A two-decades (1988–2009) record of diatom fluxes in the Mauritanian coastal upwelling: Impact of low-frequency forcing and a two-step shift in the species composition

Abstract. Eastern Boundary Upwelling Ecosystems (EBUEs) are among the most productive marine regions in the world's oceans. Understanding the degree of interannual to decadal variability in the Mauritania upwelling system is key for the prediction of future changes of primary productivity and carbon sequestration in the Canary Current EBUE as well as in similar environments. A multiannual sediment trap experiment was conducted at the mooring site CBmeso (= 'Cape Blanc mesotrophic', ca. 20° N, ca. 20°40' W) in the high-productive Mauritanian coastal waters. Here, we present results on fluxes and the species-specific composition of the diatom assemblage for the time interval between March 1988 and May 2009. The temporal dynamics of diatom populations allows to propose three main diatom productivity/flux intervals: (i) early 1988–late 1996; (ii) 1997–1999, and (iii) early 2002–mid 2009. The impact of the Atlantic Multidecadal Oscillation appears to be an important forcing of the long-term dynamics of diatom population. The impact of cold (1988–1996) and warm AMO phases (2001–2009) is reflected by the outstanding shifts in species-specific composition. This AMO-impacted, long-term trend is interrupted by the occurrence of the strong 1997 ENSO. The extraordinary shift in the relative abundance of benthic diatoms in May 2002 suggests the strengthening of offshore advective transport within the uppermost layer of filament waters, and in the subsurface and in deeper and bottom‐near layers. It is hypothesized that the dominance of benthic diatoms was the response of the diatom community to the intensification of the slope and shelf poleward undercurrents. This dominance followed the intensification of the warm phase of AMO and the associated changes of the Atlantic Meridional Overturning Circulation. Transported valves (siliceous remains) from shallow coastal waters into the deeper bathypelagial should be considered for the calculation and model experiments of bathy- and pelagial nutrients budgets (especially Si), the burial of diatoms and the paleoenvironmental signal preserved in downcore sediments. Our 1988–2009 data set contributes to the distinction between climate-forced and intrinsic variability of populations of diatoms and will be especially helpful for establishing the scientific basis for forecasting and modelling future states of this ecosystem and its decadal changes.



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We used deep-moored (>1000 m depth), large-aperture, time-series sediment traps of the Kiel and Honjo types with 20 cups and 0.5 m² openings, equipped with a honeycomb baffle (Kremling et al., 1996). As the traps were moored in deep waters (mostly below 1,200m, Table 1), uncertainties 108 with the trapping efficiency due to strong currents (e.g. undersampling) and/or due to the migration and activity of zooplankton migrators ('swimmer problem') are assumed to be minimal (Buesseler et al., 2007). Prior to the deployments, the sampling cups were poisoned with HgCl2 (1 ml of conc. HgCl2 111 per 100ml of filtered seawater) and pure NaCl was used to increase the density in the sampling cups to 40‰. Upon recovery, samples were stored at 4°C on board and wet-splitted in the home laboratory (MARUM, University of Bremen) using a rotating McLane wet splitter system. Larger 114 swimmers -such as crustaceans-were handpicked at the home lab by using forceps and were removed by filtering carefully through a 1 mm sieve. All flux data here refer to the size fraction <1 mm. In almost all samples, the fraction of particles >1 mm was negligible, only in a few samples 117 larger pteropods were found (Fischer et al., 2016).

