Biogeosciences, 13, 3071–3090, 2016 www.biogeosciences.net/13/3071/2016/ doi:10.5194/bg-13-3071-2016 © Author(s) 2016. CC Attribution 3.0 License.

Deep ocean mass fluxes in the coastal upwelling off Mauritania from 1988 to 2012: variability on seasonal to decadal timescales

Gerhard Fischer1,2, Oscar Romero2, Ute Merkel1,2, Barbara Donner2, Morten Iversen2,3, Nico Nowald2, Volker Ratmeyer2, Götz Ruhland2, Marco Klann2, and Gerold Wefer2 1Geosciences Department, University of Bremen, Klagenfurter Strasse, 28359 Bremen, Germany 2Marum Center for Marine and Environmental Sciences, Leobener Strasse, 28359 Bremen, Germany 3Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen, 27570 Bremerhaven, Germany

Correspondence to: Gerhard Fischer (gerhard.fi[email protected])

Received: 12 October 2015 – Published in Biogeosciences Discuss.: 3 November 2015 Revised: 10 May 2016 – Accepted: 12 May 2016 – Published: 27 May 2016

Abstract. A more than two-decadal sediment trap record Niño-Southern Oscillation (ENSO) in 1997–1999 where low from the Eastern Boundary Upwelling Ecosystem (EBUE) fluxes were obtained for almost 1 year during the warm El off Cape Blanc, Mauritania, is analysed with respect to deep Niño and high fluxes in the following cold La Niña phase. ocean mass fluxes, flux components and their variability on For decadal timescales, Bakun (1990) suggested an intensifi- seasonal to decadal timescales. The total mass flux revealed cation of coastal upwelling due to increased winds (“Bakun interannual fluctuations which were superimposed by fluctu- upwelling intensification hypothesis”; Cropper et al., 2014) ations on decadal timescales. High winter fluxes of biogenic and global climate change. We did not observe an increase silica (BSi), used as a measure of marine production (mostly of any flux component off Cape Blanc during the past 2 and by diatoms) largely correspond to a positive North Atlantic a half decades which might support this. Furthermore, fluxes Oscillation (NAO) index (December–March). However, this of mineral dust did not show any positive or negative trends relationship is weak. The highest positive BSi anomaly was over time which might suggest enhanced desertification or in winter 2004–2005 when the NAO was in a neutral state. “Saharan greening” during the last few decades. More episodic BSi sedimentation events occurred in several summer seasons between 2001 and 2005, when the previous winter NAO was neutral or even negative. We suggest that distinct dust outbreaks and deposition in the surface ocean 1 Introduction in winter and occasionally in summer/autumn enhanced par- ticle sedimentation and carbon export on short timescales Eastern Boundary Upwelling Ecosystems (EBUEs; Fréon et via the ballasting effect. Episodic perturbations of the ma- al., 2009) cover only about 1 % of the total ocean area but rine carbon cycle by dust outbreaks (e.g. in 2005) might contribute with about 15 % to total marine primary produc- have weakened the relationships between fluxes and large- tion (Carr, 2002; Behrenfeld and Falkowski, 1997). Roughly, scale climatic oscillations. As phytoplankton biomass is high 20 % of the marine global fish catch is provided by the four throughout the year, any dry (in winter) or wet (in sum- major EBUEs (Pauly and Christensen, 1995), the Benguela, mer) deposition of fine-grained dust particles is assumed to the Canary, the Californian and the Humboldt Current Sys- enhance the efficiency of the biological pump by incorpo- tems. Continental margins may be responsible for more than rating dust into dense and fast settling organic-rich aggre- 40 % of the carbon sequestration in the ocean (Muller-Karger gates. A good correspondence between BSi and dust fluxes et al., 2005) and are thus highly relevant for the global carbon was observed for the dusty year 2005, following a period cycle. In the literature, multiple factors with potential influ- of rather dry conditions in the Sahara/Sahel region. Large ence on upwelling systems have been mentioned. To discuss changes of all bulk fluxes occurred during the strongest El all of them would be beyond the scope of this paper and we therefore focus on three major factors.

Published by Copernicus Publications on behalf of the European Geosciences Union. 3072 G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania

In the 1990s, a discussion began whether global warming depths was observed in the subtropical north Atlantic (33◦ N, may lead to intensified coastal upwelling in the EBUEs (e.g. 22◦ W; Brust et al., 2011). Fischer and Karakas (2009) pro- Bakun, 1990: “Bakun upwelling intensification hypothesis”; posed that high dust supply may increase particle settling Cropper et al., 2014). Since then, various studies showed rates by ballasting and result in relatively high organic car- contradicting results, depending on the timescales regarded, bon fluxes in the Canary Current system compared to other the area studied and the methods applied. The longer-term EBUEs. Wintertime African dust transport is suggested to time series analysis of wind stress and sea surface tempera- be affected by the NAO (Chiapello et al., 2005; Hsu et al., ture (SST) by Narayan et al. (2010) from coastal upwelling 2012). As dust plays a major role in the Cape Blanc area areas seems to support the “Bakun upwelling intensifica- with respect to deep ocean fluxes and the intensity of coastal tion hypothesis”, but correlation analysis showed ambigu- upwelling is affected by the NAO as well, the major focus of ous results concerning the relationships of upwelling to the this long-term study will be on the relationship between deep North Atlantic Oscillation (NAO). With some modification, ocean mass fluxes and NAO forcing. the “Bakun hypothesis” is supported for the Canary Current From the mesotrophic Cape Blanc study site CBmeso lo- (CC) coastal upwelling system by Cropper et al. (2014). Us- cated about 200 nautical miles off the coast (Fig. 1a), we ing an upwelling index derived from SSTs and remote sens- obtained an almost continuous sediment trap record of fluxes ing wind stress, Marcello et al. (2011) obtained increased (mostly from about 3500 m water depth) for the past 25 years offshore spreading of upwelled waters off Cape Blanc from (1988–2012, only interrupted between 1992 and 1993). Long 1987 to 2006. Other authors, however, found a warming trend time series of particle fluxes are rare, in particular from of the Canary Current System (e.g. Aristegui et al., 2009). coastal upwelling sites with high productivity. Although Bode et al. (2009) observed a continuous decrease in up- SSTs and wind data analyses over longer timescales (e.g. welling intensity in the northern CC around the Canary Is- decades) for the NW African upwelling system and other lands during the past 40 years, associated with the warming EBUEs are very important to test the “Bakun upwelling of surface waters, a decrease in zooplankton abundance, and, intensification hypothesis” (Bakun, 1990; Cropper et al., locally, in phytoplankton abundance. Studying a sediment 2014), any potential increase of upwelling intensity does core off Cape Ghir, Morocco, a cooling of the northern Ca- not necessarily result in an increase of phytoplankton stand- nary Current in the 20th century was inferred (McGregor et ing stock and/or productivity and/or deep ocean mass fluxes al., 2007). (e.g. Ducklow et al., 2009). Hence, for studying the poten- An influence of tropical Pacific interannual variability tial changes of the biological pump and carbon sequestra- on EBUEs has also been proposed earlier. A link between tion in the deep ocean over decades and over a larger area, the cold La Niña period (1997–1999 ENSO cycle) and the sediment traps are a primary and probably the best choice. Mauritanian upwelling via a strengthening of the north- As deep ocean sediment traps have a rather large catchment easterly (NE) trade winds in autumn and winter was de- area for particles formed in the surface and subsurface wa- scribed by Pradhan et al. (2006). Helmke et al. (2005) corre- ters (e.g. Siegel and Deuser, 1997), they integrate rather lo- lated these anomalous events with deep-ocean carbon fluxes cal and small-scale effects, events and processes in the highly at the mesotrophic Cape Blanc study site. Using ocean colour dynamic EBUE off Mauritania. data, Fischer et al. (2009b) showed a large extension of the Cape Blanc filament from autumn 1998 to spring 1999 when comparing it to the rest of the record (1997–2008). Using 2 Study area remote-sensing data, Nykjaer and Van Camp (1994) found a weak northwest upwelling south of 20◦ N during and after 2.1 Oceanographic and biological setting the strong 1982–1983 El Niño event. The NW African margin and the low-latitude North At- The sediment trap mooring array CBmeso is deployed in lantic are heavily influenced by Saharan dust transport, depo- the Canary Current System within one of the four major sition (e.g. Kaufman et al., 2005) and sedimentation (Brust EBUEs (Fréon et al., 2009) (Fig. 1a). Coastal upwelling is et al., 2001). Dust particles influence the earth’s radiation driven there by alongshore trade winds, leading to offshore balance and supply micro-nutrients (e.g. iron) and macro- advection of surface waters, which are replaced by colder nutrients to the ocean surface waters (e.g. Jickells et al., and nutrient-rich subsurface waters. Around 21◦ N off Cape 2005; Neuer et al., 2004). Additionally, dust acts as ballast Blanc, a prominent cold filament leads to offshore streaming mineral (Armstrong et al., 2002; Klaas and Archer, 2002) for of cold and nutrient-rich waters from the coast to the open organic carbon-rich particles (e.g. Fischer et al., 2009a, b; ocean up to about 450 km offshore (Fig. 1a). This cold tongue Bory and Newton, 2000; Iversen and Ploug, 2010; Iversen et is named the “giant Cape Blanc filament” (Van Camp et al., al., 2010; Bressac et al., 2014). Dunne et al. (2007) suggested 1991), being one of the largest filaments within all EBUEs. that dust may be the major carrier for organic carbon to the The relationship between the coastal winds, SST and the seafloor. A clear coupling between atmospheric dust occur- biological response (e.g. changes in chlorophyll) off Mauri- rence and deep-sea lithogenic particle fluxes at 2000 m water tania seems to be strong and almost immediate (Mittelstaedt,

Biogeosciences, 13, 3071–3090, 2016 www.biogeosciences.net/13/3071/2016/ G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania 3073

-25° -20° -15° 30° MODIS winter 2006-2007 (NAO+) Weak Permanent Annual Upwelling Zone Canary Islands CC

25°

Permanent Annual Upwelling Zone b

CB meso Cape Blanc MODIS winter 2009-2010 20° (NAO-) NEC Mauritania Cape Verde FZ

Cape Verde I. Mauritania- MC Senegalese 15° Upwelling Zone c 0 m 200 m 4000 m a 10°

SeaWiFs fall 1997 SeaWiFs fall 1998 (El Nino) (La Nina)

de

Figure 1. General setting of the study area: (a) Oceanographic setting in the area of the long-term mooring site Cape Blanc (CBmeso) within the Cape Blanc filament (green arrow), dissolving into eddies (indicated as circles with arrows) further offshore. The Cape Verde Frontal Zone (CVFZ) separating the subsurface water masses of the NACW and the SACW (Zenk et al., 1991) is shown. Upwelling zones are marked according to Cropper et al. (2014). Ocean colour map (chlorophyll, 9 km resolution) from MODIS is shown for 2 extreme years, winter 2006–2007 (b, NAO+) and winter 2009–2010 (c, NAO−). SeaWiFS ocean colour during two contrasting situations for the strongest ENSO cycle 1997–1999: autumn 1997 during the warm El Niño phase (d), and autumn 1998 during the cold La Niña event (e). The study site CBmeso is indicated by a square box in the ocean colour pictures, green arrow indicates the Cape Blanc filament, yellow arrows the major dust transport. MC = Mauritanian Current, CC = Canary Current, NEC = North Equatorial Current.

1991; Pradhan et al., 2006). Trade winds persist throughout bringing warmer surface water masses from the equatorial the year and intensify in late winter to reach their highest realm into the study area. Where the CC departs from the intensity in spring (Barton et al., 1998; Nykjaer and Van coast, a NE-SW orientated salinity front in the subsurface Camp, 1994; Meunier et al., 2012). According to Lathuil- waters is observed, the Cape Verde Frontal Zone (CVFZ, ière et al. (2008), our study area is located within the Cape Zenk et al., 1991) (Fig. 1a), which separates the salty and Blanc inter-gyre region (19–24◦ N) which is characterized by nutrient-poor North Atlantic Central Water (NACW) from a weaker seasonality (peaks in winter–spring and autumn). the nutrient-richer and cooler South Atlantic Central Water Following the definition by Cropper et al. (2014), our study (SACW). Both water masses may be upwelled and mixed area is situated on the southern rim of the strong and perma- laterally and frontal eddies develop off Cape Blanc (Meu- nent coastal upwelling zone (21–26◦ N) (Fig. 1a). nier et al., 2012) (Fig. 1a). Lathuilière et al. (2008) offered a The cold and nutrient-rich southward flowing CC departs comprehensive overview of the physical background, i.e. the from the coastline south of Cape Blanc, later forming the ocean circulation off NW Africa. North Equatorial Current (NEC) (Fig. 1a). South of about 20◦ N, a recirculation gyre drives a poleward coastal current fed by the North Equatorial Counter Current (NECC) dur- ing summer. The Mauritanian Current (MC) flows northward along the coast to about 20◦ N (Fig. 1a; Mittelstaedt, 1991), www.biogeosciences.net/13/3071/2016/ Biogeosciences, 13, 3071–3090, 2016 3074 G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania

6 NAO Hurrell index 1989– 90 zone separates the warm and moist SW monsoon flow from (1864– 2014) the dry NE trade winds coming from the Sahara. The trop- 1995 2007– 4 2008 2000 ical rainbelt in the Atlantic realm originates from the con-

2 vergence of the NE and SE trade wind systems and migrates roughly between ∼ 3◦ S (boreal winter) and ∼ 15◦ N (boreal 0 summer) in the course of the year (Lucio et al., 2012). On longer timescales, severe Sahel drought intervals occurred in -2 the 1980s (Chiapello et al., 2005; Nicholson, 2013). Recent

