Organic carbon budget for the eastern boundary of the North Atlantic subtropical gyre: Major role of DOC in mesopelagic respiration

1 SCIen IFIC REPORTS | 7: 10129 | DOI:10.1038/s41598-017-10974-y www.na u e.com/scien i ic epo s O ganic ca bon budge o he eas e n bounda y o he No h A lan ic sub opical gy e: majo ole o DOC in mesopelagic espi a ion Ye ay San ana-Falcón 1, Xosé An ón Ál a ez-Salgado2, Ma ía Dolo es Pé ez-He nández 1,3, Alonso He nández-Gue a1, E an Mason 4 & Ja ie A ís egui1 T anspo s o suspended pa icula e (POCsusp) and dissol ed (DOC) o ganic ca bon a e in e ed om a box-model co e ing he eas e n bounda y o he No h A lan ic sub opical gy e. Co esponding ne espi a ion a es (R) a e ob ained om a ne o ganic ca bon budge ha is based on he anspo es ima es, and includes bo h e ical and la e al luxes. The o e all R in he mesopelagic laye (100–1500 m) is 1.6 ± 0.4 mmol C m−2 d−1. DOC accoun s o up o 53% o R as a esul o d awdown o o ganic ca bon wi hin Eas e n No h A lan ic Cen al Wa e (ENACW) ha is en ained in o sinking Medi e anean O e low Wa e (MOW) ha leads o o ma ion o Medi e anean wa e (MW) a in e media e dep hs (~900 m). DOC ep esen s 90% o he espi ed non-sinking o ganic ca bon. When con e ed in o oxygen uni s, he compu ed ne espi a ion a e ep esen s less han hal he oxygen u iliza ion a es (OUR) epo ed o he mesopelagic wa e s o he sub opical No h A lan ic. Mesoscale p ocesses in he a ea, no quan i ied wi h ou app oach, could accoun in pa o he OUR di e ences obse ed be ween ou ca bon budge and o he published s udies om he No h A lan ic, al hough seasonal o in e annual a iabili y could also be esponsible o he di e ence in he es ima es. Oceanic espi a ion occu s o e he en i e wa e column1, 2, as opposed o pho osyn hesis, which is es ic ed o he epipelagic laye ( oughly abo e 150 m). The non- espi ed ac ion o he biogenic o ganic ca bon p oduced in he epipelagic laye is suscep ible o downwa d expo in he o m o dissol ed (DOC) and pa icula e (POC) o ganic ca bon, suppo ing espi a ion in he da k ocean2, 3. A he global scale, he ela i e con ibu ion o DOC o oxygen consump ion in he da k ocean has been es ima ed o be 10–20%4, 5, bu i may inc ease signi ican ly in egions o deep-wa e con ec ion5–7. Pa adoxically, he sum o DOC and sinking POC collec ed wi h sedimen aps does no equen ly accoun o he es ima ed da k ocean espi a ion a es8–10. This con lic ing imbalance could be a consequence o me hodological unce ain ies in he es ima ion o espi a ion a es and/o ca bon luxes10–12. Fo ins ance, he bulk o pa icula e ma e ial in he wa e column is composed o slow-se ling o sus- pended pa icles13–15 ha can escape om sedimen aps due o hei buoyancy16. Se e al s udies ha e sugges ed ha suspended pa icles a e an impo an sou ce o o ganic ma e o p oka yo ic o ganisms in he mesopelagic zone17, 18. The e o e, suspended pa icles would ep esen an unquan i ied sou ce o “missing” o ganic ca bon, especially in a eas whe e la e al ad ec ion is in ense, such as in Eas e n Bounda y Cu en sys ems19. Indeed, i has been al eady shown ha , using a box-model app oach, suspended pa icula e o ganic ca bon (POCsusp) could suppo up o 59% o he o al mesopelagic espi a ion in he sou hwes e n sec o o he Cana y Cu en 20, a egion s ongly a ec ed by he coas al-ocean expo o pa icula e ma e ial om he NW A ican coas al upwelling sys em21, 22. 1Ins i u o de Oceanog a ía y Cambio Global, IOCAG, Uni e sidad de Las Palmas de G an Cana ia, ULPGC, 35017, Las Palmas de G an Cana ia, Spain. 2CSIC, Ins i u o de In es igaciones Ma inas, Edua do Cabello, 6, 36208, Vigo, Spain. 3Depa men o Physical Oceanog aphy, Woods Hole Oceanog aphic Ins i u ion, Woods Hole, Massachuse s, USA. 4Ins i u o Medi e áneo de Es udios A anzados, CSIC-UIB. C. Miquel Ma quès, 21, 07190, Espo les, Illes Balea s, Spain. Co espondence and eques s o ma e ials should be add essed o Y.S.