, and land-use [e.g., Hedges et al., 1986b;Kao and Liu, 1996; Raymond and Bauer, 2001b; Raymondet al., 2004; Zeng et al., 2011]. Controls on inputs from agedC sources in this study are discussed in the companion paper[Hossler and Bauer, 2013].

[23] The mean age of POC from these eight rivers over-laps with POC ages observed for MAB coastal waters towhich these rivers discharge (Figure 2a) [Bauer et al., 2001,2002]. Shelf water circulation patterns suggest that muchof MAB POC derives from water inflowing from the north

5


4500 yr BP

0 yr BP500 yr BP

1500 yr BP

-600

-400

-200

0

200

1100

Δ14C

-DO

C (

‰)

Δ14C

-PO

C (

‰)

Δ14C

-DIC

(‰

)

4500 yr BP

0 yr BP500 yr BP

1500 yr BP

-400

-200

0

200

400

600

800

1950

0 yr BP500 yr BP

1500 yr BP-200

0

200

400

600

1500

1700

Con

nect

icut

Hud

son

Del

awar

eS

chuy

lkill

Sus

queh

anna

Pot

omac

Pam

unke

yR

oano

keC

ombi

ned

Com

bine

d*M

AB

(N

orth

)M

AB

(C

entr

al)

MA

B (

Sou

th)

a

b

c

Figure 2. Barplots of (a) �14C-POC, (b) �14C-DOC, and (c) �14C-DIC for the eight rivers of thisstudy (lightly shaded bars); the flux-weighted means of all eight rivers combined and (*) all except theSchuylkill and Pamunkey rivers combined (darkly shaded bars); and literature values (open bars) forthe north, central, and south regions of the MAB shelf and slope [source: Bauer et al., 2001, 2002].For each C pool, the vertical bar boundaries indicate the range of observed values, while the hori-zontal black bar within each vertical bar indicates either the flux-weighted mean (this study) or theunweighted mean (MAB values). The solid and dashed horizontal lines indicate conventional radiocar-bon years before 1950 A.D. (yr BP; solid line is 0 yr BP and dashed lines are 500 yr BP, 1500 yr BP, and4500 yr BP). (Note that the exceptionally enriched�14C signatures in the Schuylkill and Pamunkey riverswere likely a result of radiocarbon contamination from upstream nuclear power plants; see section S5 inHossler and Bauer [2013].)

(i.e., Gulf of Maine and Georges Bank; [Beardsley andBoicourt, 1981; Townsend et al., 2006]), with additionalcontributions from contemporary aged primary production,moderately-aged riverine POC (e.g., this study), and morehighly aged sedimentary POC brought in by intrusions andupwellings from the eastern and southeastern MAB slope[Anderson et al., 1994; Bauer et al., 2001, 2002; DeMasteret al., 2002].

[24] In contrast, the mean riverine �14C-DOC signatureof this study was much more enriched than the bulk DOCfractions reported for the MAB (e.g., mean riverine �14C-

DOC of 17� versus mean MAB �14C-DOC of –251�;Figure 2b) [Bauer et al., 2001, 2002]. Similar to POC, how-ever, previous studies suggest that the MAB DOC reflects apredominant contribution from DOC of inflowing northernwaters, with further DOC inputs from degradation of MABPOC, primary production, riverine DOC (e.g., this study),and highly aged MAB slope water DOC [Peterson et al.,1994; Raymond and Bauer, 2000, 2001a; Bauer et al., 2002;Raymond and Hopkinson, 2003].

[25] The highly-aged riverine DIC of this study also con-trasted with the modern-aged DIC reported for the MAB

6


Table 2. Mean Annual Exports of POC, DOC, and DIC From theEight Study Rivers to the Nearshore MABa

Mean Annual Export (Gg�yr–1)

POC DOC DIC

Connecticut RiverMean 13 54 12095% CI (8, 20) (36, 75) (89, 160)

Hudson RiverMean 13 57 25095% CI (9, 20) (42, 79) (200, 300)

Delaware RiverMean 6.6 31 15095% CI (4.0, 11) (21, 46) (120, 210)

Schuylkill RiverMean 1.3 7.7 6395% CI (0.9, 1.9) (5.1, 12) (49, 91)

Susquehanna RiverMean 26 74 46095% CI (12, 46) (48, 120) (330, 680)

Potomac RiverMean 26 42 23095% CI (8, 60) (22, 88) (140, 470)

Pamunkey RiverMean 2.3 6.2 3.995% CI (0.2, 5.7) (1.6, 14) (1.9, 10)

Roanoke RiverMean 8.1 28 5895% CI (2.4, 21) (12, 62) (29, 120)

All Eight Rivers CombinedMean 100 300 133095% CI (60, 160) (210, 460) (1050, 1940)

a For each river, estimates for mean annual exports of POC, DOC, andDIC based on the discharge-weighted mean concentrations (Table 1) andmean annual discharge (Table 1 in Hossler and Bauer [2013]) are pro-vided. The 95% CI (obtained by bootstrap with 10,000 resamplings) is alsoindicated for each estimate (“range” row, with values in parentheses).