-4 evidence shows that Sahel rainfall may have recovered dur- 2 SD

NAOindex station-based (DJFM), Hurrell 1996 2010 ing the last 2 decades and that the region is now “greening” -6 (Fontaine et al., 2011; Lucio et al., 2012). 1900 1950 2000 Y ears S ediment trap records 2.3 Large-scale teleconnections affecting the study area (1988– 2012)

Figure 2. The NAO Hurrell index (DJFM, station-based, Lisbon- Ocean-atmosphere dynamics at our study site is influ- Rejkjavik, Hurrell, 1995) plotted from 1864–2014. Grey shading enced by large-scale atmospheric teleconnections and cli- indicates the time period covered by the long-term flux record off mate modes. Here, such teleconnections are illustrated based Cape Blanc, Mauritania. A 5-point running mean is shown by the on results from a long-term present-day climate control run thick line. which was performed using the Comprehensive Climate Sys- tem Model version 3 (CCSM3; Collins et al., 2006; Yeager et al., 2006). Atmospheric sea-level pressure (SLP) patterns 2.2 Importance of dust supply and Sahel rain fall for describe the near-surface air flow which affects ocean up- the study area welling and currents as well. We therefore correlated simu- lated SLP with prominent teleconnection indices such as the Dust supply from land to the low-latitude North Atlantic NAO SLP index (Hurrell, 1995) and the Niño3 area-averaged Ocean is not only dependent on the strength of the trans- (150–90◦ W, 5◦ S–5◦ N) SST index, both calculated from the porting wind systems (NE trade winds at lower levels and model results (Fig. 3). Boreal winter is the season where the Saharan Air Layer above) but also on the rainfall and dry- NAO is strongest and where tropical Pacific SST anomalies ness in the multiple source regions in West Africa (Goudie associated with ENSO events tend to peak. and Middleton, 2001; Nicholson, 2013). During long periods Correlations during winter show that NAO and ENSO may of droughts (e.g. in the 1980s), dust loadings over the Sa- have opposite effects on the NW African/eastern Atlantic hel experienced extraordinary increases (N’Tchayi Mbourou realm (Fig. 3a, b), for instance on wind fields, and conse- et al., 1997). As mass fluxes and settling rates of larger ma- quently on upwelling with potential implications for deep rine particles (i.e. marine snow) are assumed to be influenced ocean mass fluxes. A positive phase of the NAO is associ- by mineral dust particles via the ballasting effect (Armstrong ated with anomalous high pressure in the Azores high re- et al., 2002; Fischer et al., 2009a, 2010; Iversen and Ploug, gion (Fig. 3a) and stronger northeasterly winds along the NW 2010; Bressac et al., 2014; Dunne et al., 2007; Thunell et African coast. In contrast, a positive phase of ENSO (El Niño al., 2007), climatic conditions on land need to be consid- event) goes along with a weakening of the northeasterlies in ered. The contribution of dust to the settling particles in the the study area (Fig. 3b). It should be noted, however, that deep ocean off Cape Blanc amounts to one-third on aver- the magnitude of correlation in our study area is larger for age of the total mass flux (Fischer et al., 2010), but it may the NAO than for ENSO. This should be taken into account be as high as 50 % during particular flux events (Nowald et when disentangling the relative importance of these climate al., 2015). As shown by Jickells et al. (2005), modelled dust modes. Apart from seasonal-to-interannual timescales, low- fluxes from the Saharan region and their variability may be frequent climate variability may impact on our study area influenced by ENSO and NAO cycles (see also Goudie and as well and is probably linked to Atlantic SST variations Middleton, 2001; Chiapello et al., 2005; Hsu et al., 2012; Di- on decadal-to-interdecadal timescales, e.g. the Atlantic Mul- atta and Fink, 2014). During the time period of this study tidecadal Oscillation (AMO). The correlation of SLP with (1988–2012, Fig. 2), the wintertime (December–January– area-averaged (0–70◦ N, 60–10◦ W) SST fluctuations over February–March = DJFM) NAO index after Hurrell (Hurrell, periods above 10 years highlights a centre of action in the 1995) is characterized by switches from extremely positive tropical Atlantic with SLP reductions (weaker northeasterly (e.g. 1989, 1990) to extremely negative values (e.g. in 1996, winds) along with higher Atlantic basin-wide SST during a 2010). positive AMO phase (Fig. 3c). This shows the potential im- Climate over West Africa is also influenced by the conti- portance of longer-term Atlantic basin-scale SST variations nental Inter-Tropical Convergence Zone (ITCZ; also named for alongshore winds and upwelling (trends) at our trap loca- Intertropical Front, Nicholson, 2013). This low-pressure tion.

Biogeosciences, 13, 3071–3090, 2016 www.biogeosciences.net/13/3071/2016/ G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania 3075

ment was small in winter–spring 1997–1998 and unusually high from autumn 1998 to spring 1999 (Figs. 7b, 1e). This is documented by reduced (warm El Niño) and elevated (cold La Niña) deep ocean mass fluxes of all components. In cer- tain years, the filament area was more than twice as large in spring as in fall (e.g. 1999 La Niña Event). Tropical Pacific variability on interannual ENSO timescales is also an im- portant factor in driving ecosystem variability in the Califor- nia Current System (for a summary see Checkley and Barth, 2009).

3 Material and methods

3.1 Sediment traps and moorings

We used deep-moored (> 1000 m), large-aperture time series sediment traps of the Kiel and Honjo type with 20 cups and 0.5 m2 openings, equipped with a honeycomb baffle (Krem- ling et al., 1996). Mooring and sampling dates are given in Table 1. As the traps were moored in deep waters (mostly below 1000 m), uncertainties with the trapping efficiency due to strong currents (e.g. undersampling, Yu et al., 2001; Bues- seler et al., 2007) and/or due to the migration and activ- ity of zooplankon migrators (“swimmer problem”) are as- sumed to be minimal. Prior to the deployments, the sampling cups were poisoned with HgCl2 (1 mL of conc. HgCL2 per 100 mL of filtered seawater) and pure NaCl was used to in- crease the salinity and density in the sampling cups (salin- ity = 40 ‰). Upon recovery, samples were stored at 4 ◦C and wet-splitted in the home laboratory using a rotating McLane wet splitter system. Larger swimmers such as crustaceans were picked by hand with forceps and were removed by fil- tering carefully through a 1 mm sieve and all flux data there- fore refer to the size fraction of < 1 mm. In almost all sam- ples, the fraction of particles > 1 mm was negligible, only in a few samples, larger pteropods were found.

3.2 Mass fluxes

Analysis of the fraction < 1 mm, using 1/4 or 1/5 wet splits, Figure 3. Teleconnections affecting the study site off Cape Blanc. was performed according to Fischer and Wefer (1991). Sam- Correlation of simulated sea-level pressure (SLP) with (a) the NAO ples were freeze-dried and the homogenized samples were SLP index after Hurrell (Hurrell, 1995; boreal winter season), analysed for bulk (total mass), organic carbon, total nitro- (b) the Nino3 SST index (boreal winter season), and (c) North gen, carbonate and biogenic opal (BSi = biogenic silica). Or- Atlantic SST (low pass-filter applied considering periods above ganic carbon, nitrogen and calcium carbonate were measured 10 years). Analysis based on the last 100 model years of a present- by combustion with a CHN-Analyser (HERAEUS). Organic day control simulation using the CCSM3 model. carbon was measured after removal of carbonate with 2 N HCl. Overall analytical precision based on internal lab stan- dards was better than 0.1 % (±1σ). Carbonate was deter- ENSO-related teleconnections in the NW African up- mined by subtracting organic carbon from total carbon, the welling system have been described by several authors latter being measured by combustion without pre-treatment (Behrenfeld et al., 2001; Pradhan et al, 2006; Zeeberg et with 2N HCl. BSi was determined with a sequential leaching al., 2008) and can be illustrated by the negative correlation technique with 1M NaOH at 85 ◦C (Müller and Schneider, of SLP with eastern tropical Pacific SST (Fig. 3b). Fischer 1993). The precision of the overall method based on replicate et al. (2009b) showed that the size of the Cape Blanc fila- analyses is mostly between ±0.2 and ±0.4 %, depending on www.biogeosciences.net/13/3071/2016/ Biogeosciences, 13, 3071–3090, 2016 3076 G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania

Table 1. Deployment data of the moorings and traps at the mesotrophic sediment trap site CB, Cape Blanc, Mauritania. Associated ships’ cruises and references to earlier publications on fluxes are indicated.

Trap LAT LONG water depth trap depth sampling no remark, relevant cruise recovery/ name N W m m start end of samples reference to fluxes GeoB no. CB-1 lower 20◦45.3’ 19◦44.50 3646 2195 22.03.88 08.03.89 13 Fischer et al. (1996, 2003) Meteor 9/4/ GeoB 1121-4 CB-2 lower 21◦08.70 20◦41.20 4092 3502 15.03.89 24.03.90 22 Fischer et al. (1996, 2003) Meteor 12/1/ GeoB 1230-1 CB-3 lower 21◦08.30 20◦40.30 4094 3557 29.04.90 08.04.91 17 Fischer et al. (1996, 2003, 2010) Polarstern ANT IX/4 CB-4 lower 21◦08.70 20◦41.20 4108 3562 03.03.91 19.11.91 13 Fischer et al. (1996, 2003, 2010) Meteor 20/1/ GeoB 1602-1 CB-5 lower 21◦08.60 20◦40.90 4119 3587 06.06.94 27.08.94 19 Meteor 29/3/ GeoB 2912-1 CB-6 upper 21◦15.00 20◦41.80 4137 771 02.09.94 25.10.95 20 Fischer et al. (2010) Polarstern ANT XIII/1 CB-7 lower 21◦15.40 20◦41.80 4152 3586 20.11.95 29.01.97 20 Meteor 38/1/ GeoB 4302-7 CB-8 upper 21◦16.30 20◦41.50 4120 745 30.01.97 04.06.98 20 Fischer et al. (2010) Meteor 41/4/ GeoB 5210-2 CB-9 lower 21◦15.20 20◦42.40 4121 3580 11.06.98 07.11.99 20 Helmke et al. (2005) Metero 46/1/ GeoB 6103-3 CB-10 lower 21◦17.20 20◦44.10 4125 3586 10.11.99 10.10.00 3 mostly no seasonal sampling Polarstern ANT XVIII/1 CB-11 upper 21◦16.80 20◦43.00 4113 1003 11.10.00 30.03.01 20 Poseidon 272/ GeoB 7401-1 CB-12 lower 21◦16.00 20◦46.5 4145 3610 05.04.01 22.04.02 14 Meteor 53/1c/ GeoB 7917-1 CB-13 lower 21◦16.80 20◦46.70 4131 3606 23.04.02 08.05.03 20 Fischer et al. (2009b) Meteor 58/2b/ GeoB 8628-1 Fischer and Karakas (2009) CB-14 upper 21◦17.20 20◦47.60 4162 1246 31.05.03 05.04.04 20 Poseidon 310/ no number CB-15 lower 21◦17.90 20◦47.80 4162 3624 17.04.04 21.07.05 20 Meteor 65/2/ no number CB-16 lower 21◦16.80 20◦47.80 4160 3633 25.07.05 28.09.06 20 Poseidon 344/1/ GeoB 11401-1 CB-17 lower 21◦16.40 20.48.20 4152 3614 24.10.06 25.03.07 20 Merian 04/b/ GeoB 11833-1 CB-18 lower 21◦16.90 20◦48.10 4168 3629 25.03.07 05.04.08 20 Poseidon 365/2/ GeoB 12907-1 CB-19 lower 21◦16.20 20◦48.70 4155 3617 22.04.08 22.03.09 20 Merian 11/2/ GeoB 13616-4 CB-20 upper 21◦15.60 20◦50.70 4170 1224 03.04.09 26.02.10 19 Poseidon 396/ GeoB 14201-3 CB-21 lower 21◦15.60 20◦50.90 4155 3617 28.02.10 04.04.11 20 Merian 18/1/ GeoB 15709-1 CB-22 lower 21◦16.10 20◦50.90 4160 3622 05.05.11 11.01.12 15 Poseidon 425/ GeoB 16101-1 CB-23 lower 21◦15.80 20◦52.40 4160 3622 20.01.12 22.01.13 18 Poseidon 445/ GeoB 17102-5 the material analysed. For a detailed table of standard devi- trap data (around 1000 m) were used, which mostly match ations for various samples we refer to Müller and Schnei- the lower trap fluxes with respect to seasonality (Fischer et der (1993). Lithogenic fluxes or the non-biogenic material al., 2009b). When plotting all available lower and upper trap was estimated according to total mass fluxes for winter, a close correspondence is ob- served (r2 = 0.84, N = 10), with slightly higher fluxes in the lithogenic material = dust = total mass − carbonate deeper trap due to lateral particle advection processes (Fis- − opal − 2 × Corg. cher et al., 2009b; Karakas et al., 2006, 2009). However, considering the entire record presented here, it seems that We estimated organic matter by multiplying organic carbon the upper trap fluxes of the winter seasons 1998 and 2004 by a factor of two as about 50–60 % of marine organic mat- may be critical due to smaller filament areas. As a conse- ter is constituted by organic carbon (Hedges et al., 1992). quence, the area/filament with elevated chlorophyll and high Some studies have shown a clear linear relationship between particle concentrations may not have reached the upper off- lithogenic fluxes and particulate aluminum (e.g. Ratmeyer et shore trap. Because of lateral particle advection from the east al., 1999a), the latter being derived from clay minerals as (Karakas et al., 2006) and the larger catchment area of the part of the lithogenic (non-biogenic) component. Grains size deeper traps (Siegel and Deuser, 1997), particle fluxes might studies from Ratmeyer et al. (1999a, b) and further micro- have been higher in the deeper water column in winter 1998 scopic analysis provide evidence that most of the lithogenic and 2004. In general, the seasonal patterns and the compo- material in the study area was derived from quartz grains in sition of the particle fluxes were rather similar between the the fine silt fraction (10–30 µm, see also Friese et al., 2016). upper and lower traps (Fischer et al., 2009b). The long-term Here we attribute the lithogenic flux to dust-derived mate- means and standard deviations were calculated using only the = rial ( mineral dust flux) as no large rivers supply suspended available deeper trap flux values. The seasonal anomalies of material to the study area off Cape Blanc. the bulk fluxes were calculated using the deviations from the Due to logistical reasons, we had very different time res- mean values of the respective seasons. olutions of the sediment trap collections (a few days to sev- eral weeks) which limits comparisons between specific inter- 3.3 Carbonate producers vals and years. Seasonal fluxes were calculated and shown to allow comparison between the seasons mainly with re- To determine the major carbonate producers, the trap ma- spect to interannual variability. Seasons were defined us- terial was carefully wet-sieved with a 1 mm screen and ing the dates of opening and closure of the sampling cups split into aliquots by a rotary liquid splitter. Generally closest to the start of the astronomical seasons (21 March, a 1/5 split of the < 1 mm fraction was used to pick 21 June, 23 September, 21 December) (Table 2). Where planktonic foraminifers and pteropods from the wet so- lower trap data (around 3500 m) were not available, the upper lution. Foraminifers and pteropods were picked by hand

