A total of 20 moorings were deployed off Mauritania (Fig. 1) between March 1988 and June 2009. Details on sampling intervals and trap depths are given in Table 1. Major gaps in the diatom record are between (i) December 1991 and June 1994 (no traps deployed), (ii) October 1994 and November 1995 (malfunctioning of the trap CBmeso6 upper), (iii) October 1997 and June 1998 (malfunctioning of the trap CBmeso8 upper), and November 1999 and March 2001 (malfunctioning of traps CBmeso10 and 11 lower) (Table 1). The entire study interval extended over 7734 d between March 1988 and June 2009. During this interval, samples were collected for 5574 d. The gaps that occurred total 2160 d of the entire trap experiment.

We used deep-moored (>700 m water 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 below intermediate water masses (CB1lower: 2195 m; CB2–5, 7, 9–12, 15–20lower: 3502–3633 m; and CB6, 8 and 14upper: 745–1246 m, Table 1), uncertainties 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 per 100 mL 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-split in the home laboratory (MARUM, University of Bremen) using a rotating McLane splitter system. Larger swimmers – such as crustaceans – were handpicked at the home lab by using forceps and were removed by filtering the sample 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 (larger pteropods were found only in a few samples; Fischer et al., 2016).

We compare our data with those previously published at the mooring location CBeu (ca. 20∘45′ N, 18∘45′ W), also deployed off Mauritania (Romero and Fischer, 2017; Romero et al., 2020). It locates ca. 80 nautical miles (∼150 km) offshore over the continental slope in ca. 2750 m water depth. For site CBeu, only the upper trap fluxes are shown (Romero and Fischer, 2017; Fischer et al., 2019; Romero et al., 2020).

The 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 study, a total of 282 sediment trap samples were 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., 1999a, b, 2002, 2009a, b, 2020). Qualitative and quantitative analyses of the diatom community were carried out on permanent slides of acid-cleaned material (Mountex® mounting medium) at ×1000 magnification by using a Zeiss® Axioscope with phase-contrast illumination (MARUM, University of Bremen). Depending on valve abundances in each sample, several traverses across each slide were examined. The counting procedure and definition of counting units for valves follow Schrader and Gersonde (1978). The total number of counted valves per slide ranged between ca. 400 and 1000. Two cover slips per 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 valvesm-2d-1 (DF), calculated according to Sancetta and Calvert (1988), as follows: 1DF=[N]×[A/a]×[V]×[Split][days]×[D], where [N] is number of valves in a known area [a] as a fraction of the total area of a petri dish [A] and the dilution volume [V] in milliliters. This value is multiplied by the sample split [Split], representing the 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].

Correspondence analysis (CA) was used to explore diatom communities' changes. CA is an ordination technique that enables description of the community structure from multivariate contingency tables with frequency-like data (i.e., abundances derived from counting with integers and zeros) that 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 hierarchical dendrogram was generated using Ward's aggregation link (Legendre and Legendre, 2012). This approach has been computed by using the vegan package included in the R software. In addition, Kruskal–Wallis tests, coupled with multiple comparison tests (pairwise Wilcoxon rank sum test), have been performed on climatic indexes and total diatom flux according to sample groups highlighted by the clustering analysis with the aim of identifying relationships between environmental forcing indices and diatom communities.

The CC EBUE is in the eastern part of the North Atlantic subtropical gyre (Fig. 1, Chavez and Messié, 2009; Arístegui et al., 2009; Cropper et al., 2014). Both the temporal occurrence and the intensity of the upwelling along northwestern Africa depend on the shelf width, seafloor topography, wind direction and strength (Mittelstaedt, 1983; Hagen, 2001), Ekman-mediated transport, and mesoscale heterogeneity (Chavez and Messié, 2009; Fréon et al., 2009; Cropper et al., 2014). The Mauritanian shelf is wider than the shelf northward and southward along the CC 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). Because of the coastal topography, shelf and slope bathymetry, ocean currents, and wind system, surface waters off Mauritania are characterized by almost permanent upwelling with varying intensity year-round (Lathuilière et al., 2008; Cropper et al., 2014). The site CBmeso locates at the westward end of this permanent upwelling zone.

The surface hydrography off Mauritania is influenced by two major surface currents: the southwestward-flowing CC and the poleward-flowing coastal countercurrent or Mauritania Current (MC) (Fig. 1). The surficial CC detaches from the northern African continental slope between 25 and 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 brings warmer surface waters from the equatorial realm into waters overlying the 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 strip of less than 100 km wide 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).

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-round, but surface waters remain stratified, and the coastal Poleward 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 convergence of the northward-flowing MC–PUC system with the southward-flowing currents builds the Cabo Verde Frontal Zone (Zenk et al., 1991; Fig. 1), and the large offshore water export is visible as the giant Mauritanian chlorophyll filament (Gabric et al., 1993; Pelegrí et al., 2006, 2017).