(Figure 2c) [Bauer et al., 2001, 2002]. As demonstratedin earlier studies, however, most of the MAB DIC derivesfrom waters near atmospheric equilibrium (i.e., modern-aged) such as the Gulf of Maine and Georges Bank waters tothe north (�14C � –50� to 50�; [Sherwood et al., 2008])and MAB slope waters to the south and southwest (�14C� –50� to 80�; [Bauer et al., 2002]). Additional atmo-spheric exchange of CO2 can also be expected during theapproximately 100 days residence time on the MAB shelf[Mountain, 1991].

3.2. Total, Allochthonous, and Aged C Exports3.2.1. Total C Exports

[26] Mean annual total (i.e., allochthonous, autochtho-nous, aged, and modern) C export from the eight rivers com-bined was estimated at 100 Gg�yr–1 as POC, 300 Gg�yr–1

as DOC, and 1330 Gg�yr–1 as DIC (Table 2). Approxi-mately one third of the total river C exports to the MABwere attributable to the Susquehanna River alone, the largestof the study rivers. Areal yields (i.e., export normalizedto drainage area; not shown) ranged from 0.2 Mg�km–2 �yr–1 to 0.8 Mg�km–2 � yr–1 of C for POC, and from

1.0 Mg�km–2 � yr–1 to 2.0 Mg�km–2 � yr–1 of C for DOC,which compared favorably to the broad range of estimatesfor similar regions and bioclimates (0.04 Mg�km–2 � yr–1 to4.7 Mg�km–2 � yr–1 of C for POC and 0.3 Mg�km–2 � yr–1 to41.7 Mg�km–2 � yr–1 of C for DOC) [Meybeck, 1993; Hopeet al., 1994, 1997b; Ludwig et al., 1996; Worrall et al., 2007;Ogrinc et al., 2008; Lauerwald et al., 2012]. For DIC, arealyields were more variable, ranging from 1.3 Mg�km–2 � yr–1

to 13.0 Mg�km–2 �yr–1 of C, but were within the range of esti-mates reported by Moosdorf et al. [2011] for North Americaof 0.05 Mg�km–2 � yr–1 to 25.8 Mg�km–2 � yr–1 of C (note,the Moosdorf et al. [2011] estimates were actually for bicar-bonate, which was typically �80% of the DIC pool in thisstudy).3.2.2. Allochthonous C Contributions to RiverExport Fluxes

[27] Of particular interest in the present study werethe contributions of allochthonous and aged materials (fordefinitions, see section 2.3.3) to the regional C exportsfrom the study rivers. Contributions were estimated for sixof the eight study rivers (i.e., the Connecticut, Hudson,Delaware, Susquehanna, Potomac, and Roanoke), but wereunattainable for two of the rivers (i.e., Schuylkill andPamunkey) because obvious radiocarbon contamination pre-cluded application of the isotope mixing models used todifferentiate C source contributions (see Hossler and Bauer[2012] and section S5 in Hossler and Bauer [2013]).

[28] Estimated allochthonous contributions to exportedPOC ranged from 11% to 67% (95% CI) in the DelawareRiver up to 61% to 100% (95% CI) in the RoanokeRiver (Figure 3a and Table S2). POC exports from the sixrivers combined were derived 46% to 91% (95% CI) fromallochthonous sources. The predominance of allochthonousPOC in these rivers agreed with previous studies that havequalitatively identified terrestrial source materials mainlythrough isotope analysis, C:N ratios, and lignin biomark-ers [Hedges et al., 1986a; Pocklington and Tan, 1987; Tanand Edmond, 1993; Hope et al., 1997a; Onstad et al., 2000;Kanduc et al., 2007; Wu et al., 2007; Ogrinc et al., 2008].(A more in-depth discussion of allochthonous sources ofPOC (as well as of DOC and DIC) can be found in Hosslerand Bauer [2012].)

[29] DOC tended to be composed of allochthonous mate-rials to a much greater extent than POC in the six rivers(Figure 3b and Table S2). Allochthonous C represented overthree quarters of exported DOC in four of the six rivers. TheSusquehanna River was the primary exception, with esti-mated allochthonous contributions as low as 8% (95% CI)to exported DOC (Figure 3b and Table S2). This reducedallochthonous contribution may be attributable in part toboth greater presence of finely textured soils and fewerwetlands within the Susquehanna watershed. (Factors con-trolling allochthonous contributions to DOC (as well as toPOC and DIC) are addressed more thoroughly in the com-panion paper [see Hossler and Bauer, 2013].) For the sixrivers combined, the estimated allochthonous contributionto DOC was 67% to 96% (95% CI), the majority of whichderived from either recently-fixed plant material or youngSOC [Hossler and Bauer, 2012]. Allochthonous materialshave also been found to constitute the majority of DOCin various other streams and rivers through correlationsbetween DOC flux and extent of peat cover, isotope analysis,

7


020406080

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awar

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awar

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Figure 3. Percent contributions of (a–c) allochthonous Cand (d–f) aged C to river POC, DOC, and DIC exports fromthe six rivers to the MAB. Mean estimates for each riverare indicated by the smaller horizontal black bars, while theshaded vertical bars span the 95% CI. The dashed horizon-tal lines indicate the mean estimates across all six rivers,with the dotted horizontal lines indicating the approximate95% CI. (See section 2.3.3 for estimation details. Note thatestimates could only be obtained for six of the eight studyrivers because radiocarbon contamination in the Schuylkilland Pamunkey rivers precluded application of the isotopemixing models used to differentiate C source contributions(see text for additional details; see also Hossler and Bauer[2012] and section S5 in Hossler and Bauer [2013]).)