Biogeosciences, 13, 3071–3090, 2016 www.biogeosciences.net/13/3071/2016/ G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania 3077 %%%%% 2 − g m 2 − g m 2 − g m 2 − g m 2 − g m 2 − no sampling #1–2 winter 1997#1 spring 82 14.24 81 0.78 gap 0.77 0.097 1.57 0.01 5.37 0.06 6.55 0.008 5.46 1.21 5.41 0.20 0.68 37.70 0.45 46.00 3.70 0.51 76.80 12.62 #1–2 fall 75 gap 2.20 0.09 0.09 0.010 1.11 0.75 4.26 3.90 0.45 50.36 34.07 + + + #1–5 spring 71.5 gap 4.87 0.24 0.38 0.053 2.09 1.77 4.89 7.83 1.09 43.02 36.36 #1–8 fall 87 8.32 0.41 0.56 0.087 5.13 1.68 4.93 6.73 1.05 61.66 20.19 + + start end11.06.8827.09.88 27.09.8817.12.88 17.12.88 #4–7 of trap 08.03.89 #8–10 #11–1325.06.8918.09.89 18.09.8929.12.89 29.12.89 #7–11 summer fall 24.03.90 #12–17 winter #18–2203.07.90 198927.09.90 27.09.90 summer22.12.90 22.12.90 #5–8 fall 18.03.91 #9–12 winter #13–16 1990 108 81 summer fall 81 winter days23.06.94 1991 85 27.08.94 85 #5–19 10221.12.94 23.0119.03.95 19.03.95 11.5015.06.95 15.06.95 #6–9 1.57 11.09.95 g #10–13 0.89 86 m 86 14.12 summer #14–1719.12.95 0.86 13.62 8616.03.96 16.03.96 14.06 1.0712.06.96 0.45 12.06.96 #2–5 winter spring 16.29 0.4930.09.96 30.09.96 #6–9 0.75 summer 0.135 1995 0.055 0.51 27.12.96 #10–14 0.67 #15–18 13.20 10.89 10.83 0.056 0.39 0.46 winter 5.0920.06.97 65 0.40 0.58 spring 0.48 summer 0.04602.10.97 02.10.97 9.76 1996 0.055 8.47 6.78 fall14.12.97 14.12.97 88 #7–10 4.63 0.05622.03.98 22.03.98 0.40 #11–13 7.48 0.39 88 0.73 18.06.98 #14–17 6.81 6.81 88 5.46 #18–20 7.70 8.2109.09.98 0.044 0.091 summer 0.4428.12.98 28.12.98 88 4.87 fall 5.58 6.1120.03.99 20.03.99 4.66 #5–8 7.30 winter 6.45 0.052 11011.06.99 3.92 11.06.99 #9–11 7.35 4.50 12.44 3.60 88 1998 29.09.99 0.16 #12–14 5.33 0.59 3.60 #15–18 0.48 4.11 0.96 47.07 4.02 3.50 88 4.67 4.34 0.24 44.21 36.81 fall 2.86 winter 0.14 0.40 3.27 40.23 0.22 spring 0.00 48.00 8.02 4.46 3.05 5.33 2.95 98 0.74 summer 1999 98 0.023 0.34 38.70 0.39 73.5 7.44 0.37 54.92 0.113 48.44 0.24 0.34 4.08 9.55 0.00 35.76 2.98 6.73 39.69 5.97 50.40 0.2027.06.01 0.042 0.63 39.59 8.59 0.00101.10.01 01.10.01 5.89 82.5 0.34 0.33 0.84 4.5217.12.01 17.12.01 #5–9 0.89 0.38 55.64 41.35 21.03.02 0.29 #10–13 0.044 110 1.59 82.5 4.25 0.00 0.61 35.35 110 4.12 39.87 #14 1.68 0.53 sammel 0.036 2.19 0.49 41.21 0.04 0.080 3.80 0.40 0.05 1.20 45.66 7.69 winter 0.00 summer fall 0.01 4.72 0.049 16.33 2002 1.92 4.76 6.29 3.16 0.20 0.00 0.15 5.91 1.19 17.06 19.55 0.04 1.95 3.73 0.32 0.026 16.88 0.024 4.55 2.94 81.78 6.86 1.07 1.08 0.91 0.006 94.25 0.00 0.51 12.19 2.66 47.33 0.62 3.69 2.91 96.25 6.58 0.84 33.14 1.20 4.28 0.42 0.25 0.073 0.77 45.43 0.65 0.57 4.40 0.00 77 0.90 3.90 34.29 0.45 0.55 6.38 0.086 0.083 0.00 7.18 0.12 0.068 47.40 0.48 0.92 39.48 4.64 3.21 0.84 0.88 11.77 63.39 9.31 11.80 2.86 12.49 49.83 6.71 26.15 0.05 0.57 30.82 4.66 1.03 5.40 8.87 43.38 5.15 3.43 7.27 7.90 7.14 42.91 0.61 0.04 1.43 5.53 0.53 6.25 68.50 0.63 3.02 1.22 50.12 3.81 21.20 0.009 51.84 26.49 0.091 24.08 3.33 0.43 4.48 0.45 3.35 0.75 43.99 6.47 0.050 0.42 41.11 0.50 0.40 60.17 54.59 69.92 0.02 27.61 3.75 30.19 3.72 20.35 5.68 8.25 2.79 4.20 6.71 5.04 1.02 0.73 85.23 5.44 51.80 2.27 30.02 0.63 47.09 35.32 16.12.9921.03.00 21.03.0021.06.00 21.06.00 #3 21.09.00 #3 #3 winter spring 2000 summer 94 92 92 8.92 0.14 8.74 8.74 0.14 0.14 0.45 0.076 0.44 0.44 0.075 7.26 0.075 7.11 0.93 7.11 1.59 0.91 0.91 1.59 1.59 5.04 0.85 5.05 5.05 81.39 10.46 0.86 0.86 81.33 81.33 10.45 10.45 22.06.9120.09.91 20.09.91 19.11.91 #6–14 #15–20 summer fall 90 60 9.06 0.48 2.67 0.13 0.83 0.110 0.17 0.023 3.66 1.3118.12.00 3.25 22.03.01 #9–19 0.89 5.30 4.87 winter 9.16 2001 6.37 1.21 40.40 0.86 35.87 49.06 93.5 33.33 6.51 0.39 0.60 0.093 3.57 1.34 6.01 9.25 1.43 54.84 20.58 Seasonal flux data and percentages of major bulk components of total flux at the mesotrophic sediment trap site CB from 1988–2012. CB mesoCB-1 lower interval 22.03.88 11.06.88 #1–3CB-2 lower sample no. 15.03.89 25.06.89 season #1–6 springCB-3 lower year 1988 29.04.90 03.07.90 #2–4 springCB-4 lower duration remark 18.03.91 81 22.06.91 TTL mass #17 springCB-5 lower BSi 06.06.94 org.CB-6 carbon upper 23.06.94 102 nitrogen #1–4 24.09.94 carbonate 15.64 21.12.94 lithogenic 1.91 #2–5CB-7 lower 64.5 BSi spring 20.11.95 org. carbon 0.59 19.12.95 1994 12.91 nitrogen #1 fall 0.069 carb.CB-7/8 0.52 lower lith. CB-8 27.12.96 upper 4.89 12.68 20.03.97 0.40 17 #19–20 20.03.97 fall 0.49 20.06.97 7.66 0.051 #3–6CB-9 lower 12.23 0.39 6.92 88 18.06.98 0.047 09.09.98 spring 3.77 1.76 4.68 #2–4 6.78CB-9/10 lower 0.05 0.44 4.03 29.09.99 29 31.25 16.12.99 48.96 4.64 11.58 #19–20 summer 0.05 3.10CB-11 upper 0.26 3.87 0.007 11.10.00 98 0.40CB-12 lower 18.12.00 53.60 0.70 1.27 #3 3.04 2.91 05.04.01 36.25 27.06.01 0.104 0.18 82.5 #1–4 0.37 0.35 53.49 6.82 36.55 17.72 2.84 0.12 0.62 spring 3.10 0.015 2.84 17.67 2.27 1.22 1.05 0.40 0.61 72.16 0.131 1.26 19.89 6.02 0.58 9.69 0.90 6.26 83 0.074 58.92 26.74 5.30 12.57 4.26 3.50 0.52 3.34 42.06 6.50 43.33 5.94 3.45 0.32 0.74 54.68 3.29 0.25 29.92 0.42 0.034 71.11 18.87 2.91 2.76 4.92 3.85 0.52 44.77 42.46 Table 2. www.biogeosciences.net/13/3071/2016/ Biogeosciences, 13, 3071–3090, 2016 3078 G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania al 2. Table B2 oe 00.02.61 26srn 0 .402 .100747 .332 .606 51 18.12 65.12 0.64 0.67 4.26 88.59 0.62 3.27 4.56 1.33 2.36 4.78 25.24 0.07 62.87 25.24 0.54 64.87 0.047 0.48 8.63 4.24 0.31 3.30 0.060 3.40 0.24 0.44 3.28 7.34 2.83 1.09 0.23 7.05 26.90 9.74 2.81 60.64 0.47 0.061 100 0.021 23.52 3.58 0.48 50.80 0.14 0.92 #15 88 5.30 0.38 24.03.12 29.55 7.32 0.14 25.12.11 58.42 11.22 spring 3.36 lower 0.48 CB-22/23 4.34 #20 21.06.11 11.06 3.78 15.03.11 7.58 lower CB-21/22 #2–6 2.67 spring 28.06.10 0.059 4.46 20.03.10 92 5.77 0.45 60 lower CB-21 #1–5 1.78 30.06.09 0.104 03.04.09 0.66 3.53 0.83 upper 12.49 CB-20 #20 spring lith. 23.06.08 0.029 1.26 17.03.08 carb. fall 0.23 11.35 nitrogen lower CB-19 #1–5 carbon org. 25.06.07 BSi 0.27 25.03.07 69 #1–8 23.12.06 6.03 lower CB-18 lithogenic 24.10.06 carbonate 93 nitrogen lower CB-17 carbon org. #20 BSi 27.09.05 spring 28.06.05 mass 57 TTL lower CB-16 summer remark #1–3 duration 25.06.04 17.04.04 year #2–7 lower CB-15 16.09.03 season spring 15.06.03 no. sample upper CB-14 #1–3 19.06.02 23.04.02 interval lower CB-13 meso CB Continued. 20.22.21 1–6fl 0. 11 .807 .9 09 .146 .504 19 36.54 25.97 44.09 51.94 67.21 43.80 0.46 0.36 0.47 16.13 71.25 3.45 11.77 2.37 4.17 0.46 75.22 0.46 25.99 4.43 4.62 2.09 3.77 61.16 18.02 8.94 5.16 0.42 69.64 77.85 7.71 3.65 0.59 3.35 5.97 0.52 22.31 5.13 2.63 61.54 10.96 4.13 6.43 2.02 3.48 0.77 8.67 5.93 28.85 80.90 1.23 2.65 0.097 57.45 0.65 6.92 8.92 33.60 4.05 0.046 0.064 0.46 3.50 0.73 51.45 15.47 2.55 7.86 12.58 4.95 0.50 0.31 0.56 69.04 0.057 0.89 5.07 71.12 2.31 0.69 0.53 0.57 30.56 6.00 0.048 0.98 4.87 0.56 2.74 56.31 0.06 0.27 0.51 3.44 4.44 3.54 6.01 33.22 32.96 4.99 0.46 0.54 21.10 0.041 51.37 55.94 12.90 13.54 0.21 5.17 0.46 0.58 0.029 0.47 0.16 5.42 3.87 0.040 6.60 0.50 6.51 0.28 12.52 0.20 3.03 2.63 24.03 4.69 3.83 0.27 0.002 10.78 10.45 1.47 62.47 5.38 1.58 0.26 25.51 26.11 40.63 0.56 0.02 107.5 4.64 0.021 59.64 0.20 58.51 43.97 0.086 6.03 3.43 0.27 9.81 86 86 2.20 6.56 0.56 30.48 0.51 0.47 8.95 4.23 0.16 4.94 0.95 58.46 7.72 0.045 0.01 3.51 24.94 2.83 102 0.38 4.37 3.97 35.14 3.94 0.050 4.05 64.72 0.072 0.44 50.60 0.26 5.03 85 0.09 0.53 0.66 3.11 0.42 0.52 5.43 4.79 0.63 31.59 21.34 0.033 6.09 fall 3.25 7.38 7.51 summer 18.77 4.78 3.65 51.69 70.64 0.47 spring 100 4.25 30.87 0.28 0.65 0.061 0.35 0.040 4.82 0.63 3.87 80 1.58 6.45 0.82 48.14 9.01 80 #12–16 27.48 12.42 0.50 0.74 0.33 3.04 fall 9.51 4.76 #8–11 2.54 29.12.12 49.91 12.59 summer 5.72 4.54 0.39 36.23 #4–7 12.09.12 9.04 84 12.09.12 0.87 3.55 6.98 30.66 0.112 5.48 47.73 18.06.12 2011 18.06.12 1.93 58.16 7.19 0.64 2.93 0.29 0.47 24.03.12 7.18 90 2.80 67.5 8.71 #9–14 5.22 0.84 0.44 winter 0.085 29.72 0.031 0.074 17.23 #4–8 25.12.11 fall 10.58 8.57 59.37 5.00 summer 10.16 3.54 72.94 14.09.11 14.09.11 7.44 4.22 3.25 0.66 102 3.36 0.24 0.63 0.44 0.056 2010 0.37 21.06.11 5.39 1.00 #16–19 85 85 2.56 #11–15 0.041 15.03.11 0.46 winter 3.07 0.043 12.18 8.92 10.75 19.89 #7–10 2.69 25.12.10 25.12.10 fall 0.92 4.42 0.45 1.19 0.16 summer 0.28 16.09.10 76 16.09.10 3.99 0.36 0.152 2009 15.16 28.06.10 0.053 6.07 2.79 15.87 0.72 #11–19 95 4.78 2.92 95 4.44 0.29 #11–15 26.02.10 winter 1.12 0.39 0.061 0.24 14.90 #6–10 21.12.11 21.12.11 fall summer 0.48 3.68 3.39 28.09.09 28.09.09 5.53 3.98 0.45 0.005 7.72 2008 30.06.09 8.34 90 #16–20 1.69 0.43 #10–15 0.036 22.03.09 0.03 winter 6.78 0.042 16.85 #5–9 27.12.08 86 27.12.08 fall 23.56 86 15.21 0.84 107.5 summer 0.27 16.09.08 16.09.08 0.32 2007 0.050 23.06.08 0.085 8.28 86 0.03 #15–19 winter #11–14 17.03.08 0.35 0.62 0.69 0.58 #6–10 13.12.07 92 13.12.07 0.42 92 28.09.07 28.09.07 2006 7.71 summer 25.06.07 9.51 #9–20 spring 92 m 92 0.55 g 1.03 winter 23.03.07 fall 23.12.06 11.41 23.10 #16–20 93 #12–15 28.09.06 2005 #8–11 12.06.06 12.06.06 spring 93 #4–7 18.03.06 18.03.06 winter fall 22.12.05 38 22.12.05 summer days 95 27.09.05 #16–19 2004 #12–15 28.06.05 95 95 #8–11 28.03.05 28.03.05 winter fall #4–7 26.12.04 26.12.04 25.09.04 25.09.04 2003 25.06.04 #14–19 spring #8–13 20.03.04 winter fall 18.12.03 18.12.03 summer 16.09.03 #19–20 #14–18 08.05.03 #9–13 31.03.03 31.03.03 trap of #4–8 26.12.02 26.12.02 22.09.02 22.09.02 19.06.02 end start + + + + 13srn 7gp49 .802 .2 .304 .142 .78.29.39 80.22 0.57 4.23 3.61 0.46 3.93 13.92 73.67 0.53 0.028 4.03 0.21 4.43 0.18 4.90 0.76 4.04 gap 29.62 57.38 67 0.029 0.43 0.22 3.43 0.24 #1 6.13 spring 5.49 5.40 #1–3 gap 10.46 81 0.078 0.63 1.12 spring 18.23 #1–4 gap 87.5 summer #1–3 + 3wne 028. 9.)gp1.108 .00061.857 .633 .85.832.05 56.28 0.48 3.35 4.86 5.74 10.08 0.086 0.60 0.87 17.91 gap (90.5) 81.5 2012 winter #3 − 2 m g − 2 m g − 2 m g − 2 m g − 2 m g − 2 % % % % %