Assessment of diatom fluxes and species identification
1/64 and 1/125 splits of the original samples were used. Samples were rinsed with distilled water and prepared for diatom studies following standard methods (Schrader and Gersonde, 1978). For this 126 study, a total of 282 sediment trap samples was processed. Each sample was chemically treated with potassium permanganate, hydrogen peroxide (33%) and concentrated hydrochloric acid (32%) following previously used methodology (Romero and Fischer, 2017;Romero et al., 1999aRomero et al., , b, 2002Romero et al., , 129 2009a. Qualitative and quantitative analyses of the diatom community were carried out on permanent slides of acid cleaned material (Mountex® mounting medium) at x1000 magnifications by using a Zeiss ® Axioscop with phase-contrast illumination (MARUM, University of Bremen). Depending 132 on valve abundances in each sample, several traverses across each slide were examined. The counting procedure and definition of counting units for valves follows Schrader and Gersonde (1978).
Total amount of counted valves per slide ranged between ca. 400 and 1,000. Two cover slips per 135 sample were scanned in this way. Counts of valves in replicate slides indicate that the analytical error of valve concentration estimates is 10 %.
The resulting counts yielded abundance of individual diatom taxa as well as daily fluxes of valves 138 per m-2 d-1 (DF), calculated according to Sancetta and Calvert (1988), as follows:
[days] x [D] where, [N] number of valves in an known area [a], as a fraction of the total area of a petri dish [A] and the dilution volume [V] in ml. This value is multiplied by the sample split [Split], representing the 141 fraction of total material in the trap, and then divided by the number of [days] of sample deployment and the sediment trap collection area [D].

Statistical analysis 144
Correspondence Analysis (CA) was used to explore diatoms community's changes. CA is an ordination technique that enables describing the community structure from multivariate contingency tables with frequency-like data (i.e. abundances derived from counting with integers and zeros) that 147 are dimensionally homogeneous (Legendre and Legendre, 2012). Based on the CA samples' scores, a hierarchical clustering analysis was performed to classify the samples date according to the diatom composition similarities. Euclidean distance was used to compute the distance matrix from which a 150 hierarchical dendrogram was generated using Ward's aggregation link (Legendre and Legendre, 2012

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The CC-EBUE is in the eastern part of the North Atlantic subtropical gyre ( Fig. 1; Arístegui et al., 2009;Chavez and Messié, 2009;Cropper et al., 2014). Both the temporal occurrence and the intensity of the upwelling along northwestern Africa depend on the shelf width, the seafloor 162 topography, wind direction and strength (Mittelstaedt, 1983;Hagen, 2001), the Ekman-mediated transport and strong mesoscale heterogeneity (Chavez and Messié, 2009;Cropper et al., 2014;Fréon et al., 2009). The Mauritanian shelf is wider than the shelf northward and southward along the CC-

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EBUE and gently slopes from the coastline into water depths below 200 m (Hagen, 2001). The shelf break zone with its steep continental slope extends over approximately 100 km from the coastline (Hagen, 2001). As a consequence of the coastal topography, and the shelf and slope bathymetry, the 168 ocean currents and the wind system, surface waters off Mauritania are characterized by almost permanent upwelling with varying intensity year-through (Lathuilière et al., 2008;Cropper et al., 2014). Site CBmeso locates at the westward end of this permanent upwelling zone.

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The surface hydrography off Mauritania is strongly influenced by two surface currents: the southwestward-flowing CC and the poleward-flowing coastal countercurrent or Mauritania Current https://doi.org/10.5194/bg-2020-336 Preprint. Discussion started: 13 October 2020 c Author(s) 2020. CC BY 4.0 License. 6 (MC) (Fig. 1). The surficial CC detaches from the northern African continental slope between 25° and 174 21°N and supplies Si-poor waters to the North Equatorial Current. CC waters are relatively cool because it entrains upwelled water from the coast as it moves southward (Mittelstaedt, 1991). The Si-rich MC gradually flows northward along the coast up to about 20°N (Mittelstaedt, 1991), and 177 brings warmer surface waters from the equatorial realm into waters overlying site CBmeso. Towards late autumn, the MC is gradually replaced by a southward flow associated with upwelling water due to the increasing influence of trade winds south of 20°N (Zenk et al., 1991), and becomes a narrow 180 strip of less than 100 km width in winter (Mittelstaedt, 1983). The MC advances onto the shelf in summer and is enhanced by the relatively strong Equatorial Countercurrent and the southerly trade winds (Mittelstaedt, 1983).