-F. (email: ye ay[email p o ec ed]) Recei ed: 26 Ap il 2017 Accep ed: 14 Augus 2017 Published: xx xx xxxx OPEN www.na u e.com/scien i ic epo s/ 2 SCIen IFIC REPORTS | 7: 10129 | DOI:10.1038/s41598-017-10974-y In he p esen s udy, ollowing he app oach used by Alonso-González and cowo ke s in he sou he n Cana y Cu en 20, we es ima e he con ibu ion o DOC and POCsusp luxes o ne espi a ion in he eas e n bounda y o he No h A lan ic sub opical gy e (Fig.1). Ou o e a ching hypo hesis is ha , con a y o he sou he n Cana y Cu en whe e POCsusp is an impo an componen o he ca bon budge 20, 23, he DOC con ibu ion o o al espi a ion in he s udy a ea is la ge han ha o POCsusp. The wo a eas a e oceanog aphically dis inc . On he one hand, he coas line o ien a ion in ou egion is un a o able o coas al upwelling, which is absen o limi ed jus o he summe mon hs24, 25. On he o he hand, ou egion is cha ac e ized by he densi y-d i en exchange o wa e masses be ween he Medi e anean Sea and he eas e n No h A lan ic26. As a consequence, Eas e n No h A lan ic Cen al Wa e (ENACW) is en ained in o he dense as - lowing Medi e anean O e low Wa e (MOW) ha exi s h ough he S ai o Gib al a 27–29 and is subduc ed along he slope in o he Gul o Cádiz (ca. 33–37°N)30–33 o below 1000 m dep h, whe e i sp eads in o he A lan ic as Medi e anean Wa e (MW)27–29, 34–37. This en ainmen o ENACW in o MOW may p omo e he downwa d lux o DOC o in e media e laye s in his pa icula egion, as has al eady been shown o an h opogenic ca bon dioxide38. In a p e ious s udy39, based on a ough es ima ion o o ganic ca bon luxes in a box egion ha included he S ai o Gib al a , i has been sugges ed ha DOC emine aliza ion could explain abou 90% o he ino ganic ca bon p oduced in he wa e column. Ou s udy es s his hypo hesis, p o iding di ec measu emen s o POCsusp and DOC luxes o in eg a ion wi hin a box-model analysis. We aim o con ibu e o he unde s anding o he o ganic ca bon dynamics in his complex egion, which is cha ac e ized by he con luence o wa e masses o di e en o igin, wa e mass en ainmen and subduc ion, and in e mi en upwelling24, 40–43. Resul s O ganic ca bon dis ibu ions. The DOC and POCsusp da a used in his wo k we e collec ed du ing a esea ch c uise in au umn 2009. The c uise consis ed o a se o hyd og aphic s a ions dis ibu ed egula ly along h ee ansec s (no he n, wes e n and sou he n) ha delimi he box p esen ed in Fig.1 (see Ma e ials and Me hods o u he de ails on sampling and analysis). POCsusp and DOC dis ibu ions om 0 o 4000 m (Fig.2) show ha su ace POCsusp concen a ions along he no he n ansec dec ease o sho e om 4–6 µmol C L−1 in he coas al a ea o abou 2 µmol C L−1 in he open ocean. Below 500 m POCsusp concen a ions a e 1 µmol C L−1 o less, excep a ound a seamoun a ~14° W. Wa e s wi h ela i ely high POCsusp deepen a s a ion 30. Su ace DOC concen a ions a e highe han 60 µmol C L−1 o he en i e ansec . A deepening o DOC occu s a s a- ions 30–37. Su ace POCsusp concen a ions along he wes e n ansec a e highe han 1–2 µmol C L−1. A deepening o POCsusp- ich wa e s a s a ions 56–58 coincides wi h he shoaling o isoneu al su aces be ween wo mesoscale Figu e 1. Loca ion o he CTD s a ions. Biogeochemical s a ions a e numbe ed. AC, ACC, CanC, and PC e e o Azo es Cu en , Azo es Coun e -Cu en , Cana y Cu en , and Po ugal Cu en . AI and MO indica e he su ace A lan ic in low and bo om Medi e anean o e low. Blue/ ed a ows indica e anspo in o/ou o he s udy egion. S a ion 75 is a he ESTOC si e. The map was p oduced using M_Map oolbox (h ps://www.eoas. ubc.ca/~ ich/map.h ml) on MATLAB R2013a, h ps://www.ma hwo ks.com/p oduc s/ma lab/. Global ETOPO is used o ba hyme ic da a (h ps://www.ngdc.noaa.go /mgg/global/). www.na u e.com/scien i ic epo s/ 3 SCIen IFIC REPORTS | 7: 10129 | DOI:10.1038/s41598-017-10974-y eddies44. POCsusp concen a ions emain a ound 1 µmol C L−1 along he es o he sec ion wi h li le a ia ion. DOC is abou 60–70 µmol C L−1 nea he su ace, and dec eases o less han 50 µmol C L−1 wi h dep h. Cen al wa e s wi h high DOC concen a ions descend along sloping isoneu als a s a ions 53–56. The deepening o DOC no h o 35°N coincides wi h he loca ion o he Azo es Cu en sys em44. The sou he n ansec shows su ace POCsusp concen a ions o 2–3 µmol C L−1. A wes wa d dec ease is obse ed below 250 m. Su ace DOC concen a ions show alues abo e 70–80 µmol C L−1. A pa ch o ela i ely high POCsusp below 1000 m dep h coincides wi h cool and esh wa e s44. DOC shows alues abo e 70–80 µmol C L−1 a he su ace, wi h maximum concen a ions abo e 100 µmol C L−1 a subsu ace dep hs (~200 m) in he eas e n s a ions close o he Lanza o e Passage. In e media e wa e s (below he 27.38 isoneu al) show low DOC concen a ions, excep a s a ions 64–65 whe e a on be ween a cyclonic eddy and he sou hwa d Cana y Cu en is pe iodically obse ed44. Mass anspo s. Mass anspo s ac oss he no he n, wes e n, and sou he n ansec s a e aken om a p e- ious s udy by Pé ez-He nandez and co-wo ke s who de eloped an in e se box model wi h da a om he same c uise44 (see Ma e ial and Me hods o u he de ails abou he in e se model and i s eliabili y). A summa y o he in eg a ed la e al mass anspo a each ansec is p esen ed in Fig.3. No e ha luxes in o/ou o he box a e indica ed by posi i e/nega i e signs, espec i ely. A p edominan inwa d (sou hwa d) anspo is obse ed ac oss he no he n ansec (~37°N; blue line), in e up ed only by he ou wa d (no hwa d) anspo o in e media e wa e s (isoneu als 27.38 o 27.92). The mass anspo ac oss he wes e n ansec (~24°W; ed line) is s ongly inwa ds (eas wa d) in he su ace and Figu e 2. Ve ical sec ion (0–4000 m) o POCsusp (µmol C L−1; uppe panels) and DOC (µmol C L−1; lowe panels) o he no he n (le ), wes e n (middle), and sou he n ( igh ) ansec s. Biogeochemical s a ions a e indica ed a he op o he uppe panels. G ay do ed lines indica e neu al densi y. www.na u e.com/scien i ic epo s/ 4 SCIen IFIC REPORTS | 7: 10129 | DOI:10.1038/s41598-017-10974-y cen al wa e s, and p edominan ly ou wa ds (wes wa d) below. The mass anspo ac oss he sou he n ansec (~29°N; g een line) is ou wa ds (sou hwa d) in he uppe 700 me e s o he wa e column, while he low o in e - media e and deep wa e s a ies by less han ±2 S . POCsusp and DOC anspo s. O ganic ca bon anspo s along isoneu als a e calcula ed by mul iplying he co esponding POCsusp and DOC concen a ions (Fig.2) by he mass anspo s (Fig.3) (see Ma e ials and Me hods o mo e de ails on he es ima ion and obus ness o he ca bon anspo s). Fo he case o DOC, anspo s o he non- e ac o y pool DOC (e-DOC) a e calcula ed. The e-DOC pool is ob ained by sub ac ing a backg ound e ac o y DOC concen a ion (~45 mmol C m−³ in he s udy a ea) om he measu ed DOC. We only conside he e-DOC ac ion because he e ac o y ac ion enewal is housands o yea s45, 46, a pe iod much longe han he decadal enewal ime o he s udy box. POCsusp and e-DOC anspo s in he a ea (Fig.4) may be di ided in o a sou hwa d lux o su ace and cen al wa e s, in e up ed by a weak no hwa d anspo o he uppe in e media e wa e s, ha is obse ed bo h a he no he n and sou he n ansec s. Sou hwa d anspo p e ails again in he deepes wa e s. The zonal ci cula ion is p edominan ly eas wa d a he su ace and cen al wa e s. The end e e ses in he in e media e and deep lay- e s whe e a wes wa d anspo is obse ed, especially a he bo om o he in e media e laye . POCsusp and e-DOC luxes in he su ace, cen al, in e media e, and deep wa e s a each ansec a e summa- ised in Table1. The inpu s (posi i e alues) o o ganic ca bon h ough he no he n and wes e n ansec s exceed he ou pu s (nega i e alues) o he sou he n ansec a bo h su ace and cen al laye s. Weak ho izon al ou pu luxes a e ound bo h in he in e media e and he deepes laye s. Discussion To e alua e he o ganic ca bon sou ces wi hin he box we build a mass balance (Fig.5a) and an associa ed o ganic ca bon budge (Fig.5b). Fo he su ace wa e s (0–100 m), 1.2 ± 0.1 S en e he box h ough he no he n, sou h- e n and wes e n ansec s, and −0.78 ± 0.1 S lea e he box as he A lan ic in low in o he Medi e anean Sea (see Ma e ials and Me hods). As a esul , he mass balance o inpu s minus ou pu s o he box would be 0.4 ± 0.2 S . We p opose ha his olume is anspo ed om he su ace o he cen al wa e s o pa ially balance he loss o ENACW o in e media e wa e s when en ained in o he MOW (see below he discussion abou he mass balance o he cen al wa e s). The balance o o ganic ca bon, i.e. POCsusp and e-DOC, anspo ed ac oss he no he n, wes e n and sou he n ansec s indica es ha he su ace wa e s o he box ecei e an ex e nal inpu om he Figu e 3. In eg a ed mass anspo s o he no he n (blue diamonds o e a dashed line), wes e n ( ed c osses o e a do ed line) and sou he n (g een ci cle dashed line) ansec s. The sign o he ne anspo is nega i e/posi i e o di e ging/con e ging low ou o /in o he box. No h A lan ic Cen al Wa e (NACW), Medi e anean Wa e (MW), sub-A ic In e media e Wa e (SAIW), An a c ic In e media e Wa e (AAIW), Lab ado Sea Wa e (LSW), and No h A lan ic Deep Wa e (NADW) wa e masses a e indica ed bo h by colo and ac onym. Y-axis is neu al densi y. www.na u e.com/scien i ic epo s/ 5 SCIen IFIC REPORTS | 7: 10129 | DOI:10.1038/s41598-017-10974-y A lan ic Ocean o 30.0 ± 2.0 × 108 mol C d−¹. A he same ime, acco ding o ou es ima es (see Ma e ials and Me hods), −11.4 ± 2.9 × 108 mol C d−¹ is expo ed wi hin su ace wa e s o he Medi e anean Sea. This numbe lies wi hin he ange o he A lan ic in low o TOC p e iously measu ed in Ap il 1998 (7.6 × 108 