C:N ratios, and UV adsorption [Hope et al., 1997a; Schiffet al., 1997; Palmer et al., 2001; Hélie and Hillaire-Marcel,2006; Sanderman et al., 2009].

[30] Allochthonous contributions to the DIC pool of thesix rivers were lower compared to POC and DOC (Figure 3cand Table S2). The notable exception was the Connecti-cut River, in which allochthonous C comprised 38% to99% (95% CI) of the exported DIC. Factors leading togreater allochthonous DIC in the Connecticut River water-shed likely relate to apportionment between surface andsubsurface runoff, such as soil texture (e.g., Connecticutwatershed soils tend to be more coarsely textured) and landuse cover (e.g., the Connecticut watershed is more forestedand less developed than the other five watersheds) [for addi-tional details, see Hossler and Bauer, 2013]. For the otherfive rivers, estimated contributions tended to be in the rangeof 30% to 60%, which was similar to the estimated com-bined allochthonous contribution to exported DIC (95% CI:37% to 52%) . Both carbonate dissolution and OC rem-ineralization have been identified as major allochthonoussources of DIC in previous studies, with the balance betweenthe two sources depending on factors such as lithology,hydrologic flow path, and OC bioavailability [e.g., Telmer

and Veizer, 1999; Finlay, 2003; Raymond and Hopkinson,2003; Raymond et al., 2004; Zhang et al., 2009; Zeng et al.,2011]. (Factors controlling allochthonous contributions toDIC (as well as to POC and DOC) are addressed more thor-oughly in the companion paper [see Hossler and Bauer,2013].) For the six rivers in the present study, allochthonouscontributions to DIC were approximately one third carbon-ate dissolution (note that this proportion includes IC fromboth the carbonate rock and the weathering agent carbonicacid) and two thirds remineralization of allochthonous OC[Hossler and Bauer, 2012]. The remaining 40% to 70%of exported DIC (95% CI) derived from atmospheric CO2exchange (i.e., was in isotopic equilibrium with atmosphericCO2), which has also been established as an important pro-cess affecting DIC composition in other riverine systemsbased on 13C analyses [Pawellek and Veizer, 1994; Hélie etal., 2002; Finlay, 2003; Brunet et al., 2005; Wachniew, 2006;Zhang et al., 2009].3.2.3. Aged C Contributions to River Export Fluxes

[31] Aged C typically represented less than 30% of thedifferent C fluxes for the six rivers in the present study(Figure 3d–3f and Table S2). For POC, aged C comprised7% to 18% (95% CI) of the combined river exports. Theaged POC fraction was lowest in the Potomac River (95%CI: 1% to 8%) and highest in the Susquehanna River(95% CI: 9% to 34%; Figure 3d). On average, 74% ofthe aged POC fraction was estimated to derive from fossilOC sources, with the remainder from passive-turnover SOC(turnover time 5000 years; [Hossler and Bauer, 2012]). Theproportions of aged—and particularly fossil—POC weremuch lower for these east coast rivers than has been reportedfor small mountainous rivers, such as the Eel River inthe northern California, USA, (�50%; [Blair et al., 2003])and the Lanyang Hsi River in Taiwan (>70%; [Kao andLiu, 1996]). This is consistent with the hypothesis of Blairet al. [2003] that aged contributions would be lower forrivers draining “passive” continental margins (e.g., U.S. eastcoast). An aged POC component has also been identified innumerous other rivers, with sources including soil erosion,rock weathering, and fossil-fuel combustion [Hedges et al.,1986b; Masiello and Druffel, 2001; Raymond and Bauer,2001b; Raymond and Hopkinson, 2003; Raymond et al.,2004; Longworth et al., 2007].