Biogeosciences, 13, 3071–3090, 2016 www.biogeosciences.net/13/3071/2016/ G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania 3079 with a pipette under a ZEISS Stemi 2000 microscope and Following the strong ENSO cycle 1997–1999, total flux rinsed with fresh water for three times and dried at 50 ◦C anomalies were low or negative over a longer period (autumn overnight and counted. The mass fluxes of total carbon- 1999 to autumn 2004), only interrupted by an episodic peak ate producers expressed as mg m−2 d−1 are mainly consti- in summer 2002 (Fig. 5a, b). Other episodic peaks in sed- tuted of planktonic foraminifera, pteropods and nannofos- imentation were found in winter/spring 1996–1997 and in sils/coccolithophorids. Masses of foraminifera and pteropods the winter seasons 2004–2005, 2006–2007 and 2009–2010 were determined with a Sartorius BP 211D analytical bal- (Fig. 5a, b). Longer intervals (several seasons) of negative ance. flux anomalies were obtained in 1997–1998 and 2009–2011 (Fig. 5b). Total fluxes decreased from 1988 to 1991, from 3.4 Additional web-based data spring 2007 to 2010, later increasing from 2010 to 2012 (Fig. 5). To put our flux results from the deep ocean into a broader In general, the major bulk flux components followed the context, we used several observational data sets available total flux and were well inter-correlated, except that the re- from several of the websites given below. For ocean colour, lationship between organic carbon and carbonate was weak time series from the MODIS or SeaWiFS sensors based on in summer (Table 3). However, the regression-based rela- ◦ × ◦ ◦ ◦ a 1 1 box from 20–21 N and 21–20 W (9 km resolu- tionships (i.e. the slope) varied interannually (e.g. Fischer tion) slightly to the east of the study site CB have been cho- et al., 2009a). The matrix in Table 3 shows the correla- sen due to the generally prevailing E-W directed current sys- tion coefficients between organic carbon and nitrogen, BSi, tem, transporting particles to the west (Helmke et al., 2005). carbonate and lithogenic fluxes for the four seasons (lower ◦ × ◦ ◦ × ◦ Larger boxes, e.g. 2 2 or 4 4 , revealed similar re- traps only). Organic carbon and BSi (mainly diatoms) were sults. For the aerosol optical thickness (AOT, 869 nm, 9 km highly correlated during the major upwelling events in win- ◦ × ◦ resolution), a 1 1 box was chosen from the SeaWiFS and ter (R2 = 0.70) and summer/autumn, whereas the relation- MODIS data. ship between organic carbon and total carbonate in sum- mer was weak (R2 = 0.16, Table 3). Dust fluxes peaked to- 4 Results gether with organic carbon, preferentially in winter and au- tumn (R2 = 0.63 and 0.67 , respectively Table 3). The tight In the long term, seasonal bulk fluxes were highest in boreal coupling between organic carbon and nitrogen is not surpris- winter and summer and slightly lower in spring and autumn ing as both elements constitute organic matter formed during (Figs. 4, 5, 6a; Table 2). Total bulk fluxes reached 23.6 and photosynthesis, which is later degraded in the upper water 23.1 g m−2 in winter and summer, respectively (Table 2). For column forming sinking phytodetritus. The slope is almost spring and autumn, total mass fluxes were as high as 19.6 constant (0.13–0.11) and the reciprocal value reflects the and 21.1 g m−2, respectively (Table 2). However, the sea- Redfield Ratio (Redfield et al., 1963) of the sinking organic- sonal differences in the bulk fluxes are not statistically signif- rich particles (Table 3). The molar C : N varied seasonally icant. Along with the highest mass fluxes, winter and summer between 8.9 and 10.6, typical for sinking detritus collected seasons also exhibit the highest standard deviations (Fig. 4), in deep sediment traps. pointing to a high interannual variability. In general, this in- In the long term, the composition of settling particles in terannual variability is clearly higher than the seasonal differ- the deeper traps off Cape Blanc consisted of roughly 57 % ences in bulk fluxes. Only the lithogenic components, i.e. the carbonate, ca. 30 % lithogenic particles, 4 % organic carbon, mineral dust particles, did not show an increase during sum- 0.5 % nitrogen and 5 % BSi (Fig. 4). BSi contained mostly mer and only peaked in winter (up to 7.4 g m−2) when dust a mixture of coastal and open-ocean diatoms (Romero et plumes were most frequent (Goudie and Middleton, 2001). al. 1999, 2002, and unpublished data). The BSi flux pat- High summer fluxes of up to 16.9 g m−2 were mostly due to tern (Fig. 6) was influenced by switches from a positive to high carbonate sedimentation (Fig. 4), both of primary (coc- a negative NAO index which were reflected in decreasing colithophores) and secondary producers (foraminifera and winter fluxes, e.g. from 1989 to 1991, 1995 to 1996 and pteropods). Organic carbon and BSi showed a rather simi- 2007 to 2010. From 2001 through 2006, NAO variability was lar pattern (Figs. 4, 6a) with a maximum in winter (up to rather low and the index was around zero or slightly negative 1.1 and 1.7 g m−2, respectively) and a secondary maximum (Fig. 6c; Table 4). Nevertheless, BSi fluxes varied consider- in summer/autumn. This is reflected in the close correspon- ably and showed episodic peaks in the summer seasons 2001, dence between both flux components for these seasons (Ta- 2002 and 2003 (Fig. 6a, b). BSi flux was high and showed ble 3). Highest mass fluxes coinciding with highest positive positive anomalies in 2005, except for spring 2005 (Fig. 6a, flux anomalies lasting for several seasons occurred in 1988– b; Table 4). 1989, 1998–1999, and 2005–2006 (Fig. 5). The general flux pattern of BSi (Fig. 6a, b) with values from almost zero to 1.91 g m−2 did not match the SeaWiFS ocean colour time series trend which showed an overall de- crease in chlorophyll from 1997 to 2010 (Fig. 6d). The or- www.biogeosciences.net/13/3071/2016/ Biogeosciences, 13, 3071–3090, 2016 3080 G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania

Cape Blanc 1– 23 Winter lower traps 20 W inter Summer 1 (1988– 2012) F all 16 Spring 0.8

12 0.6 -2 -2 g m g m

8 flux BSi 0.4 Total massflux 4 0.2 (a) (d) 0 0 5.6 5.3 4.3 5.1 %

1 10 Summer Winter 0.8 8

-2 0.6 6 -2 g m g m

Corg flux 0.4 4 carbonate flux 0.2 2 (b) (e) 0 0 4.2 4.0 3.8 4.1 % 54.7 55.0 63.8 52.2 % Winter 0.1 10

0.08 8 Winter 0.06 6 -2 -2 g m g m 0.04 4 N total flux lithogenic flux 0.02 2 (c) (f) 0 0 0.56 0.53 0.49 0.47 % 32.3 30.6 24.4 34.4 %

Figure 4. Seasonal means of major bulk fluxes of the lower traps only (a total, b organic carbon, c nitrogen, d biogenic silica (= BSi), e carbonate, and f lithogenic = mineral dust) and the respective standard deviations (1 SD), which reflect interannual variability. Relative contributions ( %) of BSi, organic carbon, nitrogen, carbonate and lithogenic materials to total mass in the respective seasons are indicated by numbers below the bars.

Table 3. Correlation coefficients between organic carbon flux and colour values generally mimicked the SeaWiFS pattern, ex- major bulk flux components for the four different seasons (lower cept for the discrepancy in summer 2010 (Fig. 6d). trap data only). Number of data points (N) and the slopes (s) for the regression lines are given as well. Statistically significant values for R2 at a 99.9 % confidence level are indicated in bold. 5 Discussion Organic carbon Winter Spring Summer Autumn 5.1 Particle transport processes in the water column Nitrogen 0.96 0.93 0.92 0.93 N = 16 N = 19 N = 19 N = 18 Mass fluxes and particle transport processes off Cape Blanc s = 0.13 s = 0.12 s = 0.12 s = 0.11 (Mauritania) have been described by summarising articles BSi 0.70 0.46 0.63 0.75 of Fischer et al. (2009b) and Karakas et al. (2006). Com- N = 16 N = 19 N = 19 N = 18 mon flux patterns were the increase of fluxes in late winter– s = 1.3 s = 1.7 s = 1.4 s = 1.4 spring and late summer of all components at both trap lev- els. This matched the seasonal intensification of coastal up- Carbonate 0.56 0.56 0.16 0.82 N = 15 N = 19 N = 19 N = 18 welling (e.g. Meunier et al., 2012) due to wind forcing and a s = 8.9 s = 10.9 s = 6.0 s = 13.2 stronger offshore streaming of the Cape Blanc filament (e.g. Fischer et al., 2009b). The increase of fluxes in late summer Lithogenic 0.63 0.53 0.43 0.67 to autumn was mostly due to enhanced biogenic carbonate = = = = = ( mineral dust) N 16 N 19 N 19 N 18 sedimentation (Fig. 4e), associated with the northward flow- s = 6.9 s = 8.5 s = 5.6 s = 8.6 ing warm MC, coming from tropical regions (Mittelstaedt, 1991). In the Canary Current upwelling system, which is dominated by carbonate producers, particle settling rates are rather high (around 300 m d−1), compared to EBUEs domi- ganic carbon flux pattern (not shown, values from almost nated by BSi sedimentation (Fischer and Karakas, 2009; Fis- zero to 1.1 g m−2) did not follow the ocean colour data from cher et al., 2009a). As suggested by Fischer et al. (2009a), MODIS/SeaWiFS either. Peak chlorophyll values were ob- the relatively high organic carbon flux in the deep ocean off served mostly during spring, except in 1998 (autumn max- NW Africa may be due to the high availability of mineral imum) and 2007 (summer maximum). The MODIS ocean ballast, i.e. from coccolithophorids and fine-grained mineral

Biogeosciences, 13, 3071–3090, 2016 www.biogeosciences.net/13/3071/2016/ G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania 3081

Table 4. Summary of important flux changes between 1988 and 2012 which are related to large-scale climate modes such as NAO and ENSO. The record is divided into six major periods, including the outstanding year 2005 (see text).