mol C d−¹)47, and in Sep embe 1997 (27.1 × 108 mol C d−¹)48, a e sub ac ion o e ac o y DOC. Fu he mo e, he non- espi ed o ganic ca bon in su ace wa e s may be anspo ed o he da k ocean by u bu- len di usion, passi e sinking, and ac i e anspo 49, 50. We es ima ed i s he e ical u bulen di usion a 100 m dep h as F = −ε × R/(N2 × (1 − R)) × (DOCz2 − DOCz1)/(z2 − z1)51. We conside ed (1) he a e aged DOC in he uppe 100 m (DOCz1 cen e ed a z1 = 50 m) and om 100 o 200 m dep h (DOCz2, cen e ed a z2 = 150 m) o he calcula ion o he e ical DOC g adien , (2) he squa e o he B un -Väisälä equency (N2) be ween 50 and 150 m, and (3) cons an alues o he dissipa ion a e (ε = 10−8 m3 s−2) and he Richa dson numbe (R = 0.2). We ob ained an a e aged alue o 0.4 ± 0.3 mmol C m−² d−¹, (a e age ± SD o 47 s ns) ha would gi e a o al downwa d DOC lux o abou 5.7 ± 3.7 × 108 mol C d−¹, when mul iplied by he su ace a ea o he box (1.4 × 106 km2). Figu e 4. POCsusp (uppe panels) and DOC (lowe panels) luxes (108 mol C d−¹) a each ansec . Posi i e/ nega i e alues indica e inpu s/ou pu s o/ om he box. Wa e masses a e indica ed bo h by colo and ac onym: No h A lan ic Cen al Wa e (NACW), Medi e anean Wa e (MW), sub-A c ic In e media e Wa e (SAIW), An a c ic In e media e Wa e (AAIW), Lab ado Sea Wa e (LSW), and No h A lan ic Deep Wa e (NADW). Y-axes is neu al densi y. No he n T ansec Wes e n T ansec Sou he n T ansec Global POCsusp e-DOC POCsusp e-DOC POCsusp e-DOC POCsusp e-DOC Su ace 1.79 ± 0.02 15.29 ± 0.04 7.27 ± 0.2 85.20 ± 2.8 −7.45 ± 0.5 −70.46 ± 3.1 1.48 ± 0.2 28.51 ± 2.0 Cen al 2.52 ± 0.3 17.83 ± 2.3 0.87 ± 0.03 9.97 ± 0.7 −1.34 ± 0.1 −17.46 ± 2.9 1.93 ± 0.2 −0.12 ± 0.06 In e media e −0.12 ± 0.06 −0.39 ± 0.2 −1.03 ± 0.09 −7.73 ± 0.9 0.22 ± 0.01 0.88 ± 0.02 −0.96 ± 0.1 −7.53 ± 0.5 Deep 4.04 ± 0.4 16.04 ± 1.5 −2.41 ± 0.3 −13.24 ± 0.9 −1.68 ± 0.1 −5.95 ± 0.5 −0.24 ± 0.3 −3.79 ± 1.0 Table 1. POCsuspand e-DOC luxes (108 mol C d−1) o su ace, cen al, in e media e, and deep wa e s o he no he n, wes e n, and sou he n ansec s. E o ba s a e gi en. Posi i e/nega i e alues indica e inpu /ou pu s o/ om he box. www.na u e.com/scien i ic epo s/ 6 SCIen IFIC REPORTS | 7: 10129 | DOI:10.1038/s41598-017-10974-y Sinking POC luxes ob ained wi h su ace e he ed aps a 200 m dep h and ex apola ed o 150 m wi h he Ma in equa ion52 we e app oxima ely 0.55 mmol C m−² d−¹ a he Eu opean S a ion o Time-se ies in he Ocean (ESTOC)53. In he no heas A lan ic (39–43°N, 17–19°W), he minimum e ical POC luxes we e es ima ed o be 0.36 ± 0.09 mmol C m−² d−¹ om d i ing aps a 200 m dep h du ing la e summe 54. Al hough he use o d i ing aps has been equen ly c i icized o unde es ima ing expo luxes55–57, compa ison wi h POC luxes a he Be muda Time Se ies S a ion (BATS) de i ed om bo h su ace- e he ed aps and Tho ium-23456 shows ea- sonable ag eemen du ing low p oduc i i y pe iods. A he ESTOC si e, he lowes p oduc i i y alues ha e been ob ained du ing au umn (~10 mmol C m−² d−¹)57; conside ing an e- a io (expo /p oduc ion a io) o 5–10%55, he e ical POC lux du ing he less p oduc i e season would be 0.5–1.0 mmol C m−² d−¹. The e o e, an a e age alue o he expo a e om d i ing aps o 0.36 o 1.0 mmol C m−² d−¹ is ep esen a i e o he sinking lux om su ace wa e s wi hin he box a ha ime o he yea . Agg ega ion and disagg ega ion o sinking pa icles by bio ic and abio ic mechanisms could modi y he sinking a es and magni ude o POC luxes wi h dep h ac oss he da k ocean58–62, al hough ou box model app oach does no allow iden i ica ion o hese p ocesses. In addi ion, he es ima ed a e aged e ical lux o POC media ed by mig an zooplank on (ac i e lux) was 0.22 mmol C m−² d−¹ sou h o he island o G an Cana ia63, accoun ing o 25% o he sinking POC lux (passi e lux) in he egion. Mo e ecen ly, a mean ac i e lux o 0.28 mmol C m−² d−¹ (29% o he passi e lux) has been es ima ed no h o G an Cana ia64; hese es ima es ag ee wi h hose obse ed a BATS (0.17 mmol C m−² d−¹)65 and he Hawaiian Ocean Time Se ies (HOT; 0.26 mmol C m−² d−¹)66. Howe e , he ac i e lux accoun ed o only 8% and 11–44% o he passi e lux a hei espec i e egions65, 66. I we assume ha he ac i e lux ep esen s a bes 20% o he passi e lux in ou egion, he passi e + mig a o y POC lux om he su ace o he cen al wa e s would be a ound 0.82 mmol C m−² d−¹ which, mul iplied by he a ea o ou box, yields a o al POC lux om he su ace o he cen al wa e s o 11.4 × 108 mol C d−¹. The cen al wa e s (100–700 m) wi hin he box ecei e an o