[32] Of the three major river C pools (i.e., POC, DOC,and DIC), aged C sources contributed least to the exportedDOC (Figure 3e and Table S2). Per river, the estimatedaged DOC fraction ranged from 0% (the lower range forfour of the six rivers) to as much as 23% in the DelawareRiver (95% CI). For the six rivers combined, the esti-mated aged C contribution was 3% to 11% (95% CI) ofthe exported DOC. Of this aged DOC export, only 18%was derived from fossil OC sources [Hossler and Bauer,2012]. In other riverine studies measuring both DOC andPOC �14C, the DOC component has been observed to bemore modern in origin [Masiello and Druffel, 2001; Ray-mond and Bauer, 2001b; Raymond and Hopkinson, 2003;Longworth et al., 2007]. This age disparity suggests signif-icant differences in the sources and processes controllingcatchment POC and DOC contributions to rivers [Raymondand Bauer, 2001b; Raymond and Hopkinson, 2003]. In thepresent study, for example, while fossil OC contributed themajority of aged POC, passive SOC comprised the major-

8


0

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Figure 4. Time series plots showing (a–c) total C exports (kg � s–1) and percent contributions to totalC exports from (d–f) allochthonous C and (g–i) aged C for the rivers of the present study. Exports (i.e.,Figures 4a–4c) are plotted against the leftmost y axes and scales vary by parameter; percent contribu-tions (i.e., Figures 4d–4i) are plotted against the rightmost y axes. Regions of 95% CI are indicated byshading and estimates are indicated by thick black lines. Vertical shaded bars indicate winter months (i.e.,December (D) through March (M)); spring, summer, and fall months are unshaded (e.g., June (J) andSeptember (S)). (Note: in Figures 4a–4c, data have been compiled from all eight study rivers, whereas inFigures 4d–4i, data have been compiled from only six of the eight study rivers; allochthonous and aged Cestimates could not be obtained for the Schuylkill and Pamunkey rivers because of model violations (seetext for additional details; see also [Hossler and Bauer, 2012]).)

ity of aged DOC [see also Hossler and Bauer, 2012]. Anaged DOC fraction originating primarily from soils hasalso been suggested in other studies [Schiff et al. 1997,Longworth et al. 2007, Sanderman et al. 2009, Sickmanet al. 2010]. Possible controlling factors for these differ-ences in source contributions are explored in Hossler andBauer [2013].

[33] Aged C contributions to river DIC comprised 11%to 16% (95% CI) of the combined six river export to theMAB (Figure 3f and Table S2). Per river, the aged DICfraction was lowest in the Roanoke River (95% CI: 0%to 9%) and highest in the Delaware River (95% CI: 14%to 24%). Of the aged DIC fraction, approximately 60%derived from fossil IC (e.g., carbonates), with the remaining40% contributed by remineralization of aged POC and DOCfractions (24% originating from fossil OC and 16% orig-inating from passive-turnover SOC) [Hossler and Bauer,2012]. Moosdorf et al. [2011] estimated that �36% of river-ine bicarbonate flux stemmed from carbonate weathering inNorth America. Among the rivers of the present study, 80%of the DIC flux was in the form of bicarbonate on average,of which an estimated 10% to 30% resulted from carbon-ate weathering [see also Hossler and Bauer, 2012]. Thisestimate is less than that of Moosdorf et al. [2011]; how-ever, the northeastern U.S. region is not particularly rich incarbonate-bearing rocks compared to other North Americanregions such as the midwestern U.S. (e.g., see lithologic mappresented in Jansen et al., [2010]).

3.3. Seasonal Patterns in Total, Allochthonous, andAged C Exports

[34] Riverine C flux dynamics in this study were domi-nated by discharge and tended to be highest in the late falland winter months during peak flows (Figures 4a–4c andS2), although there was considerable site-to-site variabil-ity (Figures S1 and S3). The dominating discharge effectalso accounted for the synchronous temporal behavior ofthe various C pools (i.e., POC, DOC, and DIC, and total,allochthonous, and aged). The control of discharge on Cflux dynamics has been well established in the literature[e.g., Meybeck, 1982; Bluth and Kump, 1994; Ludwig et al.,1996; Moosdorf et al., 2011; Alvarez-Cobelas et al., 2012;Lauerwald et al., 2012] and is primarily due both to its directeffect on flux (i.e., C flux is the product of discharge and Cconcentration, hence as discharge increases, C flux shouldalso increase) and its larger coefficient of variation over thatof C concentration [see e.g., Benson, 1965; Kenney, 1982].

[35] The discharge effect on flux can also be medi-ated through differences in C concentration. For POC,the discharge-concentration relationship is typically direct[Pocklington and Tan, 1987; Correll et al., 2001]. For DOC,the observed relationship with discharge has been foundto be direct [Correll et al., 2001; Wilson and Xenopoulos,2008; Sanderman et al., 2009; Raymond and Saiers, 2010],inverse [Wilson and Xenopoulos, 2008], or not significant[Pocklington and Tan, 1987; Raymond and Hopkinson,2003; Wilson and Xenopoulos, 2008]. In the present study,

9


discharge correlated positively with POC concentration forthree of the rivers (Connecticut: r = 0.75, p = 0.05; Potomac:r = 0.89, p = 0.01; and Pamunkey: r = 0.96, p = 0.02;not shown), and with DOC concentration for only one river(Pamunkey: r = 0.83, p = 0.04; not shown). In contrast,the discharge-concentration relationship has generally beenfound to be inverse for DIC [Bluth and Kump, 1994; Finlay,2003; Brunet et al., 2005; Baker et al., 2008; Barnes andRaymond, 2009; Zeng and Masiello, 2010]. However, onlytwo of the rivers in the present study demonstrated negativecorrelations between discharge and DIC concentration (Con-necticut: r = –0.93, p = 0.003; and Delaware: r = –0.87,p = 0.01; not shown).