Period/years 1988–1991 1997–1999 2001–2005/6 2005 2007–2010 2010–2012 El Niño – La Niña FORCING NAO decreasing increasing negative or neutral decreasing increasing neutral ENSO strongest weak ENSOs neutral ENSO FLUX RESPONSE BSi decreasing first episodic peaks High throughout, decreasing increasing decreasing, except spring then increasing Carbonate decreasing generally high, major episodic pteropod peaks pteropod peaks Lithogenic (dust) decreasing first decreasing, high throughout, decreasing increasing then increasing except spring dust (Iversen et al., 2010; Iversen and Ploug, 2010; Ploug et tation and, finally, the documentation of increasing fluxes in al., 2008). Direct evidence for the influence of the deposi- the deep traps in spring. tion of dust particles on the settling rates of larger particles From 2001 to 2006 when the winter NAO index became and the flux attenuation in the epi- and mesopelagic has been close to zero (Fig. 6c), the BSi flux showed rather unusual found on short timescales, i.e. days. This was observed dur- (episodic) peaks either in summer, autumn or in winter 2004– ing a severe dust outbreak in January 2012 (Iversen, unpub- 2005 (Fig. 6a, b). This suggests increasing coastal upwelling lished observations) by deploying drifting traps before and in summer and autumn (e.g. Cropper et al., 2014) and/or after the dust outbreak (Fig. 8a, insert image). a strengthening of the northward flowing and warmer MC, combined with an enhanced supply of a nutrient- and Si- 5.2 Influence of the NAO on biogenic silica richer source water (SACW instead of NACW). We favour sedimentation the latter scenario as there is evidence of unusual warm sur- face water conditions (SST anomalies of +3 ◦C) related to The NAO both affects coastal upwelling and productiv- weak trade wind intensity between 2002 and 2004 (Zeeberg ity off Mauritania through wind forcing (upwelling) and et al., 2008; Alheit et al., 2014). These conditions might have dust/nutrient supply (Chiapello et al., 2005), mainly during led to a stronger influence of the northward flowing MC and winter (DJFM) (Goudie and Middleton, 2001; Cropper et the silicate-richer SACW which mixes into the Cape Blanc al., 2014). Indeed, we observed an increase of both the win- upwelling filament and, thus, contributed to higher BSi pro- ter NAO index and associated winter BSi fluxes (Figs. 6, ductivity and sedimentation. Such a scenario was proposed 7a), the latter known to be indicative of coastal upwelling by Romero et al. (2008) to explain the extraordinary high strength and productivity. When plotting winter BSi fluxes content of BSi in Late Quaternary sediments deposited off versus the Azores pressure alone (DJFM Ponta Delgada SLP, Cape Blanc during Heinrich Event 1 and Younger Dryas fol- 2 = 1989–2002), the relationship improves slightly (R 0.19 lowing the Last Glacial Maximum. = N 11, not shown) but remains statistically insignificant. The 2004–2005 winter BSi flux clearly falls off the re- Since upwelling is wind-driven and large-scale wind patterns gression line of winter BSi flux versus the winter NAO in- in the study area are positively correlated to NAO variabil- dex (Fig. 7a). Exceptional conditions in 2005 are also indi- ity (Fig. 3a), a close linkage between a positive (negative) cated when plotting the area with high chlorophyll (> 1 mg NAO and higher (lower) BSi fluxes can be expected. Or- Chl a m−3) covered by the Cape Blanc filament (Fischer et ganic carbon flux showed less correspondence to the winter al., 2009b) versus the BSi fluxes (Fig. 7b). In general, a larger NAO index (not shown). No clear relationship can be seen (smaller) Cape Blanc filament area has been associated with between the winter (DJFM) NAO index and BSi and organic higher (lower) BSi fluxes (Fig. 7b) and also with higher total carbon fluxes later in spring, if we consider a time delay of a mass fluxes (not shown). However, in winter of 2004–2005 few weeks between wind forcing of coastal upwelling, high (a relatively cold season with negative SST anomalies), the chlorophyll standing stock, particle formation and sedimen- www.biogeosciences.net/13/3071/2016/ Biogeosciences, 13, 3071–3090, 2016 3082 G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania

CB-1 to CB-23 CB-1 to CB-23 upper and lower traps 2 25 (a) upper and lower traps

1.6 20 -2

-2 1.2

15 0.8

10 m (g flux BSi Seasonal 0.4 )

No S easonal total flux (g m(g flux total S easonal ) sampling (a) 0 5 1988 19901995 2000 2005 2010

No sampling 0 1988 19901995 2000 2005 2010 ñ )

-2 El Ni o/ ñ ñ El Ni o/ 15 ñ 1 La Ni a (b) La Ni a -2 10 0 5

0 -1

BSi flux anomalies (g m (b) -5 1988 19901995 2000 2005 2010 -10

S easonal flux anomalies (g m (g anomalies flux S easonal ) 6 -15 4 1988 19901995 2000 2005 2010 2 0 (c) -2 0.4 -4 (c) -6 NAO Index (DJFM) Hurell 0 1988 19901995 2000 2005

-3 3 SeaWiFs

-0.4

AMO Index (unsmoothed) 2

1 MODIS 1988 19901995 2000 2005 2010 (d)

Seasonal chl (mg m(mg chl Seasonal ) 0 Figure 5. (a) Total mass fluxes of the lower traps (grey-shaded). 1988 19901995 2000 2005 2010 Gaps were filled with upper trap data (light grey bars). Deviations Figure 6. (a) Seasonal flux of biogenic silica (BSi, green) with of the seasonal total mass fluxes from the long-term seasonal means gaps filled from the upper trap data (light green bars). Deviations of (anomalies), fitted with a 9-order polynomial (b). (c) Atlantic Multi- the long-term seasonal means (anomalies, b). (c) The NAO Hurrell decadal Oscillation (AMO) Index based on monthly SST fitted with index (DJFM). (d) Seasonal chlorophyll concentration both from a 9th-order polynomial fit (dashed blue line). The strong ENSO cy- the MODIS (light green) and the SeaWiFS (dark green) sensors cle 1997–1999 with a warm El Niño and a cold La Niña phase is at 9 km resolution. Note that high chlorophyll biomass generally indicated. occurs in spring but sometimes in summer/autumn as well (e.g. in 1998, 2007). SeaWiFS chlorophyll reveals a decreasing trend from 1997–2010, not mimicked in any flux data. The strong ENSO cy- filament area was smaller and chlorophyll standing stock was cle 1997–1999 with a warm El Niño and a cold La Niña phase is lower (Figs. 6d, 7b). Nevertheless, BSi fluxes were the high- indicated. est of the entire record. The seasonal variability of chloro- phyll from the entire SeaWiFS record (1997–2010, Fig. 6d) indicates no relationship between the chlorophyll standing fore, that the linear relationship between the NAO index and stock and deep ocean BSi flux (or organic carbon flux, not BSi fluxes may be biased in years of anomalous dust input shown). These observations point to additional regulators into the surface ocean. for organic carbon and BSi export to the deep sea. Ocean colour imagery even revealed a decreasing trend from 1997 5.3 Interaction between mineral dust and the to 2010 (Fig. 6d), which suggests a decrease in upwelling. biological pump This is not consistent with the “Bakun upwelling intensifica- tion hypothesis” (Bakun, 1990) nor with studies from Kahru Fischer and Karakas (2009) stated that particle settling rates and Mitchell (2008). Throughout 2005, however, the posi- and organic carbon fluxes in the Canary Current system tive BSi flux anomalies corresponded well with positive dust were unusually high compared to other EBUEs. This was flux anomalies (Figs. 6b, 8b). As seen from Aerosol Optical mainly attributed to particle loading by dust particles (see Thickness (AOT, Fig. 8c), dust availability was rather high in also Fischer et al., 2009a, b; Iversen et al., 2010). BSi and 2005 and corresponded to high dust sedimentation in summer lithogenic (mineral dust) fluxes point to a close linear rela- and autumn 2005 (Fig. 8; see Sect. 5.3). We suggest, there- tionship (R2 = 0.78, N = 21, Fig. 9a) mainly in winter where

Biogeosciences, 13, 3071–3090, 2016 www.biogeosciences.net/13/3071/2016/ G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania 3083

2 CB 1-23 18-19 Jan 2012

2005 10 1.6 CB-1 to CB-23 upper and lower traps -2

) 8 -2

1.2 1999 6 2007 1995 1989 2012 4 0.8 1997 2006 1990 2010 2008 winter BSi flux (g m (g flux BSi winter 2009 R²=0.14 20031991 N=20 2 Seasonal lithogenic flux (g m(g flux lithogenic Seasonal ) No 0.4 2001 1996 sampling (a) 0 2011 1988 19901995 2000 2005 2010 2000 200419982002 a ñ 0 El Ni o/ ñ -2 La Ni a -6 -4 -2 0 2 4 6 4 NAO Hurrell Index (DJFM) 2 2

0 2005 1.6 -2

) -4

-2 (b) R²=0.63 m (g anomalies flux Lithogenic ) 1999 1.2 N=10 1988 19901995 2000 2005 2010 2007 0.3

0.8 2006 0.2 2008 SeaWiFs MODIS winter BSi flux (g m (g flux BSi winter 2003 0.1 0.4 2001 (c) 0 2000 Seasonal AOT (unitless) 1988 19901995 2000 2005 2010 2002 19982004 b 0 2345 Figure 8. (a) Seasonal flux of lithogenic (= mineral dust) parti- winter (DJFM) filament size (x 105 km2) cles (orange) with gaps filled from the upper trap data (light or- Figure 7. (a) The NAO Hurrell index (DJFM, Hurrell, 1995) plotted ange bars). Deviations from the long-term seasonal means (anoma- against winter BSi fluxes from Fig. 6. Note the increase of BSi with lies, b). Note the large positive anomalies with longer duration in increasing NAO index. However, the relationship is weak due to 1988–1989, 1997–2000 and 2005–2006. From about 2000 to 2004– unusual sedimentation events in the years 1998–1999, 2002, 2004, 2005, lithogenic fluxes remain rather low. In 2005, dust sedimenta- and, in particular in 2005. When omitting the data point from 2005, tion and BSi flux (Fig. 6b) were high throughout the year. (c) The the correlation coefficient increases, but remains low (R2 = 0.14, AOT from the SeaWiFS (brown) and MODIS (light brown) sensors N = 20). Upper trap flux data from winter 1998 and 2004 may be shows repeatedly high values in summer, but not in winter when too low as the filament with elevated chlorophyll was small and dust sedimentation is highest in the study area. A typical short-term the particles did not reach the upper trap (see text). Omitting these (2-day) dust storm in January 2012 is shown as insert in the upper two data point would slightly improve the relationship. (b) The right. The strong ENSO cycle 1997–1999 with a warm El Niño and size of the Cape Blanc filament (Fischer et al., 2009b) during win- a cold La Niña phase is indicated. ter months (DJFM) versus winter BSi fluxes shows higher fluxes with larger filament size. When omitting the BSi flux from winter 2005, a statistically significant relationship between filament size et al., 2008; Alheit et al., 2014). These conditions might and fluxes is obtained (R2 = 0.63, N = 10). Years given in the figure have allowed the later wind-induced mobilization of larger denote the respective winter seasons (e.g. 1999 = December 1998– March 1999). amounts of dust particles into the atmosphere and led to a dust-enriched atmosphere during the entire year 2005, com- bined with elevated deep ocean mass fluxes. Typically, highest dust flux off Cape Blanc occurs in win- dust availability and deposition is high (Goudie and Middle- ter (Fig. 8) whereas part of the summer dust load is trans- ton, 2001), but not in summer (R2 = 0.56, N = 23, Fig. 9b). ported further westward and deposited in the Caribbean Sea High supply of dust into the surface ocean is often associ- (Goudie and Middleton, 2001; Prospero and Lamb, 2003). ated with dry conditions in the Sahel/Sahara in the previous However, the rainfall pattern exhibits elevated precipitation year (Engelstaedter et al., 2006; Prospero and Lamb, 2003; in summer and autumn 2005 when the tropical rainbelt was Moulin and Chiapello, 2004). Indeed, the interval 2002–2004 far north; this might have led to unusual wet deposition of in particular is known to have been much warmer and drier dust in summer over our study site (Friese et al., 2016). As during summer/autumn on land and in the ocean (Zeeberg shown earlier, BSi fluxes show positive anomalies in sum- www.biogeosciences.net/13/3071/2016/ Biogeosciences, 13, 3071–3090, 2016 3084 G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania

20 20 CB 1-23 CB-1 to CB-23 upper and lower traps -2 16 16

-2 12

12 8

R²=0.78 N=21 4

8 m (g flux carbonate S easonal ) No 2005 sampling (a) 0 W inter lithogenic flux (g m (g flux lithogenic W inter ) 1988 19901995 2000 2005 2010 4 ñ El Ni o/

-2 ñ 8 La Ni a (a) 0 4 0 0.4 0.8 1.2 1.6 2 0 W inter BSi flux (g m-2 )

20 -4

-8 (b) Carbonate flux anomalies (g m (g anomalies flux Carbonate ) 16 1988 19901995 2000 2005 2010 -2 15

-2 su 2002 12 10 su 1998 su 2004 8 2005 R²=0.56 5 N=23 Summer lithogenic flux (g m(g flux lithogenic Summer )

4 m(g flux pteropod S easonal ) (c) 0 1988 19901995 2000 2005 2010 (b) 0 Figure 10. (a) Seasonal flux of total carbonate (blue) with gaps 0 0.4 0.8 1.2 1.6 2 Summer BSi flux (g m-2 ) filled from the upper trap data (light blue bars). Deviations from the long-term seasonal means (anomalies, b). (c) Seasonal flux of Figure 9. Relationships between BSi and lithogenic (= mineral pteropods. During the strongest ENSO cycle 1997–2000, longer pe- dust) fluxes for the winter (a) and summer (b) seasons. Note the riods of low and high carbonate fluxes occurred. Note the episodic high correspondence in winter (R2 = 0.78, N = 21); a lower coeffi- sedimentation pattern of pteropods with maxima e.g. in sum- cient is found for the summer season. During the outstanding year mer 1998, 2002 and 2004. The strong ENSO cycle 1997–1999 with of 2005 (see Fig. 7), both points for winter and summer are close to a warm El Niño and a cold La Niña phase is indicated. the linear regression line.

biological pump produces sufficient organic particles to al- mer and autumn 2005 (Fig. 6b), pointing to a stronger dust- low the formation of larger particles which then settle into the influenced biological pump. deep ocean (Bory et al., 2002; Ternon et al., 2010; Nowald In contrast to BSi, winter sedimentation of mineral dust et al., 2015). Cape Blanc dust particles have predominant did not show any common trend with the winter NAO in- grain sizes between 10 and 20 µm (Ratmeyer et al., 1999a, dex (not shown). Using satellite-derived AOT, Chiapello et b; Friese et al., 2016) and, thus, would sink too slowly to al. (2005) suggested a close relationship of atmospheric dust build a deep ocean flux signal. We propose that only the content and the NAO index. High AOT, however, does not close coupling between the organic carbon pump, dust par- necessarily correspond with high dust deposition into the ticles and the formation of dense and larger particles led to ocean. Moreover, dust deposition into the ocean surface does elevated export and sedimentation (Bory et al., 2002; Fis- not unavoidably and directly result in particle export and cher et al., 2009a; Fischer and Karakas, 2009). Thunell et transfer to the deep ocean. Dust deposition is not only con- al. (2007) found that organic carbon fluxes strongly corre- trolled by wind strength and direction in the trap area but also lated with mineral fluxes in other upwelling-dominated con- by source region conditions and precipitation over the trap tinental margin time series such as the Santa Barbara Basin site. Consequently, considering the NAO as the only control- located within the California Current System. However, the ling factor for dust deposition and sedimentation even if the detailed processes and interaction between different groups correlation between SLP (and thus winds) and NAO is strong of phytoplankton and types of ballast minerals (e.g. quartz in the study area (Fig. 3a), would be an oversimplification. versus clay minerals etc.) are largely unknown and need clar- Another explanation for the missing relationship could be ification. Laboratory experiments with different ballast min- that fine-grained dust accumulates in surface waters until the erals (e.g. Iversen and Roberts, 2015) and measurements of

Biogeosciences, 13, 3071–3090, 2016 www.biogeosciences.net/13/3071/2016/ G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania 3085 organic carbon respiration and particle settling rates suggest riod of near-neutral NAO together with an almost permanent a significant influence of ballast minerals on particle settling El Niño phase during 2002–2004 might have acted in con- rates, carbon respiration and flux (Ploug et al., 2008; Iversen cert towards weakening trade winds which allows a stronger and Ploug, 2010). In a time series study with optical mea- influence of the warm and northward flowing MC, supplying surements, addressing particle characteristics (e.g. sizes) and high amounts of pteropods from tropical waters. In summary, using fluxes at the nearby eutrophic sediment trap off Cape ENSO may impact differently on different flux components. Blanc (CBeu), Nowald et al. (2015) suggested an influence Whereas an increase in pteropod fluxes is found during the of dust outbreaks on particle sedimentation down to 1200 m. El Niño phase, La Niña induces an increase in total carbonate Interestingly, settling organic-rich particles off Cape Blanc flux. were only around 1 mm in size during the two-year deploy- ment from 2008 to 2010 (Nowald et al., 2015). Higher fluxes 5.5 Decadal variability and potential trends in mass were mostly attributed to higher numbers of small particles fluxes rather than to larger particle sizes during blooms in the Cape Blanc area (Nowald et al., 2015). Our records allow a first estimate of deep-ocean mass flux variations beyond seasonal-to-interannual timescales. The 5.4 Carbonate fluxes and potential ENSO “Bakun upwelling intensification hypothesis” (Bakun, 1990) teleconnections has been supported by other studies using long-term SSTs, wind stress records or upwelling indices (e.g. Cropper et al., Deep ocean total mass and carbonate fluxes (Figs. 5, 10) 2014; Narayan et al., 2010). Kahru and Mitchell (2008) ap- showed elevated values over more than a year from sum- plied satellite derived chlorophyll time series from SeaWiFS mer 1998 to autumn 1999 during a La Niña event, whereas to conclude that chlorophyll standing stock in major up- BSi and dust fluxes showed positive anomalies of shorter du- welling regions of the world oceans had increased since ration (autumn 1998 to spring 1999) (Fig. 6b). Investigat- September 1997. However, these records are rather short ing SeaWiFS-derived ocean colour in the Mauritanian up- (1997–2006) and started in an unusual period with the welling region, Pradhan et al. (2006) obtained a link between strongest ENSO ever reported (1997–1998). In our record, the multivariate ENSO index, the strength of upwelling and no long-term trend in any mass flux component from 1988 the chlorophyll standing stock (250 % increase) during the through 2012 is seen, which would indicate a long term in- 1998–1999 La Niña. They also observed that during the ma- crease or decrease in the strength of coastal upwelling off ture La Niña phase in the Pacific Ocean, NW African trade Cape Blanc. The 1997–2010 chlorophyll time series from winds increased in winter–spring. Coincidentally, Helmke et SeaWiFS (Fig. 6d) shows a decreasing standing stock, which al. (2005) obtained a more than doubling of the deep ocean might indicate a decrease in the strength of coastal upwelling organic carbon fluxes in autumn 1998 to summer 1999 dur- in the Cape Blanc area. The upwelling indices used by Crop- ing the major La Niña phase. per et al. (2014) showed a decreasing trend from 1980–2013 We obtained positive carbonate flux anomalies with a for the Mauritanian-Senegalese upwelling zone (12–19◦ N), longer duration in summer 1998 to autumn 1999 and sum- while observing some interdecadal variability. All these ob- mer 2005 to spring 2006 (Fig. 10b). During autumn 1998 servations together point to regional differences within the (La Niña phase), the area of the Cape Blanc filament was un- upwelling system along the NW African coast (Cropper et usually large compared with autumn 1997 (El Niño phase) al., 2014) with respect to long-term trends in upwelling (Fig. 1d, e). The contribution of major carbonate producers and chlorophyll standing stock. According to these findings, to total carbonate flux varied both on seasonal and interan- only the southernmost weak permanent upwelling zone (21– nual timescales (Fischer et al., 2009a). These authors ob- 26◦ N) would be in concert with the “Bakun upwelling in- served that nannofossils contributed almost 95 % to carbon- tensification hypothesis”. Another implication is that trends ate sedimentation in 1991 (a relatively cold year) but only detected from near-surface data/indices are not necessarily 64 % in 1989 (a relatively warm year). In the long term, nan- reflected in changes of deep-ocean mass fluxes and organic nofossils showed a rather low seasonality. Among the cal- carbon sequestration. No evidence of decreasing dust fluxes careous microorganisms, pteropods had the strongest sea- from the Sahara/Sahel is seen in our lithogenic (dust) flux sonal signal which did not quite match the pattern of carbon- record (Fig. 8a), which might indicate “Saharan greening” ate flux (Fig. 10a, c). As previously observed by Kalberer and reduced dust plumes during the past 2 decades (Zhao et al. (1993), a possible explanation is the high pteropod et al., 2010; Fontaine et al. 2011). Thus, mass flux patterns flux (mostly Limacina inflata) in summer 1989 due to un- might be partly independent from chlorophyll standing stock usual high SSTs. In our record, we found distinct pulses of or the size of the Cape Blanc filament. pteropods in the summer seasons of 1998, 2002 and 2004 Long-term model simulations under present-climate (Fig. 10c). In particular, the peaks in 2002 and 2004 can boundary conditions allow to study the linkages within the be attributed to anomalously warm conditions in the study climate system on decadal timescales and beyond. Climate area (Zeeberg et al., 2008; Alheit et al., 2014). Here, a pe- modes such as the AMO are operating in this frequency www.biogeosciences.net/13/3071/2016/ Biogeosciences, 13, 3071–3090, 2016 3086 G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania band, and a correlation between large-scale patterns of SLP In particular, ENSO might confound the relationship be- and North Atlantic SST (AMO) index (both lowpass-filtered tween the NAO and BSi fluxes. for periods above 10 years, Fig. 3c) suggests that even on these long timescales, climate modes such as the AMO 6. Fluxes from 1988–2012 point to a long-term decadal might impact on climate variables such as SLP, SST and variability, probably related to the Atlantic Multi- wind patterns, specifically through a weakening of the trade decadal Oscillation. However, the time series record is winds over the eastern Atlantic during the AMO warm phase too short to reproduce AMO cycles with statistical ro- (Fig. 3c). This response of the winds to low-frequency SST bustness. variations is consistent with earlier findings on interdecadal 7. No long-term trend of any flux component was observed Atlantic SST variability (Kushnir, 1994; Alexander et al., in the Mauritanian upwelling off Cape Blanc and there- 2014), and could influence the main characteristics of parti- fore does not support the “Bakun upwelling intensifica- cle fluxes at our study site (Fig. 5). However, as current parti- tion hypothesis” (Bakun, 1990; Cropper et al., 2014). cle flux records from sediment traps only cover a few decades and cannot resolve AMO cycles with statistical robustness, 8. We found no evidence of an increasing/decreasing sup- continuation of trap experiments are essential to capture all ply of dust and its deposition off Mauritania between relevant timescale variations. They will help to understand 1988 and 2012. modern particle settling rates and the interpretation of ma- rine sediment records used in paleoclimate reconstructions. The long-term flux record allows insights into the influ- ences of major climatic oscillations such as the NAO and on particle export and transfer of particles to the deep ocean 6 Summary and conclusions and might help to evaluate how the ecosystem off Mauritania could develop in the future. We have some indications that In our study, we presented a sediment trap record from the the relationships between major Northern Hemisphere cli- Eastern Boundary Upwelling Ecosystem area off Cape Blanc mate oscillations (e.g. the NAO) and deep ocean mass fluxes (Mauritania) for the period 1988–2012. Our major findings are weakened by short-term ecosystem perturbations, e.g. can be summarized as follows (also see Table 4): due to dust outbreaks, the latter probably leading to episodic 1. Winter BSi fluxes showed a trend of increasing values sedimentation pulses into the deep ocean. The complex pro- with an increasing NAO Hurrell Index and the increas- cesses of the interaction of non-biogenic particles (e.g. differ- ing Azores SLP as well. However, both relationships are ent minerals within dust, e.g. Iversen and Roberts, 2015) with statistically insignificant. organic materials produced by photosynthesis, aggregate for- mation and disintegration in the epi- and mesopelagic, parti- 2. Episodic BSi flux peaks occurred between 2000 and cle characteristics (e.g. Nowald et al., 2015), settling rates 2005 when the NAO was neutral or negative. Dust out- and remineralization require further process studies, com- breaks, followed by dry (winter) and wet (summer) de- bined with laboratory experiments and different modelling position (e.g. in 2005) into the ocean, might have modi- approaches (e.g. particle (dis-) aggregation, Karakas et al., fied the efficiency of the biological pump and resulted in 2009). increased downward fluxes (e.g. of BSi or organic car- Additionally, our record provides information on poten- bon) which were not related to any large-scale forcings. tial long-term changes or trends of mass fluxes which point to ecosystem changes or an intensification/weakening of the 3. Only the extreme 1997–1999 ENSO was documented NW African upwelling system in the study area. Considering clearly in the record, with low fluxes for almost a year the present record of bulk fluxes of more than 2 decades, we during the warm El Niño phase, followed by high fluxes have no indication of any long-term trend which might sug- of almost a year during the following cold La Niña gest a fundamental ecosystem change or a regime shift (step- phase. wise change) in this important coastal upwelling ecosystem. 4. In addition to episodic BSi fluxes, episodic peaks of pteropods occurred in the summers 2002 and 2004 Data availability (Fig. 10c, Table 4). This occurred during a neutral NAO phase and weakening trade winds, allowing a stronger Text data availalbility The flux data shown in the tables of influence from tropical surface waters from the south the manuscript and the sediment trap metadata will be made via the Mauritanian Current (MC) and an entrainment available under: www.pangaea.de. of Si-richer subsurface waters.