e all la e al inpu o POCsusp and e-DOC o 10.7 ± 1.9 × 108 mol C d−¹ om he su ounding A lan ic Ocean and 2.4 ± 1.5 × 108 mol C d−¹ om he Medi e anean ou low h ough he S ai o Gib al a . In addi ion, i is well known ha MOW is subduc ed wi hin he s udy box o mo e han 1000 m and ha he cen al wa e s a e en ained in o he MOW as i sinks29, 38. In ac , he mass balance o cen al wa e s wi hin he box (Fig.5a), conside ing he ne wa e lows ac oss he no he n, sou he n, and wes e n ansec s (2.1 ± 0.1 S ), he Medi e anean ou low in o he A lan ic (0.67 ± 0.1 S ; see Ma e ials and Me hods), and he inpu o 0.4 ± 0.2 S om he su ace wa e s, is 3.1 ± 0.4 S . The e o e, in o de o main ain conse a ion o olume in he cen al wa e s, 3.1 ± 0.4 S should en e he in e media e laye by Figu e 5. Schema ic diag am showing (a) he mass anspo and (b) he o ganic ca bon budge s in he box egion. The wa e column is di ided in o su ace (0–100 m), cen al (100–700 m), in e media e (700–1500 m), and deep (below 1500 m) laye s. A ows indica e ne anspo (S ) o o ganic ca bon (108 mol C d−¹) ac oss he open bounda ies. Ligh blue indica es la e al inpu s (posi i e alues), ligh ed indica es la e al ou pu s (nega i e alues), ligh iole indica es e ical en ainmen , ligh g een indica es e ical luxes om he laye abo e. Ne espi a ion a es o each compa men a e indica ed. www.na u e.com/scien i ic epo s/ 7 SCIen IFIC REPORTS | 7: 10129 | DOI:10.1038/s41598-017-10974-y en ainmen o he cen al wa e s in o he subduc ing Medi e anean ou low. Ou olume calcula ion is in ag ee- men wi h ea lie es ima es (2–3 S 67; 2.6 S 68; 1.3 S 28; 2.3 S 38). To ob ain he o ganic ca bon anspo ed by his olume, we mul iply he a e age o ganic ca bon concen a ion a he bo om o he cen al laye by 3.1 ± 0.4 S o ob ain a downwa d POC + e-DOC lux o 14.8 ± 3.7 × 108 mol C d−¹, 90% o which is DOC. Simila ly, we also calcula e he o ganic ca bon anspo associa ed wi h he 0.4 ± 0.2 S o su ace wa e s ha en e he cen al wa e s yielding 6.6 ± 2.2 × 108 mol C d−¹, 10% as POCsusp and 90% as DOC (Fig.5b). Fo he in e media e wa e s (700–1500 m), aside om he o ganic ca bon ha en e s his laye by en ain- men o ENACW in o he subduc ing MOW (14.8 ± 3.7 × 108 mol C d−¹), he o he o ganic ca bon anspo s associa ed wi h he wa e mass luxes lea e his laye . The ne balance o POCsusp and e-DOC anspo ac oss he no he n, wes e n and sou he n ansec s is −8.5 ± 0.6 × 108 mol C d−¹. This indica es an expo om he box o he adjacen A lan ic Ocean, and is especially in ense ac oss he wes e n ansec (see Table1), con a y o he su ace and cen al wa e s whe e o ganic ca bon was impo ed om he A lan ic Ocean. Fu he mo e, he mass balance o he in e media e wa e s (Fig.5a) indica es ha he e should be an en ainmen o 1.3 ± 0.5 S in o he deep wa e s o olume conse a ion, ha ansla es in o a d awdown o o ganic ca bon o 3.3 ± 1.4 × 108 mol C d−¹, 76% o which is DOC (Fig.5b). In addi ion, an a e age pa icle lux below 700 m o 0.46 mmol C m−² d−¹ has been ob ained om h ee si es loca ed wi hin he Cana y Cu en egion69. Assuming his alue o ou egion o s udy, we ob ain a e ical POC supply o in e media e wa e s o 6.4 × 108 mol C d−¹. Finally, he mass anspo balance o he deep wa e s (below 1500 m), 0.4 ± 0.4 S , is compa ible wi h mass conse a ion wi hin he unce ain y o he es ima es. POCsusp + e-DOC anspo ed om he in e media e wa e s by en ainmen (3.3 ± 1.4 × 108 mol C d−1), sinking POC (2.4 × 108 mol C d−1), o ganic ca bon bu ial in o he sed- imen s (0.06 × 108 mol C d−¹)70, and he ca bon balance a he open bounda ies o he box (−4.0 ± 1.3 × 108 mol C d−¹) yield a negligible ca bon excess o 1.6 ± 1.9 × 108 mol C d−¹. The sinking POC lux a 1500 m was de i ed om he lux a 700 m using he Ma in equa ion52. O e all, he o ganic ca bon balance o he su ace wa e s associa ed wi h he la e al luxes ac oss he no he n, wes e n and sou he n ansec s (30.0 ± 2.2 × 108 mol C d−¹), losses om he S ai o Gib al a (−11.4 ± 2.9 × 108 mol C d−¹), en ainmen (−6.6 ± 2.0 × 108 mol C d−¹), ac i e + passi e anspo luxes (−11.4 × 108 mol C d−¹), and u bulen di usion (−5.7 ± 3.7 × 108 mol C d−¹), yields −5.1 ± 4.1 × 108 mol C d−¹. Assuming s eady s a e condi ions, his balance esul s in a ne he e o ophy (R > 0) o abou −0.4 ± 0.3 mmol C m−² d−¹ a he ime o his s udy. The o e all balance o o ganic ca bon o he cen al wa e s includes he POCsusp + DOC la e al luxes ac oss he A lan ic bounda ies o he box (10.7 ± 2.1 × 108 mol C d−¹; 82% as DOC) and he S ai o Gib al