[36] Across all C pools, allochthonous inputs to the sixrivers assessed in this study tended to be higher in thewinter (Figures 4d–4f and S4). Various riverine studieshave also observed allochthonous contributions to be higherin the winter, while autochthonous contributions peak inthe summer when conditions are more favorable for pri-mary production [Pocklington and Tan, 1987; Atekwana andKrishnamurthy, 1998; Hellings et al., 1999; Kendall et al.,2001; Hélie et al., 2002; Hélie and Hillaire-Marcel, 2006;Wachniew, 2006].

[37] Aged C inputs were more variable than total (i.e.,aged plus modern) allochthonous C inputs, although theytended to increase in late winter/early spring (Figures 4g–4iand S5). For POC, this pattern may be attributed to increasedsoil erosion and rock weathering during high flow [Kao andLiu, 1996; Blair et al., 2003]. For DOC and DIC, how-ever, an opposite pattern might be expected: i.e., more agedcontributions during low flow because of greater contri-butions from groundwater (as opposed to surface water)which would be in contact with deeper, older soils in thecase of DOC [Schiff et al., 1997, 1998; Duan et al., 2007;Sanderman et al., 2009] and with carbonate-rich bedrockin the case of DIC [Atekwana and Krishnamurthy, 1998;Finlay, 2003; Kanduc et al., 2007] (see also Table 2 inHossler and Bauer [2013]). Instead, the positive relation-ship between discharge and aged DOC and DIC observedin the present study (Figures 4h and 4i) suggests that simi-lar mechanisms controlling aged POC may also operate foraged DOC and DIC in certain watersheds.

[38] One aspect of the observed seasonality in riverineOC fluxes is that the timing may be offset from peak primaryproductivity in coastal ocean margins, such that while on anannual basis fluvial OC inputs provide only minor subsidyto coastal ocean heterotrophy (section 3.4.3), on a seasonalbasis fluvial input becomes significant. For the MAB region,NPP on the shelf tends to peak in spring [Rowe et al., 1986],whereas fluvial OC inputs (particularly from allochthonoussources) tend to peak in the late fall and winter (this study).As a result, fall and winter fluvial OC inputs may subsidizethe net heterotrophy observed on the shelf also during thesetimes of year [Rowe et al., 1986]. A potential consequenceof this scenario is that much of the fluvial OC reaching theshelf becomes remineralized because it is delivered primar-ily in the off-seasons (i.e., fall or winter), even though it isconsidered less labile compared to autochthonous OC.

3.4. Extrapolations to the Middle Atlantic Bight Region[39] Using mean discharge data compiled for coastal sys-

tems in the contiguous United States [Coastal Assessment

and Data Synthesis System, 1999], we estimated that theeight rivers included in this study comprised approximately80% of the freshwater discharged directly to the MABannually. Under the assumption that these rivers are repre-sentative of riverine discharge and C export to the MAB ingeneral, the combined eight river exports were scaled-up tothe freshwater discharge for the entire region (section S2)yielding estimated total C exports to the nearshore MABof 80 Gg�yr–1 to 200 Gg�yr–1 of C as POC (95% CI);260 Gg�yr–1 to 570 Gg�yr–1 of C as DOC (95% CI); and1300 Gg�yr–1 to 2400 Gg�yr–1 of C as DIC (95% CI; Table 3).The upper DOC export estimate compares favorably to the566 Gg�yr–1 of C as DOC value calculated in a previousstudy by Raymond and Bauer [2000].

[40] The amounts of allochthonous and aged riverine Cexported to the nearshore MAB were determined in a man-ner similar to that for total POC, DOC, and DIC exports(Table 3). These calculations were based additionally onthe allochthonous and aged C contributions that had beenestimated for six of the eight study rivers (Table S2) andscaled up by a factor 1.29 (i.e., the six study rivers for whichallochthonous and aged C contributions could be determinedcomprised approximately 78% of the freshwater dischargeddirectly to the MAB annually) rather than by 1.24 as was thecase for the total C exports which were based on estimatesfrom all eight study rivers.

3.4.1. Terrestrial C Losses via Rivers in the MiddleAtlantic Bight Region

[41] The allochthonous riverine POC, DOC, and DICexports to the nearshore MAB (Table 3) represent conserva-tive estimates of terrestrial C losses because some fractionof the terrestrially-derived materials would have been lost(e.g., microbial and photochemical remineralization of POCand DOC; degassing and photosynthetic uptake of DIC) dur-ing downstream transport (i.e., upstream terrestrial loss of Cis greater than the C export we measure near the mouth ofthe river; see section S2 and Figure S6). Lauerwald et al.[2012], for example, estimated that �23% of DOC was lostthrough in-stream processing based on geospatial models ofNorth American rivers. Similar estimates of 20% to 30%in-stream allochthonous OC respiration have been obtainedspecifically for the Hudson River [Cole and Caraco, 2001;Maranger et al., 2004].