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North of Cape Blanc (ca. 21°N; Fig. 1), the intense northeasterly winds cause coastal upwelling to move further offshore and the upper slope fills with upwelled waters. South of Cape Blanc, northerly winds dominate year-through, but surface waters remain stratified and the coastal Poleward 186 Undercurrent (PUC) occurs as a subsurface current (Pelegrí et al., 2017). South of Cape Timiris (ca.
19°30'N), the PUC intensifies during summer-fall and remains at the subsurface during winter-spring (Pelegrí et al., 2017). The encountering of the northward flowing MC-PUC system with the southward 189 flowing currents builds the Cape Verde Frontal Zone (Zenk et al., 1991;Fig. 1) and the large offshore water export is visible as the giant Mauritanian chlorophyll filament (Gabric, 1993;Pelegrí et al., 2006Pelegrí et al., , 2017.

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The chlorophyll filament extends offshore up to 400 km (e.g., Arístegui et al., 2009;Cropper et al., 2014;Van Camp et al., 1991), carrying a mixture of North and South Atlantic Central Water (NACW and SACW, respectively) through an intense jet-like flow offshore (Meunier et al., 2012). Intense 195 offshore transport acts an important mechanism for the export of cool, nutrient-rich shelf and upper slope waters. It has been estimated that this giant filament export about 50% of the coastal new production offshore toward the open ocean during intervals of most intense upwelling (Gabric et al., 198 1993;Helmke et al., 2005;Lange et al., 1998;Van Camp et al., 1991). This transport impacts even more distant regions in the deep ocean, since sinking particles are strongly advected by lateral transport in subsurface and deeper waters (Fischer and Karakaș, 2009;Karakaș et 201 al., 2006).
The SACW occurs in layers between 100 and 400 m depth the Banc d'Arguin and off Mauritania.
The hydrographic properties of upwelled waters over the shelf suggest that they ascend from depths 204 between 100 and 200 m south off the Banc d'Arguin (Mittelstaedt, 1983). North of it, the SACW merges gradually into deeper layers (200-400 m) below the CC (Mittelstaedt, 1983 shelf (Zenk et al., 1991).  (Jungclaus et al., 2005) and/or atmospheric modes such as the North Atlantic Oscillation (e.g.,

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Buckley and Marshall, 2016). In contrast, Clement et al. (2015) found that the pattern of AMO variability can be reproduced in a model that does not include ocean circulation changes, but only the effects of changes in air temperature and winds.

El Niño/Southern Oscillation (ENSO) and La Niña: is an irregularly periodic variation in winds
and SST over the tropical eastern Pacific Ocean, affecting the climate of much of the tropics and subtropics of other ocean basins. The warming phase is known as El Niño and the cooling phase as La

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Niña. The Southern Oscillation is the accompanying atmospheric component, coupled with the SST variations. ENSO-related teleconnections in the CC-EBUE upwelling system have been described by several authors (Behrenfeld et al., 2001;Pradhan et al, 2006;Zeeberg et al., 2008) and can be 240 illustrated by the negative correlation of sea level pressure with eastern tropical Pacific SST.

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Especially during the months of November through April, the NAO is responsible for much of the variability of weather in the North Atlantic region, affecting wind speed and wind direction changes, changes in temperature and moisture distribution and the intensity, number and track of storms.

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Correlations during winter show that NAO and ENSO may have opposite effects on the CC-EBUE/northeastern Atlantic realm, for instance on wind fields, and consequently on upwelling with potential implications for deep ocean mass fluxes (Fischer et al., 2016).

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Maxima of TDF are defined here as those values that are higher than the TDF average ±1 standard deviation (STD) for the entire study period. Spring and summer show the higher amount of above-the-average TDF. Although the same number of maxima are recorded in fall as in summer and 267 spring (n=17; spring, n=16), the absolute values of fall TDF maxima were predominantly lower than those of spring and summer. Winter has the lowest amount of TDF maxima (n=12).