a (2.4 ± 1.6 × 108 mol C d−¹; 62% DOC), passi e + ac i e luxes o POC om su ace wa e s (11.4 × 108 mol C d−¹), and o in e media e wa e s (−6.4 × 108 mol C d−¹), downwa d DOC luxes by u bulen di usion (5.7 ± 3.7 × 108 mol C d−¹), and en ainmen om he su ace (6.6 ± 2.2 × 108 mol C d−¹), and o he in e media e wa e s (−14.8 ± 3.7 × 108 mol C d−¹). All hese o ganic ca bon luxes esul in a ne R o 15.6 ± 6.3 × 108 mol C d−¹ (1.1 ± 0.4 mmol C m−² d−¹). Las ly, o in e media e wa e s he POCsusp + e-DOC includes he balance ac oss he no he n, wes e n and sou he n bounda ies (−8.5 ± 0.6 × 108 mol C d−¹), en ainmen om cen al wa e s (14.8 ± 3.7 × 108 mol C d−¹) and in o deepe wa e s (−3.3 ± 1.4 × 108 mol C d−¹), and e ical downwa d POC luxes om cen al wa e s (6.40 × 108 mol C d−¹) and o deepe laye s (−2.4 × 108 mol C d−¹), esul ing in a ne R o 7.0 ± 4.0 × 108 mol C d−¹ (0.5 ± 0.3 mmol C m−² d−¹) assuming s eady-s a e condi ions. The ne R wi hin he mesopelagic laye , assumed o be equi alen o he sum o he cen al and he in e medi- a e wa e s, is 22.7 ± 5.2 × 108 mol C d−¹ (1.6 ± 0.4 mmol C m−² d−¹). This alue is lowe han hose ob ained wi h a simila box model wes o he Cana y Islands20: 2.4–5.1 mmol C m−² d−¹. Acco ding o hese au ho s, POCsusp would suppo 28–59% o he o al mesopelagic R in ha egion. Howe e , hei es ima es we e based solely on POCsusp luxes, assuming wo scena ios in which he con ibu ion o DOC was ixed a 15 and 30%. In he p esen s udy, we ha e measu ed DOC and calcula ed ha i con ibu es abou 53% o o al mesopelagic R wi hin he box. I has been hypo hesized ha DOC ep esen s abou 90% o he espi a ion o non-sinking o ganic ca bon in he mesopelagic laye om an empi ical ca bon mass balance wi hin a box bounded by he S ai o Gib al a , 24–41° N and 22° W39. Ou esul s indica e ha DOC ep esen s ha ac ion when only he non-sinking o ganic ca bon is conside ed, in ag eemen wi h he high DOC consump ion a es ob ained in a eas unde going wa e mass ans o ma ion6, 71. The es ima ed mesopelagic R is much lowe han he oxygen u iliza ion a es (OUR) calcula ed o he No h A lan ic sub opical gy e72–74 (9–11 mmol C m−² d−¹ as con e ed using a -O2/C mola a io o 1.475). In addi ion, a ca bon budge no h o ou box egion du ing summe 200976 es ima ed a o al inpu o o ganic ca bon o he wiligh zone (ca. 50–1000 m) o ~7.7 mmol C m−² d−¹, due mainly o sinking POC. OUR alues anging om 4.7 o 16.6 mmol C m−² d−¹ be ween 100 o 750 m dep h we e ob ained using i ium and 228Ra me hodologies in he No h A lan ic sub opical gy e77. Simila in eg a ed alues (4.9 o 13.0 mmol C m−² d−¹) o ca bon espi- a ion o he 100–1000 dep h zone we e ob ained bo h by 7Be and POC lux a enua ion me hodologies in he sub opical No h A lan ic78. A compila ion o da a om he Paci ic ocean79 shows OUR alues anging om 0.04 o 18.1 mmol C m−² d−¹. In he No h Paci ic, OUR alues in eg a ed o e he i s 600 m dep h80 a e - aged 4.6 mmol C m−² d−¹, while highe OUR alues o 9.8 o 11.7 mmol C m−² d−¹ ha e been ecen ly epo ed o a mon hly ime-se ies a HOT12. Values ob ained du ing he p esen s udy migh , howe e , unde es ima e he annual mean since ou s udy ook place du ing he unp oduc i e au umn when POCsusp concen a ions a e pa icula ly low56. Addi ionally, he egion wi hin he box is s ongly in luenced by mesoscale ea u es such as ilamen s and eddies, whose e ec s on ca bon seques a ion emain o be quan i ied. The enhancemen o ca bon luxes o he ocean in e io d i en by mesoscale eddies has been epo ed in a numbe o s udies81–88. Indeed, injec ion o o ganic ca bon om su ace wa e s was especially in ense in se e al a eas, such as in he an icyclonic eddy a he no hwes e n co ne o he box (s a ions 33–35), o he Azo es F on (s a ions 52–5544). Mo eo e , www.na u e.com/scien i ic epo s/ 8 SCIen IFIC REPORTS | 7: 10129 | DOI:10.1038/s41598-017-10974-y upwelling ilamen s may play a key ole in he coas al-ocean anspo o o ganic ma e 89, 90. The Cape Ghi ila- men lies wi hin he egion o s udy, and may expo o he open ocean 29 o 63% o he annual a e aged p ima y p oduc ion associa ed wi h he coas al upwelling91, 92. All o hese ea u es occu episodically wi hin he box, and could enhance he supply o o ganic ca bon o mesopelagic wa e s. Nea he ESTOC si e, ne R a es a he mesopelagic zone (ca. 150–700 m) o 2.0–3.1 mol C m−² y−¹ (5.5– 8.5 mmol C m−² d−¹) we e ob ained by using a ace conse a ion model applied o clima ological da a93. They also ob ain consis en R a es based on ETS (elec on