[42] For the region draining to the MAB (Figure 1), weestimated annual terrestrial C losses based on our estimatesof allochthonous C exports scaled up for the entire MABregion and compensating for in-stream losses during trans-port (section S2). Based on studies by Cole and Caraco[2001], Maranger et al. [2004], and Lauerwald et al. [2012],we assumed a 25% loss rate for terrestrially derived POCand DOC. For DIC, we used a 7% loss-rate from photosyn-thetic consumption based on flux-weighted isotope mixingmodel estimates [Hossler and Bauer, 2012], and a 74%loss-rate from degassing based on data presented in But-man and Raymond [2011] (see also section S2). Additionallyfor DIC, because the allochthonous export estimate includedboth external (i.e., on land) and internal (i.e., in stream)contributions from POC and DOC remineralization, the esti-mated amounts of POC and DOC remineralized in-stream(i.e., 25% of the total terrestrial POC and DOC inputs) weresubtracted from the DIC estimate. With these assumptions,

10


Table 3. Exports, Losses and Inputs of POC, DOC and DIC in the MAB Region.a

POC DOC DIC

Total C Exports to the Nearshore MAB (Gg � yr–1)Mean 120 370 165095% CI (80, 200) (260, 570) (1300, 2400)

Allochthonous C Exports to the Nearshore MAB (Gg � yr–1)Mean 90 290 78095% CI (40, 150) (200, 490) (540, 1120)

Aged C Exports to the Nearshore MAB (Gg � yr–1)Mean 10 20 22095% CI (10, 20) (10, 40) (150, 340)

Estimated Terrestrial Losses (Gg � yr–1)

Mean 120 380 396095% CI (60, 210) (260, 650) (2750, 5690)

Net Fluvial Total C Inputs to the MAB Shelf (Gg � yr–1)Mean 70 280 80095% CI (10, 200) (130, 570) (150, 2400)

Net Fluvial Allochthonous C Inputs to the MAB Shelf (Gg � yr–1)Mean 50 220 39095% CI (0, 150) (100, 490) (70, 1120)

Net Fluvial Aged C Inputs to the MAB Shelf (Gg � yr–1)Mean 10 20 10095% CI (0, 20) (0, 40) (20, 340)

a Estimates from this study of mean annual C exports, proportions of allochthonous C contributions, and proportions of aged C contributions (Tables 2and S2) were extrapolated to the entire MAB drainage region (section S2). The extrapolations include total, allochthonous, and aged C exports to thenearshore MAB coastal margin, estimated terrestrial losses from the MAB drainage region, and inputs to the MAB shelf (see also Figures S6 and S7). Forthe estimated terrestrial losses, the values reported refer to the form of C (i.e., POC, DOC or DIC) that is exported to the river. For DIC, a portion of theexported material will have originated from mineralization of terrestrial OC (section 3.4.1). (Note, for conversion to areal yields, the MAB drainage regionis approximately 3.7�105 km2 in area based on data available through the National Oceanic and Atmospheric Administration’s Coastal Assessment andData Synthesis System (http://coastalgeospatial.noaa.gov)).

terrestrial losses from the MAB drainage region were esti-mated to be 320 Gg�yr–1 to 860 Gg�yr–1 in the formof OC (i.e., POC+DOC; 95% CI); and 2750 Gg�yr–1 to5690 Gg�yr–1 in the form of DIC (95% CI; Table 3).

[43] Of the terrestrial DIC losses, approximately 83%derived from terrestrial OC remineralization (and theremaining 17% from carbonate dissolution, based on sourcecontribution estimates from Hossler and Bauer [2012]),bringing the annual OC terrestrial loss (independent ofexport form) to 3800 Gg�yr–1 on average. Based on annualterrestrial NPP data available from Imhoff et al. [2004b],the regional catchments draining to the MAB produce250,000 Gg of net OC annually. The estimated annual terres-trial OC loss would then represent less than 2% of the annualNPP in the MAB region.

[44] These estimates of terrestrial NPP exported by riverscompare favorably to previous studies. Hope et al. [1994],in their review of 117 temperate and boreal catchment stud-ies, estimated that 1% of NPP was exported as OC, whileMoeller et al. [1979], in their study of four regions across theU.S., estimated 0.1% to 0.4% of NPP was exported as DOCalone. In their review of coastal ocean OC dynamics, Smithand Hollibaugh [1993] estimated that 0.7% of terrestrialNPP was exported as riverine OC on a global basis. Otherglobal estimates of terrestrial OC losses to inland watersrange from 1.9 Pg�yr–1 to 2.9 Pg�yr–1 [Cole, 2007; Battinet al., 2009; Tranvik, 2009], which would represent 3% to6% of the estimated global terrestrial NPP of 50 Pg�yr–1 to60 Pg�yr–1 of C [Randerson et al., 2002; Imhoff et al., 2004a;Ito, 2011].