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A total of 203 diatom species were identified in CBmeso samples between March 1988 and June 2009. To better understand the temporal variations of the diverse community, we follow the same grouping approach as already applied in the nearby trap site CBeu (Romero and Fischer, 2017; 273 Romero et al., 2020). Out of 203 taxa, 109 species (whose average relative contribution is ≥0.50% for the entire studied interval) were distributed in four groups, according to the main ecological and/or habitat conditions they represent: (1) benthic, (2) coastal upwelling, (3) coastal planktonic, and (4) 276 open-ocean diatoms. Taxa assigned to each group are listed in Table 2. Below the main species of each group and their main ecological significance are shortly described.

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(1) The benthic group is dominated by Delphineis surirella. As part of the epipsammic 279 community, D. surirella is a benthic marine species that commonly thrives in the shallow euphotic zone of sandy shores, shelf and uppermost slope waters along temperate to cool seas, forming either short or long chains of small valves (length=5-15 m) (Andrews, 1981).

Vegetative cells of numerous
Chaetoceros species (mainly those assigned to the section Hyalochaete, Rines and Hargraves, 1988) rapidly respond to the weakening of upwelling intensity and nutrient depletion by forming 288 endogenous resting spores, hence their high numbers in trap samples is interpreted to represent the strongest upwelling intensity Abrantes et al., 2002;Nave et al., 2001;Romero et al., 2002).
The multivariate analyses performed on the relative abundance of diatom populations (Fig. 3) 303 confirms the strong interannual variability with significant shifts within the diatom community between 1988 and 2009. The first CA component covers 65.47% of the total variance and opposes the samples dominated by benthic and coastal planktonic diatoms (Fig. 3a). The second CA axis This shift occurred in two steps (Fig. 2b)  The impact of the environmental variables on diatom communities was investigated by simple comparison using the samples clustering and the forcing values associated (Fig 4)

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correlogram performed between CA axes and the low-frequency climate indices also confirms these trends (not shown here). A significant positive and negative correlation has been found between the first CA axis samples scores' with respectively AMO and Shannon diversity index (Fig. 3). Given that 336 the first CA is positively driven by the benthic group, this confirms that the outstanding dominance of the benthic diatom D. surirella decreased the diversity, although it also seems to be promoted by AMO strengthening. In the same way, the second CA axis samples scores are positively correlated 339 with TDF, which confirms that coastal upwelling diatoms seems to promote the TDF.

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The long-term diatom record at site CBmeso offers the possibility of discussing population dynamics in the context of the high-frequency atmospheric and hydrographic dynamics along the CC-EBUE, and low-frequency climate variability such in the North Atlantic. In 5.1, we discuss the impact

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Based on outstanding shifts in the species-specific composition of the diatom assemblage occurred throughout the study interval (Fig. 2b). We propose three main intervals in the multiyear

AMO and the two-step increase of benthic diatoms' contribution
Based on the long-term trends of our data and their statistical analysis (Figs. 2-5), we suggest that 360 the proposed intervals were the response of the diatom populations to the impact of low frequency forcing. As described above in 4.2, the benthic diatom community appears positively correlated with AMO (Fig. 5). Among the low frequency forcings affecting the subtropical North Atlantic (see above  (Fischer et al., , 2016 and observation-based model experiments conducted along the Mauritanian upwelling (Helmke et al., 2005;Karakaş et al., 2006;Nowald et al., 2015) 408 discussed already the role of intermediate and deep nepheloid layers in the lateral transport of particles and microorganisms remains upon the deeper bathypelagial. Based on the vigorous mixing in the uppermost water column due to the confluence of northward and southward water masses 411 and strong, predominantly westward winds off Mauritania ( Fig. 1; see 3.1), the offshore transport from shallow into deeper waters is most intense between 20.5°N and 23.5°N along the northwestern African margin. Erosional processes in the very dynamic coastal realm significantly contribute to the 414 downward transport of particulates and microorganism remains (Meunier et al., 2012), and are responsible for sporadic particle clouds advected up to several hundreds of kilometers offshore  Fischer et al., 417 2009, Nowald et al., 2015. This nepheloid layer-mediated transport additionally benefits from the bathymetry of the Mauritanian shelf and slope (Nowald et al., 2015). The subsurface layer (100 to 300 m water depth), in turn strongly affected by the AMOC intensification due to AMO impact (Wang