5. Teleconnections from ENSO and the NAO may have Author contributions. Gerhard Fischer prepared the ms with contri- opposite effects on the NW African upwelling (Fig. 3) butions from the co-authors. Oscar Romero investigated the diatom with potential implications for deep ocean mass fluxes. producers and contributed to the discussion, Ute Merkel contributed

Biogeosciences, 13, 3071–3090, 2016 www.biogeosciences.net/13/3071/2016/ G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania 3087 the model simulations and the analysis, Barbara Donner studied Ballesteros, S., Basterretxea, G., Escánez, J., García-Weill, L., the carbonate producers, Morten Iversen and his group did the Hernández-Guerra, A., López-Laatzen, F., Molina, P., Montero, dust experiments and provided unpublished results/observations, M. F ., Navarro-Pérez, E., Rodriguez, J. M., van Lenning, K., Nico Nowald and Volker Ratmeyer performed the optical obser- Vélez, H., and Wild, K.: Eastern Boundary of the North Atlantic: vations and analysis of particles, Götz Ruhland and Marco Klann northwest Africa and Iberia, in: The Global Coastal Ocean, Vol. designed the sediment trap experiments and analysed the sediment 11, edited by: Robinson, A. R. and Brink, K., John Wiley and trap samples, Gerold Wefer planned the entire program and con- Sons, New York, Chichester, Weinheim, Brisbane, Singapore, tributed to the discussion. Toronto, 29–67, 1998. Behrenfeld, M. J. and Falkowski, P. G.: Photosynthetic rates de- rived from satellite based chlorophyll concentration, Limnol. Oceanogr., 42, 1–20, 1997. Acknowledgements. We are greatly indebted to the masters and Behrenfeld, M. J., Randerson, J. T., McClain, C. R., Feldman, G. C., crews of many expeditions (Table 1). Many thanks also go to Los, S. O., Tucker, C. J., Falkowski, P., Field, C. B., Frouin, R., the chief scientists of the expeditions for their support during Esaias, W. E., Kolber, D. D., and Pollack, N. H.: Biospheric pri- the cruises and for the planning activities and cooperations. mary production during an ENSO transition, Science, 291, 2594– We also would like to thank the Mauritanian, Moroccan and 2597, 2001. German authorities for their help during the planning phases of the Bode, A., Alvarez-Ossorio, M. T., Cabanas, J. M., Miranda, A., and expeditions. This work was only possible because of the long-term Varela, M.: Recent trends in plankton and upwelling intensity off funding by the Deutsche Forschungsgemeinschaft through the SFB Galicia (NW Spain), Prog. Oceanogr., 83, 342–350, 2009. 261 (The South Atlantic in the Late Quaternary: Reconstruction of Bory, A. J. M. and Newton, P. P.: Transport of airborne lithogenic Mass Budget and Current Systems, 1989–2001) and the Research material down through the water column in two contrasting re- Center Ocean Margins. During about the last decade, the study is gions of the eastern subtropical North Atlantic Ocean, Global supported by the Marum Excellence Cluster, “The Ocean in the Biogeochem. Cy., 14, 297–315, 2000. Earth System”. The model simulation done by Ute Merkel has been Bory, A., Dulac, F., Moulin, C., Chiapello, I., Newton, P. P., Guelle, performed at the supercomputer of the Norddeutscher Verbund für W., Lambert, C. E., and Bergametti, G.: Atmospheric and oceanic Hoch- und Höchstleistungsrechnen (HLRN), Hannover, Germany. dust fluxes in the northeastern tropical Atlantic ocean: how close We thank the two anonymous reviewers for helpful, fair and a coupling?, Ann. Geophys., 20, 2067–2076, 2002. constructive comments and the associate editor for handling the Bressac, M., Guieu, C., Doxaran, D., Bourrin, F., Desboeufs, manuscript. K., Leblond, N., and Ridame, C.: Quantification of the lithogenic carbon pump following a simulated dust-deposition The article processing charges for this open-access event in large mesocosms, Biogeosciences, 11, 1007–1020, publication were covered by the University of Bremen. doi:10.5194/bg-11-1007-2014, 2014. Brust, J., Schulz-Bull, D. E., Leipe, T., Chavagnac, V., and Waniek, Edited by: J. Middelburg J. J.: Descending particles: from the atmosphere to the deep ocean: A time series study in the subtropical NE Atlantic, Geo- phys. Res. Lett., 38, L06603. doi:10.1029/2010GL045399, 2011. References Buesseler, K. O., Antia, A. A., Chen, M., Fowler, S. W., Gardner, W. D., Gustafsson, O., Harada, K., Michaels, A. F., Rutgers van Alexander, M. A., Kilbourne, K. H., and Nye, J. A.: Climate vari- der Loeff, M., Sarin, M., Steinberg, D. K., and Trull, T.: An as- ability during warm and cold phases of the Atlantic Multidecadal sessment of the use of sediment traps for estimating upper ocean Oscillation (AMO) 1871–2008, J. Marine Syst., 133, 14–26, particle fluxes, J. Mar. Res., 65, 345–416, 2007. 2014. Carr, M.-E.: Estimation of potential productivity in Eastern Bound- Alheit, J., Licandro, P., Coombs, S., Garcia, A., Giráldez, A., San- ary Currents using remote sensing, Deep-Sea Res. Pt. I, 49, 59– tamaría, M. T. G., Slotte, A., and Tsikliras, A. C.: Atlantic Multi- 80, 2002. decadal Oscillation (AMO) modulates dynamics of small pelagic Checkley Jr., D. M. and Barth, J. A.: Patterns and processes in the fishes and ecosystem regime shifts in the eastern North and Cen- California Current Systems, Prog. Oceanogr., 83, 49–64, 2009. tral Atlantic, J. Mar. Syst., 133, 88–102, 2014. Chiapello, I., Moulin, C., and Prospero, J. M.: Understand- Arístegui, J., Barton, E. C., Álvarez-Salgado, X. A., Santos, A. M. ing the long-term variability of African dust transport across P., Figueiras, F. G., Kifani, S., Hernández-León, S., Mason, E., the Atlantic as recorded in both Barbados surface concen- Machú,E., and Demarcq, H.: Sub-regional ecosystem variability trations and large-scale Total Ozone Mapping Spectrometer in the Canary Current upwelling, Prog. Oceanogr., 83, 33–48, (TOMS) optical thickness, J. Geophys. Res., 110, D18S10, 2009. doi:10.1029/2004JD005132, 2005. Armstrong, R. A., Lee, C., Hedges, J. I., Honjo, S., and Wakeham, Collins, W. D., Bitz, C. M., Blackmon, M. L. Bonan, G. B., Brether- S. G.: A new mechanistic model of organic carbon fluxes in the ton, C. S., Carton, J. A., Chang, P., Doney, S. C., Hack, J. J., Hen- ocean based on the quantitative association of POC with ballast derson, T. B., Kiehl, J. T., Large, W. G., McKenna, D. S., Santer, minerals, Deep-Sea Res. Pt. II, 49, 219–236, 2002. B. D., and Smith, R. D.: The Community Climate System Model Bakun, A.: Global climate change and intensification of coastal Version (CCSM3), J. Climate, 19, 2122–2143, 2006. ocean upwelling, Science, 247, 198–201, 1990. Barton, E. D., Arístegui, J., Tett, P., Cantón, M., García-Braun, J., Hernández-León, S., Nykjaer, L., Almeida, C., Almunia, J., www.biogeosciences.net/13/3071/2016/ Biogeosciences, 13, 3071–3090, 2016 3088 G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania

Cropper, T. E., Hanna, E., and Bigg, G. R.: Spatial and temporal circulation over North Africa (1979–2007), Int. J. Climatol., 31, seasonal trends in coastal upwelling off Northwest Africa, 1981– 633–648, 2011. 2012, Deep-Sea Res. Pt. II, 86, 94–111, 2014. Fréon, P., Barange, M., and Aristegui, J.: Eastern Boundary Up- Diatta, S. and Fink, A. H.: Statistical relationship between remote welling Ecosystems: integrative and comparative approaches, climate indices and West African monsoon variability, Int. J. Cli- Prog. Oceanogr., 83, 1–14, 2009. matol., 34, 3348–3367, 2014. Friese, C., van der Does, M., Merkel, U., Iversen, M., Fischer, G., Ducklow, H. W., Doney, S. C., and Steinberg, D. K.: Contribu- and Stuut, J.-B.: Environmental factors controlling the seasonal tions of long-term research and teim-sereis observations to ma- variation in particle size of modern Saharan dust deposited off- rine ecology and biogeochemistry, Annu. Rev. Mar. Sci., 1, 279– shore Cape, Blanc, Aeolian Res., in press, 2016. 302, 2009. Goudie, A. S. and Middleton, N. J.: Saharan dust storms: nature and Dunne, J. P., Sarmiento, J. L., and Gnanadesikan, A.: A synthesis of consequences, Earth Sci. Rev., 56, 179–204, 2001. global particle export from the surface ocean and cycling through Hedges, J. I., Baldock, J. A., Gelinas, Y., Lee, C., Peterson, M. L., the ocean interior and on the seafloor, Global Biogeochem. Cy., and Wakeham, S. G.: The biochemical and elemental composi- 21, GB4006, doi:10.1029/2006GB002907, 2007. tions of marine plankton: a NMR perspective, Mar. Chem., 78, Engelstaedter, S., Tegen I., and Washington, R.: North African dust 47–63, 2002. emissions and transport, Earth Sci. Rev., 79, 73–100, 2006. Helmke, P., Romero, O., and Fischer, G.: Northwest African Feldman, G.: NASA/Goddard Flight Space Center, Ocean Color upwelling and its effect on off-shore organic carbon ex- Web, http://oceancolor.gsfc.nasa.gov/cgi/l3?ctg=Standard&sen= port to the deep sea, Global Biogeochem. Cy., 19, GB4015, A&prd=CHL_chlor_a&per=SN&date=21Jun2002&res= doi:10.1029/2004GB002265, 2005. 9km&num=24, 2015. Hsu, N. C., Gautam, R., Sayer, A. M., Bettenhausen, C., Li, C., Feldman, G.: NASA/Goddard Flight Space Center, Ocean Color Jeong, M. J., Tsay, S.-C., and Holben, B. N.: Global and re- Web, http://oceancolor.gsfc.nasa.gov/cgi/l3/S19972641997354. gional trends of aerosol optical depth over land and ocean us- L3m_SNAU_CHL_chlor_a_9km.png?, 2015. ing SeaWiFS measurements from 1997 to 2010, Atmos. Chem. Fischer, G. and Karakas, G.: Sinking rates and ballast composi- Phys., 12, 8037–8053, doi:10.5194/acp-12-8037-2012, 2012. tion of particles in the Atlantic Ocean: implications for the or- Hurrell, J. W.: NAO Index Data provided by the Cli- ganic carbon fluxes to the deep ocean, Biogeosciences, 6, 85– mate Analysis Section, NCAR, Boulder, USA, Up- 102, doi:10.5194/bg-6-85-2009, 2009. dated regularly, http://climatedataguide.ucar.edu/guidance/ Fischer, G. and Wefer, G.: Sampling, preparation and analysis of hurrell-north-atlantic-oscillation-nao-index-station-based (last marine particulate matter, in: The Analysis and Characterization access: 01 January 2012), 1995. of Marine Particles, edited by: Hurd, D. C. and Spencer, D. W., Hurrell, J.: National Center for Atmospheric Re- Geophys. Monogr. Serie., 63, 391–397, 1991. search, NCAR/UCAR Climate Data Guide, Fischer, G., Donner, B., Ratmeyer, V., Davenport, R., and Wefer, https://climatedataguide.ucar.edu/climate-data/ G.: Distinct year-to-year particle flux variations off Cape Blanc hurrell-north-atlantic-oscillation-nao-index-station-based, during 1988–1991: Relation to delta δ18O-deduced sea-surface 2015. temperatures and trade winds, J. Mar. Res., 54, 73–98, 1996. Ichoku, C.: NASA/Goddard Flight Space Center, NASA Earth Fischer, G., Wefer, G., Romero, O., Dittert, N., Ratmeyer, V., and Observatory, Global Maps, http://earthobservatory.nasa.gov/ Donner, B.: Transfer of particles into the deep Atlantic and the GlobalMaps/?eocn=topnav&eoci=globalmaps, 2015. global Ocean: control of nutrient supply and ballast production, Iversen, M. H., and Ploug, H.: Ballast minerals and the sinking in: The South Atlantic in the Late Quaternary: Reconstruction carbon flux in the ocean: carbon-specific respiration rates and of material budget and current systems, edited by: Wefer, G., sinking velocities of marine snow aggregates, Biogeosciences 7, Mulitza, S., and Ratmeyer, V., Springer, Berlin, Heidelberg, New 2613–2624, doi:10.5194/bg-7-2613-2010, 2010. York, 21–46, 2003. Iversen, M. H. and Robert, M. L.: Ballasting effects of smectite on Fischer, G., Karakas, G., Blaas, M., Ratmeyer, V., Nowald, N., aggregate formation and export from a natural plankton commu- Schlitzer, R., Helmke, P., Davenport, R., Donner, B., Neuer, S., nity, Mar. Chem., 175, 18–27, 2015. and Wefer, G.: Mineral ballast and particle settling rates in the Iversen, M. H., Nowald, N., Ploug, H., Jackson, G. A., and Fischer, coastal upwelling system off NW Africa and the South Atlantic, G.: High resolution profiles of vertical particulate organic matter 98, 281–298, 2009a. export off Cape Blanc, Mauritania: degradation processes and Fischer, G., Reuter, C., Karakas, G., Nowald, N., and Wefer, G.: ballasting effects, Deep-Sea Res. Pt. I, 57, 771–784, 2010. Offshore advection of particles within the Cape Blanc filament, Jickells, T. D., An, Z. S., Andersen, K. K., Baker, A. R., Bergametti, Mauritania: Results from observational and modelling studies, G., Brooks, N., Cao, J. J., Boyd, P. W., Duce, R. A., Hunter, K. Prog. Oceanogr., 83, 322–330, 2009b. A., Kawahata, H., Kubilay, N., laRoche, J., Liss, P. S., Mahowald, Fischer, G., Neuer, S., Davenport, R., Romero, O., Ratmeyer, V., N., Prospero, J. M., Ridgwell, A. J., Tegen, I., and Torres, R.: Donner, B., Freudenthal, T., Meggers, H., and Wefer, G.: The Global iron connections between desert dust, ocean biogeochem- Northwest African Margin, in: Carbon and Nutrient Fluxes in istry, and climate, Science, 308, 67–71, 2005. Continental Margins: A Global Synthesis, edited by: Liu, K. K., Kahru, M. and Mitchell, B. G.: Ocean colour reveals increased Atkinson, L., Quinones, R., and Talaue-McManaus, L., IGBP blooms in various parts of the world ocean, EOS, 89, p. 170, Book Series, Springer, Berlin, 77–103, 2010. 2008. Fontaine, B., Roucou, P., Gaetani, M., and Marteau, R.: Recent changes in precipitation, ITCZ convection and northern tropical