[45] However, what must also be considered is the propor-tion of exported OC derived from modern inputs (see alsosection S2). In the MAB region, for example, modern agedsources comprise just 64% of the terrestrial OC loss, with theremaining 36% deriving from OC sources cycling over timeperiods ranging from decades (e.g., slow-turnover SOC) tomillennia (e.g., passive-turnover SOC) to over tens of thou-sands of years (e.g., fossil OC) [Hossler and Bauer, 2012].Adjusting for the proportion of annual terrestrial OC lossthat is modern in origin thus reduces the terrestrial OC exportestimate for annual NPP to around 1%, or 2400 Gg�yr–1 of Con average, for the MAB region.

[46] Similar adjustments for modern versus aged OC con-tributions will likely need to be made for other regions andglobally when relating fluvial C exports to terrestrial lossesof NPP. Across numerous riverine systems, it has becomeapparent that a significant fraction of the OC load is com-prised of aged materials such as fossil OC and aged SOC[Spiker and Rubin, 1975; Meybeck, 1993; Kao and Liu,1996; Cole and Caraco, 2001; Leithold and Blair, 2001;Masiello and Druffel, 2001; Raymond and Bauer, 2001a,b;Blair et al., 2003; Raymond et al., 2004; Longworth et al.,2007; Sickman et al., 2010]. The relative proportion of agedOC will vary from system to system depending on such fac-tors as lithology, hydrogeomorphology, and anthropogenicimpact [Hossler and Bauer, 2013].3.4.2. Net Fluvial C Inputs to the Middle AtlanticBight Shelf

[47] The riverine OC and IC exported to the nearshoreMAB (Table 3) is expected to be substantially modified dur-

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ing transport through nearshore (including estuarine) watersbefore reaching the MAB shelf (Figure S7). While nearshoretransit will both remove fluvial C from and add nearshore Cto the riverine OC and IC export [e.g., Kemp et al., 1997;Raymond and Bauer, 2000; Abril et al., 2002; Cai, 2011],we focus only on the removal processes in order to quantifythe actual fluvial C export reaching the MAB shelf.

[48] Through processes such as remineralization (micro-bial and photochemical), sedimentation, and flocculation[Fontugne and Jouanneau, 1987; Lucotte et al., 1991; Keilet al., 1997; Fisher et al., 1998; Moran et al., 1999, 2000;Raymond and Bauer, 2000, 2001a; Abril et al., 2002; Bauerand Bianchi, 2011; Bianchi and Bauer, 2011; Dai et al.,2012], up to 90% of riverine POC and 50% of riverine DOCmay be lost in nearshore transit. Actual OC loss across thenearshore will depend on such factors as residence time[Raymond and Bauer, 2000; Abril et al., 2002], tempera-ture [Raymond and Bauer, 2000], OC concentration [Abrilet al., 2002], and chemical composition [Sun et al., 1997;Hopkinson et al., 1998]. Residence times in the nearshoreMAB, for example, can range from a few days in thesmaller estuaries to half a year in the larger embayments[Pilson, 1985; Nixon, 1996; Sin et al., 1999; Hagy et al.,2000; Howarth et al., 2000; Abdelrhman, 2005; Gay andO’Donnell, 2009].

[49] For DIC, previous river studies suggest up to 90%loss through degassing during nearshore transit [Cai andWang, 1998; Abril et al., 2000; Hunt et al., 2011]. Net loss ofriverine DIC across the nearshore will depend on the amountof riverine OC remineralization and pCO2 water-to-air dif-ferential, in addition to residence time [Cai and Wang, 1998;Abril et al., 2000].

[50] For simplicity, we assumed that during nearshoretransit, the major fate for POC was a balance of sedimen-tation and remineralization to DIC; the major fate for DOCwas remineralization to DIC; and the major fate for DICwas degassing (Figure S7; see also section S2). Using thefull range of loss estimates from the literature (i.e., 0 to0.9 for POC; 0 to 0.5 for DOC; and 0 to 0.9 for DIC), theexpected net fluvial inputs to the MAB shelf were 10 Gg�yr–1

to 200 Gg�yr–1 of C as POC; 130 Gg�yr–1 to 570 Gg�yr–1 ofC as DOC (95% CI); and 150 Gg�yr–1 to 2400 Gg�yr–1 of Cas DIC (Table 3).

[51] Vlahos et al. [2002], in their DOC budget forthe MAB, estimated that the total nearshore DOC input(i.e., riverine plus estuarine) ranged from 400 Gg�yr–1 to770 Gg�yr–1 of C, with a mean input of 590 Gg�yr–1 ofC. Assuming a 40% to 60% fluvial contribution to totalnearshore input [Raymond and Hopkinson, 2003], the totalnearshore DOC input to the MAB for this study would be220 Gg�yr–1 to 1420 Gg�yr–1 of C, with a mean input of560 Gg�yr–1 of C, which is consistent with the estimate ofVlahos et al. [2002].