Mauritania
The long-term trends determined by the cold and warm phases of AMO was altered in the second half of the 1990s by the impact of the strong 1997 ENSO (McPhaden, 1999). Although caution is 432 advised in the interpretation of the record due to a few gaps between 1996 and 1999 (Table 1)

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typical of oligo-mesotrophic waters) evidences considerable changes in the physical setting of the Mauritanian upwelling. Since the interval 1996-1999 records the lowest TDF for the entire study (Fig.   2a), we argue that ENSO negatively impacted on diatom productivity off Mauritania and most of the 441 total organic carbon captured with CBmeso traps (Fischer et al., 2016), was instead delivered either by coccolithophorids or bacterioplankton.
A positive ENSO goes along with the weakening of E-NE winds off Mauritania (Pradhan et al., 444 2006;Fischer et al., 2016). Weakened E-NE trades lead to the deepening of the thermocline below the depth of the source of upwelled water, this hindering the mixing of the water column and causing upwelling intensity off Mauritania to decrease until early 1998 (Pradhan et al., 2006).

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Additionally, the size of the Mauritanian chlorophyll filament decreased between winter 1997 and spring 1998, while became unusually large from autumn 1998 to spring 1999 .
Aperiodic, pronounced decreases in the diatom flux in other ocean basins have been previously 450 associated with limiting nutrient levels due to ENSO-derived perturbations. The diatom production in https://doi.org/10.5194/bg-2020-336 Preprint. Discussion started: 13 October 2020 c Author(s) 2020. CC BY 4.0 License.
14 hemipelagial waters off northern Chile decreased extraordinarily during the strong 1997 ENSO compared to earlier years (Romero et al., 2001). Similar negative impact assigned to ENSO 453 teleconnections have been observed in the California Current (Lange et al., 2000), in the Cariaco Basin (Romero et al., 2009b) and in the pelagial Subarctic Pacific Ocean (Takahashi, 1987).
Complementary support of this ENSO-mediated impact on surface water productivity off 456 Mauritania is provided by variations of bulk biogenic fluxes at CBmeso. The almost 2.5 times higher organic carbon flux during 1998-99 than in 1997 (Helmke et al., 2005) led to propose that, after weakening due to impact of ENSO on the physical setting, upwelling intensified immediately 459 afterward during La Niña (Fischer et al., 2016). Similarly, the seasonal cycle of surface Chl-a distribution in waters above the CBmeso site reveals a noticeable event (~250% increase) in Mauritanian coastal waters (Pradhan et al., 2006).

Comparison of diatom fluxes and populations' dynamics within the giant Mauritanian chlorophyll filament (CBmeso vs CBeu)
In this subsection, we compare the diatom flux and the assemblage composition at site CBmeso 465 with previous results from the nearby trap site CBeu gained between 2003 and 2009 (Romero and Fischer, 2017;Romero et al., 2020). The CBeu site locates ca. 80 nautical miles (~150 km) offshore at the continental slope below the giant Mauritanian chlorophyll filament, and hence between the 468 coastline and the outer CBmeso site (Fig. 1). These two trap locations are under different nutrient availability and upwelling intensity between eutrophic (CBeu) and mesotrophic conditions (CBmeso) (Romero and Fischer, 2017;Fischer et al., 2016Fischer et al., , 2019.