Biogeosciences, 13, 3071–3090, 2016 www.biogeosciences.net/13/3071/2016/ G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania 3089

Kalberer, M., Fischer, G., Pätzold, J., Donner, B., Segl, M., and Muller-Karger, F. E., Varela, R., Thunell, R., Luerssen, R., Hu, Wefer, G.: Seasonal sedimentation and stable isotope records of C., and Walsh, J. J.: The importance of continental margins pteropods off Cape Blanc, Mar. Geol., 113, 305–320, 1993. in the global carbon cycle, Geophys. Res. Lett., 32, L01602, Karakas, G., Nowald, N., Blaas, M., Marchesiello, P., Frickenhaus, doi:10.1029/2004GL021346, 2005. S., and Schlitzer, R.: High-resolution modeling of sediment ero- Narayan, N., Paul, A., Mulitza, S., and Schulz, M.: Trends in coastal sion and particle transport across the northwest African shelf, J. upwelling intensity during the late 20th century, Ocean Sci., 6, Geophys. Res., 111. C06025, doi:10.1029/2005JC003296, 2006. 815–823, doi:10.5194/os-6-815-2010, 2010. Karakas, G., Nowald, N., Schäfer-Neth, C., Iversen, M. H., Bark- Neuer, S., Torres-Padron, M.E., Gelado-Caballeo, M.D., Rueda, mann, W., Fischer, G., Marchesiello, P., and Schlitzer, R.: Impact M.J., Hernandez-Brito, J., Davenport, R., and Wefer, G.: Dust of particle aggregation on vertical fluxes of organic matter, Prog. deposition to the eastern subtropical North Atlantic gyre: Does Oceanogr., 83, 331–341, 2009. ocean’s biogeochemistry respond?, Global Biogeochem. Cy., 18, Kempler, S. J.: NASA/Goddard Flight Space Center, GIOVANNI, GB4020, doi:10.1029/2004GB002228, 2004. http://giovanni.sci.gsfc.nasa.gov/giovanni/?instance_id=ocean_ Nicholson, S. E.: The West African Sahel: A review of recent stud- month, 2015. ies on the rainfall regime and its interannual variability, ISRN Kaufman, Y. J., Koren, I., Remer, L. A., Tanré, D., Ginoux, P., and Meteorology, 2013, 453521, doi:10.1155/2013/453521, 2013. Fan, S.: Dust transport and deposition from the Terra-Moderate Nowald, N., Iversen, M. H., Fischer, G., Ratmeyer, V., and Wefer, Resolution Imaging Spectroradiometer (MODIS) spacecraft G.: Time series of in-situ particle properties and sediment trap over the Atlantic Ocean, J. Geophys. Res., 110, D10S12, fluxes in the coastal upwelling filament off Cape Blanc, Maurita- doi:10.1029/2003JD004436, 2005. nia, Prog. Oceanogr. Pt. A, 137, 1–11, 2015. Klaas, C. and Archer, D. E.: Association of sinking organic mat- N’Tchayi Mbourou, G., Berrand, J. J., and Nicholson, S. E.: The ter with various types of ballast in the deep sea: Implica- diurnal and seasonal cycles of wind-borne dust over Africa north tions for the rain ratio, Global Biogeochem. Cy., 16, 1116, of the equator, J. Appl. Meteorol., 36, 868–882, 1997. doi:10.1029/2001GB001765, 2002. Nykjaer, L. and Van Camp, L.: Seasonal and interannual variability Kremling, K., Lentz, U., Zeitzschell, B., Schulz-Bull, D. E., and of coastal upwelling along northwest Africa and Portugal from, Duinker, J. C.: New type of time-series sediment trap for the 1981 to 1991, J. Geophys. Res., 99, 197–207, 1994. reliable collection of inorganic and organic trace chemical sub- Pauly, D. and Christensen, V.: Primary production required to sus- stances, Rev. Scient. Instr., 67, 4360–4363, 1996. tain global fisheries, Nature, 374, 255–257, 1995. Kushnir, Y.: Interdecadal variations in North Atlantic sea surface Ploug, H., Iversen, M. H., and Fischer, G.: Ballast, sinking veloc- temperature and associated atmospheric conditions, J. Climate, ity, and apparent diffusivity within marine snow and zooplankton 7, 141–157, 1994. fecal pellets: implications for substrate turnover by attached bac- Lathuilière, C., Echevin, V., and Levy, M.: Seasonal and in- teria, Limnol. Oceanogr., 53, 1878–1886, 2008. traseasonal surface chlorophyll-a variability along the north- Pradhan, Y., Lavender, S. J., Hardman-Mountford, N. J., and Aiken, west African coast, J. Geophys. Res.-Ocean., 13, C05007, J.: Seasonal and inter-annual variability of chlorophyll-a concen- doi:10.1029/2007JC004433, 2008. tration in the Mauritanian upwelling: observation of an anoma- Lucio, P. S., Baldicero Molion, L. C., de Avial-Valadão, C. E., lous event during 1998–1999, Deep-Sea Res. Pt. II, 53, 1548– Conde, F. C., Malheiro Ramos, A., and Dias de Melo, M. L.: 1559, 2006. Dynamical outlines of the rainfall variability and the ITCZ role Prospero, J. M.: Mineral-aerosol transport to the North Atlantic and over the West Sahel, Atmos. Clim. Sci., 2, 337–350, 2012. North Pacific: The impact of African and Asian sources, in: The Marcello, J., Hernandez-Guerra, A., Eugenio, F., and Fonte, A.,: long range atmospheric transport of natural and contaminant sub- Seasonal and temporal study of the northwest African upwelling stances, edited by: Knap, A. H., Dordrecht, Mathematical and system, Int. J. Remote Sens., 32, 1843–1859, 2011. Physical Sciences, Kluwer Academic Publishers, 19–52, 1990. McGregor, H. V., Dima, M., Fischer, H. W., and Mulitza, S.: Rapid Prospero, J. M. and Lamb, P. J.: African droughts and dust trans- 20th century increase in coastal upwelling off Northwest Africa, port to the Caribbean: climate change implications, Science, 302, Science, 315, 637–639, 2007. 1024–1027, 2003. Meunier, T., Barton, E. D., Barreiro, B., and Torres, R.: Ratmeyer, V., Fischer, G., and Wefer, G.: Lithogenic particle fluxes Upwelling filaments off Cape Blanc: interaction of the and grain size distributions in the deep ocean off northwest NW African upwelling current and the Cape Verde frontal Africa: Implications for seasonal changes of aeolian dust input zone eddy field?, J. Geophys. Res.-Ocean., 117, C08031, and downward transport, Deep-Sea Res. Pt. II, 46, 1289–1337, doi:10.1029/2012JC007905, 2012. 1999a. Mittelstaedt, E.: The ocean boundary along the northwest African Ratmeyer, V., Balzer, W., Bergametti, G., Chiapello, I., Fischer, coast: Circulation and oceanographic properties at the sea sur- G., and Wyputta, U.: Seasonal impact of mineral dust on deep- face, Prog. Oceanogr., 26, 307–355, 1991. ocean particle flux in the eastern subtropical Atlantic Ocean, Mar. Moulin, C. and Chiapello, I.: Evidence of the control of summer Geol., 159, 241–252, 1999b. atmospheric tansport of African dust over the Atlantic by Sahel Redfield, A. C., Ketchum, B. H., and Richards, F. A.: The influence sources from TOMS satellites (1979–2000), Geophys. Res. Lett., of organisms on the composition of seawater, in: The Sea, edited 31, L02107, doi:10.1029/2003GL019031, 2004. by: Hill, M. N., Wiley and Sons, Chichester, 2, 26–77, 1963. Müller, P. J. and Schneider, R.: An automated leaching method for Romero, O. E., Lange, C. B., Swap, R. J., and Wefer, G.: Eolian- the determination of opal in sediments and particulate matter, transported freshwater diatoms and phytoliths across the equato- Deep-Sea Res. Pt. I, 40, 425–444, 1993.

www.biogeosciences.net/13/3071/2016/ Biogeosciences, 13, 3071–3090, 2016 3090 G. Fischer et al.: Deep ocean mass fluxes in the coastal upwelling off Mauritania

rial Atlantic record temporal changes in Saharan dust transport US Department of Commerce: NOAA Earth System Research Lab- patterns, J. Geophys. Res., 104, 3211–3222, 1999. oratory, http://www.esrl.noaa.gov/psd/data/timeseries/AMO/, Romero, O. E., Lange, C. B., and Wefer, G.: Interannual variabil- 2015. ity (1988–1991) of siliceous phytoplankton fluxes off northwest Van Camp, L., Nykjær, L., Mittelstaedt, E., and Schlittenhardt, Africa, J. Plank. Res., 24, 1035–1046, 2002. P.: Upwelling and boundary circulation off Northwest Africa Romero, O. E., Kim, J.-H., and Donner, B.: Submillennial-to- as depicted by infrared and visible satellite observations, Prog. millennial variability of diatom production off Mauritania, NW Oceanogr., 26, 357–402, 1991. Africa, during the last glacial cycle, Paleoceanography, 23, Yeager, S. G., Shields, C. A., Large, W. G., and Hack, J. J. : The PA3218, doi:10.1029/2008PA001601, 2008. Low-Resolution CCSM3, J. Climate, 19, 2545–2566, 2006. Shanahan, T. M., Overpeck, J. T., and Anchukaitis, K. J., Beck, Yu, E. F., Francois, R., Honjo, S., Fleer, A. P., Manganini, S. J., Rut- J. W., Cole, J. E., Dettman, D. L., Peck, J. A., Scholz, C. A., gers van der Loeff, M. M., and Ittekkot, V.: Trapping efficiency and King, J. W.: Atlantic Forcing of persistent drought in West of bottom-tethered sediment traps estimated from the intercepted Africa, Science, 324, 377–380, 2009. fluxes of 230Th and 231Pa, Deep-Sea Res. Pt. I, 48, 865–889, Siegel, D. A. and Deuser, W. G.: Trajectories of sinking particles 2001. in the Sargasso Sea: modeling of statistical funnels above deep- Zeeberg, J., Corten, A., Tjoe-Awie, P., Coca, J., and Hamady, B.: ocean sediment traps, Deep-Sea Res. Pt. I, 44, 1519–1541, 1997. Climate modulates the effects of Sardinella aurita fisheries of Stokes, D. C.: NOAA, IRI/LDEO Climate Data Library, http://iridl. Northwest Africa, Fish. Res., 89, 65–75, 2008. ldeo.columbia.edu/filters/.NINO/SOURCES/.NOAA/.NCEP/ Zenk, W., Klein, B., and Schroder, M.: Cape Verde Frontal Zone, .EMC/.CMB/.GLOBAL/.Reyn_SmithOIv2/.monthly/.ssta/, Deep-Sea Res. Pt. I, 38, 505–530, 1991. 2015. Zhao, T. X.-P., Laszlo, I., Guo, W., Heidinger, A., Cao, C., Je- Ternon, E., Guieu, C., Loÿe-Pilot, M.-D., Leblond, N., Bosc, E., lenak, A., Tarpley, D., and Sullivan, J.: Study of long-term Gasser, B., Miquel, J.-C., and Martín, J.: The impact of Saha- trend in aerosol optical thickness observed from operational ran dust on the particulate export in the water column of the AVHRR satellite instrument, J. Geophys. Res., 113, D07201, North Western Mediterranean Sea, Biogeosciences, 7, 809–826, doi:10.1029/2007JD009061, 2008. doi:10.5194/bg-7-809-2010, 2010. Thunell, R., Benitez-Nelson, C., Varela, R., Astor, Y., and Muller- Karger, F.: Particulate organic carbon fluxes along upwelling- dominated continental margins: rates and mechanisms, Global Biogeochem. Cy., 21, GB1022, doi:10.1029/2006GB002793, 2007.

Biogeosciences, 13, 3071–3090, 2016 www.biogeosciences.net/13/3071/2016/