anspo sys em) espi a o y ac i i y o he same egion a he end o Ma ch. Di e ences be ween hese es ima es and ou alues could be due o highe la e al ad ec ion o DOC and POCsusp om he coas al upwelling egion a ha si e56, and highe p oduc i i y a he end o he la e win e bloom. In summa y, his s udy p o ides an es ima e o ne espi a ion in he mesopelagic wa e s wi hin a box enclos- ing he eas e n bounda y o he No h A lan ic sub opical gy e. This es ima e is based on he assump ion ha he box is in s eady s a e, i.e., he e is nei he accumula ion o o ganic ca bon en e ing he box no consump ion o p e iously accumula ed o ganic ca bon. This assump ion is accep able o he budge o he cen al, in e media e, and deep wa e s as hei decadal enewal imes p eclude seasonali y. Howe e , seasonal a iabili y is p onounced in he su ace laye and, hence, he s eady s a e assump ion could lead o biased alues o he annual ne luxes. Fu he mo e, al hough he con ibu ion o mesoscale phenomena needs o be add essed in u u e s udies, ou esul s show ha a majo ac ion o he mesopelagic o ganic ca bon demand in his a ea is uelled by DOC. Indeed, he o ma ion o MW has been ound o be he main mechanism o he expo o an h opogenic ca bon in his egion38. Despi e he inaccu acy o he s eady-s a e assump ion he ca bon budge p esen ed he e shows ha he e ical en ainmen o ENACW in o subduc ing MOW con ibu es o he expo o DOC o mesopelagic wa e s, whe e i may cons i u e he main subs a e suppo ing da k ocean espi a ion. Likewise, due o he wes - wa d ad ec ion o in e media e wa e s, his p ocess may also ep esen a signi ican supply o o ganic ca bon o he eas e n No h A lan ic ou side he box. Ma e ials and Me hods Hyd og aphy and seawa e sampling. Be ween Oc obe 15 and No embe 11, 2009 he RV Hespé ides ca ied ou an in ensi e hyd og aphic su ey in he eas e n bounda y o he No h A lan ic sub opical gy e a he con luence egion be ween he Azo es Cu en Sys em, he Po ugal Cu en and he Cana y Cu en . A g id o 81 conduc i i y- empe a u e-dep h (CTD) s a ions was dis ibu ed along a box egion de ined by 28.7–37.0°N and 24.5°W. A 47 o hese s a ions seawa e samples we e aken o analysis o dissol ed (DOC) and pa icula e o ganic ca bon (POC; Fig.1, numbe ed do s). Reade s in e es ed in a de ailed desc ip ion o he hyd og aphy and wa e mass cha ac e iza ion a e e e ed o Pé ez-He nández and cowo ke s44. O ganic ca bon. Disc e e samples o POC we e ob ained a selec ed dep hs om he su ace up o he bo - om (5, 25, 50, 100 200, 400, 600, 800, 1000, 1500, 2000, 3000, 4000 and 5000 m) by means o a ose e sample equipped wi h wen y- ou 10 L Niskin bo les. Al hough we a e awa e ha pa icles sink a di e en a es, we assume ha all he POC collec ed in oceanog aphic bo les co esponds o he slowly-se ling o suspended pool (POCsusp)20. Thus, his pool ep esen s an uppe h eshold o he pa icula e o ganic ma e ial ha is suscep ible o la e al anspo . Wa e samples (4 L) o POCsusp we e collec ed in polyp opylene bo les and il e ed h ough p ecombus ed (450 °C, 12 hou s) 25 mm Wha man GF/F- il e s (po e size 0.7 µm). The il e s we e w apped in p ecombus ed aluminium oil and ozen a −20 °C. In he labo a o y, he il e s we e hawed and d ied o e - nigh a 55 °C, hen placed o e nigh in a desicca o sa u a ed wi h HCl umes o emo e ino ganic ca bon, and d ied again in a second desicca o wi h silica gel o a leas 24 hou s. Finally, he il e s we e packed in in slee es be o e being analysed wi h a Pe kin-Elme 2400 CHN elemen al analyse ollowing s anda d p o ocols94. Unused p e-combus ed GF/F- il e s we e ea ed in he same way and used as handling blanks. Blanks anged om 0.06 o 0.7 µmol C L−1. DOC adso p ion on o he il e s was es ima ed a se e al andom s a ions, a h ee di e en dep h le els, as he amoun o ca bon e ained on a backing il e placed unde nea h he main il e . The alues anged om 0.17 o 1.73 µmol C pe 25 mm-diame e GF/F- il e , which a e simila o p e ious es ima es20, 95, 96. A he same s a ions and dep h le els, samples o he analyses o DOC we e collec ed in 250 mL acid-cleaned all-glass lasks. Samples om he uppe 100 m we e immedia ely il e ed h ough p ecombus ed (450 °C, 4 hou s) 47 mm Wha man GF/F il e s in an acid-cleaned all-glass il a ion sys em. Fil e ed su ace and un il e ed deepe wa e samples we e collec ed in 10 mL p ecombus ed glass ampules (450 °C, 12 hou s). A e acidi ica ion wi h H3PO4, he ampoules we e hea -sealed and p ese ed in he da k a 4 °C un il analysis in he labo a o y