[52] Also of note is the apportionment of total Cexports between allochthonous versus autochthonous C,and between aged versus modern C. Using the sameapproach and assumptions as for total (i.e., allochthonous,autochthonous, aged, and modern) fluvial OC and DICMAB inputs, the estimated net fluvial DIC input apportion-ments were 70 Gg�yr–1 to 1120 Gg�yr–1 as allochthonousDIC and 20 Gg�yr–1 to 340 Gg�yr–1 as aged DIC (Table 3).Net fluvial OC inputs to the MAB shelf ranged from

100 Gg�yr–1 to 640 Gg�yr–1 as allochthonous OC and10 Gg�yr–1 to 60 Gg�yr–1 as aged OC (Table 3). However,given the greater presumed recalcitrance of terrestrial OMin comparison to autochthonous OM [e.g., Enríquez et al.,1993; Kemp et al., 1997; Sun et al., 1997; McCallisteret al., 2004, 2006; Khodse and Bhosle, 2011], a dispro-portionate amount of the terrigenous OC (including agedOC) may escape remineralization across the nearshore. Suchpreservation may be particularly true for aged OC based onrecent studies indicating preferential degradation of youngerand presumably more labile OC [e.g., Raymond and Bauer,2001b; Raymond and Hopkinson, 2003; but see also McCal-lister et al., 2004].3.4.3. Net Fluvial OC Input Relative to NPP in theMiddle Atlantic Bight Shelf

[53] For perspective, we can compare the riverine OCcontributions to the MAB to estimates of annual NPP in theMAB. Across the MAB shelf, estimates of annual areal NPPrange from 100 Mg�km–2 �yr–1 to 600 Mg�km–2 �yr–1 of C witha mean of 350 Mg�km–2 �yr–1 of C [Malone et al., 1983]; Liuet al., 2000; Lohrenz et al., 2002; Mouw and Yoder, 2005;Fennel et al., 2006; Druon et al., 2010; Filippino et al.,2011]. With a total shelf area of approximately 1.5�105 km2,the annual total NPP on the MAB becomes 15,000 Gg�yr–1

to 93,000 Gg�yr–1 of C. Based on this estimate, riverine OCcontributions to the MAB shelf (Table 3) range from <1%to 5% of the annual NPP. However, we emphasize againthat the timing offset of riverine C inputs relative to peakshelf productivity could augment the importance of fluvialOC contributions to the MAB shelf at different times of theyear (although rough estimates suggest that the fluvial OCcontribution relative to MAB shelf NPP is at most 25% evenduring the winter when the fluvial OC contribution is nearmaximum (this study) and the shelf NPP is near minimum[Malone et al., 1983; Rowe et al., 1986; Mouw and Yoder,2005; Druon et al., 2010]; see also section 3.3). Indeed,seasonal significance of fluvial OC inputs to heterotrophicactivity has been demonstrated for nearshore systems [seee.g., Hopkinson, 1985; Kemp et al., 1997; Breed et al., 2004]and may extend to coastal shelf systems as well [e.g., Serretet al., 1999].

4. Summary[54] For the eight rivers of this study, concentrations

and areal yields of POC, DOC, and DIC comparedfavorably with previous observations from similar regionsand bioclimates. Allochthonous-derived C (as opposed toautochthonous C) comprised the majority of river exportedPOC and DOC and half of the total POC, DOC, and DICflux. A smaller but still significant fraction (<25%) of riverexported C was further composed of aged C materials deriv-ing from fossil C (a particularly important source for agedPOC and DIC) and aged SOC (a particularly importantsource for aged DOC).

[55] For the MAB region, it was estimated that, onaverage, 3800 Gg (or 10 Mg�km–2) of terrestrial OC wasremoved annually by fluvial transport. Of this terrestrial OCloss, an estimated �64% was contemporary C, or approx-imately 1% of the annual terrestrial NPP. The estimate offluvial removal of terrestrial NPP was lower than previ-ous global estimates. However, earlier studies had not taken

12


into consideration the apportionment between contemporary(i.e., NPP) and aged OC sources.

[56] Of the riverine C exported to the nearshore MAB,an estimated 0 to 80% was removed during transitthrough nearshore waters (based on the literature), leaving140 Gg�yr–1 to 770 Gg�yr–1 of OC (�76% as DOC) and150 Gg�yr–1 to 2400 Gg�yr–1 of DIC to be imported to theMAB shelf. This fluvial OC input is relatively small (<1% to5%) with respect to annual MAB NPP; however, the timingof riverine C fluxes, which peaked in the late fall and winter,appeared to be offset from peak NPP (at least on the MABshelf). Hence, fluvial OC inputs to the coastal (and perhapsopen) ocean may be important on a seasonal basis, althoughseemingly minor on an annual scale.

[57] Acknowledgments. This research was supported by the NationalScience Foundation’s Integrated Carbon Cycle Research Program GrantNo. EAR-0403949 and Chemical Oceanography Program Grant Nos. OCE-0327423 and OCE-0961860. For assistance with sample collection andprocessing, we thank Ed Keesee, David Perkey, and Andrew Wozniak. Wealso wish to thank Dr. James Sickman and an anonymous reviewer for theirinsightful and comprehensive comments and suggestions for improvementof this manuscript.

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