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The less favorable conditions for diatom productivity in waters overlying site CBmeso is evidenced by lower TDF than at site CBeu. On the seasonal pattern, TDF at site CBmeso are always two orders of magnitude lower than values gained at site CBeu (Fig. 6a). This also happens during fall, when the 474 highest average seasonal flux is recorded at CBmeso (5.6*10 5 valves m -2 d -1 vs 3.3*10 6 valves m -2 d -1 ).
We advocate that these flux differences reflect (i) the offshore weakening of the transport via the chlorophyll filament, (ii) the seaward decreasing concentration of nutrients within the filament 477 (Lathuilière et al., 2008;Meunier et al., 2012), (iii) the more intense upwelling in waters overlying the Mauritanian slope (Mittelstaedt, 1983(Mittelstaedt, , 1991Cropper et al., 2014), and (iv) the offshore weakening of the lateral transport (Karakaş et al., 2006;Nowald et al., 2015). According to satellite imagery (Van 480 Camp et al., 1991;Gabric et al., 1993;Fischer et al., 2016), the CBmeso mooring locates only occasionally beneath the giant chlorophyll filament and hence below nutrient-rich waters. In general, 15 stronger ballasting due to higher lithogenic input from northwestern Africa-compared to the 486 offshore CBmeso (Fischer et al., 2019).
Complementary support to the scenario of lower (higher) productivity levels at CBmeso (CBeu) is provided by the species-specific composition of the assemblage: relative contribution of groups 489 related with more oligo-mesotrophic waters is higher at CBmeso than at CBeu (coastal planktonic and open-ocean, Fig. 6d, e), while the opposite is true for diatoms typical of eutrophic waters (Fig.   6c). Despite the difference in the relative contribution, the species-specific composition of diatom 492 groups is remarkably similar at both sites. All the main group taxa at site CBmeso ( Table 2, see also 4.2) are also found in CBeu samples (see Table 2 in Romero and Fischer, 2017 Fig. 6b) also evidences the impact of particulates derived from the Mauritanian inner shallow shelf (Romero and Fischer, 2017;Fischer et al., , 2016Fischer et al., , 2019Romero et al., 2020). The simultaneous 501 occurrence of the second increase of benthic diatoms at CBmeso and the increase at the neritic site CBeu (Fig. 5) is a striking feature of the population shift over a large part of the Mauritanian upwelling system. The transport of particulates and microorganism remains from their source in 504 shallow coastal waters into the hemipelagic realm probably occurs within weeks (Karakaş et al., 2006. Phytoplankton thriving in Mauritanian surface waters can be transported as far as 400 km offshore from coastal waters (Gabric et al., 1993;Helmke et al., 2005;Barton et al., 2013). The

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MC might have helped in detaching benthic diatoms from their substrata (Romero and Fischer, 2017) and in transporting them northwestward into the hemipelagial/bathypelagial realm (where CBmeso traps are deployed). These observations offer additional evidence of the impact of AMO via the 510 strengthening of the meridional advection, the major nutrient input via the MC and the nepheloid layer-mediated transport into the deeper Mauritanian bathypelagial.   (Romero and Fischer, 2017;Romero et al., 2020) and followed two steps. The two-step increase of benthic 528 diatoms at the CBmeso site suggests that the intensification of the slope and shelf poleward undercurrents followed the intensification of the warm phase of AMO and the associated AMOC changes.

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Diatom remains sink not only vertically off Mauritania, but they are also laterally advected from the shelf to the deeper bathypelagial via the nepheloid layer-mediated transport. Transported valves (siliceous remains) from shallow coastal waters into the deeper bathypelagial should be considered

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for the calculation and model experiments of bathy-and pelagial nutrients budgets (especially Si), the burial of diatoms and the paleoenvironmental signal preserved in downcore sediments.
Understanding the degree of interannual to decadal variability in the Mauritania upwelling system is 537 key for the prediction of future changes of primary productivity along the NW African margin as well in other, economically important EBUEs. Our 1988-2009 data set might be instrumental in distinguishing between climate-forced and intrinsic variability of populations of primary producers 540 (e.g., diatoms) and are especially important for establishing the scientific basis for forecasting and modeling future states of this ecosystem and its decadal changes.

Author Contributions
OER and GF devised the study. OER collected the data and wrote the manuscript. SR performed the statistical analysis. All authors contributed to interpretation and discussion of results. https://doi.org/10.5194/bg-2020-336 Preprint. Discussion started: 13 October 2020 c Author(s) 2020. CC BY 4.0 License.