wi h a Shimadzu TOC-V o ganic ca bon analyse by high empe a u e ca aly ic oxida ion (HTCO). The sys em was calib a ed daily wi h po assium hyd ogen ph hala e (99.95–100.05%, p.a., Me ck). The p ecision o he DOC calib a ion was ± 1 μmol L−1. The pe o mance o he ins umen was es ed wi h he ca bon e e ence ma e i- als (CRM) p o ided by D. A. Hansell (Uni e si y o Miami, USA). Measu ed concen a ions o he CRM we e 45.5 ± 1.7 µmol C (n = 10); he ce i ied alue is 44–46 µmol C L−1 (lo #09-06 om he Flo ida S ai a 700 m). O ganic ca bon budge . The wa e column was di ided in o disc e e neu al densi y laye s97. The uppe ou laye s coincide oughly wi h he main he mocline wa e s (below 27.38 neu al densi y laye s; 0–700 m), he ollowing h ee laye s wi h in e media e wa e s (27.38–27.92; 700–1500 m), and he lowes se en laye s wi h deep wa e s (27.92–28.10; below 1500 m). Ou es ima es we e based on he in e se box model used by Pé ez-He nández and cowo ke s44 o ob ain mass anspo s (M) o each laye along he no he n, wes e n, and sou he n ansec s. The model uses he he mal wind equa ion a e calcula ing he e e ence le el eloci ies and hei unce ain ies. These eloci ies a e calcula ed assuming geos ophy, and mass and p ope y conse a ion. The in e se box model used has been ex ended om a p e ious one98 o include he app oxima e conse a ion o mass www.na u e.com/scien i ic epo s/ 9 SCIen IFIC REPORTS | 7: 10129 | DOI:10.1038/s41598-017-10974-y and anomalies o salini y and hea , and o allow ans e be ween laye s. This model also conside s adjus men o esh wa e luxes and Ekman anspo s in each sec ion. The in e se p oblem consis s o 37 equa ions and 130 unknowns: 99 o e e ence eloci ies, 26 o e ical eloci ies and e ical di usion, 4 o Ekman anspo s and 1 o he eshwa e lux. To sol e he in e se p oblem, he Gauss-Ma ko me hod, which p oduces a minimum e o a iance solu ion om ini ial es ima es o he unknowns, was used. M a each laye is mul iplied by he a e - aged suspended and dissol ed o ganic ca bon concen a ions (C) a he same laye o ob ain he co esponding la e al ca bon luxes (M × C). To epo hese ca bon luxes, he laye s we e g ouped in o su ace, cen al, in e - media e, and deep wa e s. The la e al ca bon luxes we e summed o ob ain he co esponding ca bon budge s o each laye . The obus ness o he ca bon luxes and budge s calcula ed in his s udy was es ed by means o he ollowing pe u ba ion es (see supplemen a y ma e ial): ca bon luxes we e calcula ed as (M ± e M) × (C ± e C), whe e e M and e C ep esen he unce ain y o he es ima ion o M and C, espec i ely. The e o e, he alues o M and C a e pe u bed wi hin he limi s o he unce ain y o hei espec i e es ima ions. A o al o 100 pe u ba ions we e pe o med o each ca bon lux and budge . The a e age o hese 100 alues is conside ed he op imum solu- ion and he co esponding s anda d de ia ion (SD) an es ima e o he unce ain y o he luxes and budge s99. See Pé ez-He nández and cowo ke s44 o a ho ough analysis o he alues o M and e M used in he p esen calcula ions. Conce ning he unce ain y o POCsusp and DOC measu emen s, hese we e ob ained by calcula ing he SD o all suspended and dissol ed o ganic ca bon measu ed in each laye a each ansec . To ob ain he o ganic ca bon luxes in o/ou o he Medi e anean Sea, mass anspo s ac oss he S ai o Gib al a we e se a −0.78 ± 0.1 S o he su ace ou low o he Medi e anean and 0.68 ± 0.1 S o he bo om in low o he eas e n No h A lan ic100 (1 S = 109 kg s−¹). These mass anspo s, which a e wi hin he ange o o he es ima es in he li e a u e101–107, we e mul iplied by he a e age concen a ions o POCsusp (3.4 mmol m−³ in he ou low and 1.6 mmol m−³ in he in low) and e-DOC (13.7 mmol m−³ in he ou low and 2.7 mmol m−³ in he in low) measu ed in he same egion by A ís egui and co-wo ke s (unpublished da a om May 2014). Ne espi a ion (R) o POCsusp and e-DOC wi hin each laye o he box is ob ained as ollows: =− − TOC d d IOR (1) Assuming s eady s a e condi ions (dTOC/d = 0), he balance o inpu s (I) minus ou pu s (O) o o al o ganic ca bon (TOC = POCsusp + e-DOC) ac oss he open bounda ies mus equal he o al ne espi a ion wi hin he box;, i.e., R = I − O. Da a A ailabili y. The da ase s gene a ed du ing and/o analysed du ing he cu en s udy a e a ailable om he co esponding au ho on easonable eques . Re e ences 1. Pome oy, L. R. & Johannes, R. E. Occu ence and espi a ion o ul aplank on in he uppe 500 me e s o he ocean. 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