The chlorophyll filament extends offshore up to 400 km (e.g., Arístegui et al., 2009; Cropper et al., 2014; Van Camp et al., 1991; Fig. 1b–d), carrying a mixture of North and South Atlantic Central Water (NACW and SACW, respectively) through an intense offshore jet-like flow (Meunier et al., 2012). Intense offshore transport acts as an important mechanism for the export of cool, nutrient-rich shelf and upper-slope waters. It has been estimated that this giant filament exports about 50 % of the coastal new production toward the open ocean during intervals of the most intense upwelling (Gabric et al., 1993; Lange et al., 1998; Van Camp et al., 1991; Helmke et al., 2005). 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 Karakaş, 2009; Karakaş et al., 2006; Fischer et al., 2009).

The SACW occurs in layers between 100 and 400 m depth around the Banc d'Arguin and off Mauritania. The hydrographic properties of upwelled waters over the shelf suggest that they ascend from depths between 100 and 200 m south off the Banc d'Arguin (Mittelstaedt, 1983). North of the Banc d'Arguin, the SACW merges gradually into deeper layers (200–400 m) below the CC (Mittelstaedt, 1983). The biological response is accelerated in upwelled waters when the SACW of the upper part of the undercurrent feeds the onshore transport of intermediate layers to form mixed-water types on the shelf (Zenk et al., 1991).

Marine diatoms are the main contributors to the siliceous fraction in samples collected with the CBmeso traps between March 1988 and June 2009. Silicoflagellates and radiolarians are secondary components of the siliceous fraction (data not shown here), with a minor contribution of land-derived freshwater diatoms and phytoliths. In terms of number of individuals, the total diatom flux was always 1 order to 4 orders of magnitude higher than that of the other siliceous organisms.

The total diatom flux ranged from 2.7×103 to 3.3×106 valves per square meter per day (average =4.0×105valves±4.4×105) and shows strong interannual variability (Fig. 2, Table 2). The highest fluxes (>1.0×106 valves per square meter per day) occurred in 1988, late 1989, early 2002, 2003, late 2004, early 2005, early 2007, and early 2008. The lowest total diatom flux was recorded between 1997 and 1999 (range =2.3×103–5.1×104).

Maxima of total diatom flux are defined here as those values that are higher than the total diatom flux average ±1 standard deviation (SD) for the entire study period. Fluxes in spring and summer show the highest number of above-the-average values. Although the same number of total diatom flux maxima is recorded in fall as in summer and spring (n=17), the absolute values of fall maxima were predominantly lower than those of spring and summer. Winter has the lowest number of total diatom flux maxima (n=12).

Estimates of annual diatom fluxes were calculated for calendar years with at least 250 dyr-1 of flux collections. Data are presented in Table 2. Values range between 3.8×108 and 2.6×107 valves per square meter per year (1SD=1.0×108 valves per square meter per year; average =1.5×108 valves per square meter per year). Values above the average occurred in 1988–1990, 2003, 2005, and 2008. The lowest value was recorded in 1997 (Table 2).

A total of 203 diatom species were identified in CBmeso samples between March 1988 and June 2009. To better understand the temporal variations in the diverse community, we follow the same grouping approach as already applied in the nearby trap site CBeu (Romero and Fischer, 2017; 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) open-ocean diatoms. Taxa assigned to each group are listed in Table 3 and described below.

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

The multivariate analyses performed on the relative abundance of diatom populations (Fig. 3) confirm 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 explains 19.16 % of the total variance and discriminate coastal upwelling and open-ocean diatoms. The clustering analysis allows the samples to be statistically grouped, and the complete time series was segmented according to the four diatom communities' affiliation (Fig. 3b). These clusters show clear changes of diatom populations' contribution throughout the time series (Fig. 3c) with the dominance of open-ocean and costal upwelling populations between 1988 and 1996. Open-ocean diatoms dominated from 1997 to 2001, while benthic taxa were main contributors from 2002 to 2009.

A major shift in the relative contribution of the diatom groups is seen from May 2002 onward. This shift occurred in two steps (Figs. 2b and 3c). The percentage of benthic diatoms strongly increased between the middle of May and the middle of June 2002 (increase from 12.5 % to 68.6 %; trap CBmeso13, samples 2 and 3, 12 May to 19 June 2002). Benthic diatom contribution decreased below 40 % in early 2004. A second increase occurred in winter 2006, with values being mostly above 50 % almost throughout until the end of the trap experiment in June 2009 (Fig. 2b). The dominance of benthic diatoms at CBmeso also prevails after 2009 (Romero, unpubl. data). The marked increase in variability of the benthic relative contribution is clearly evidenced by the highest variability among all diatom groups (1 SD of each group for the whole study interval is (1) benthic =±23.68 %, (2) coastal upwelling =±10.46 %, (3) coastal planktonic =±11.88%, and (4) open ocean =±16.00 %).

The impact of the environmental variables on diatom communities was investigated by comparing the sample clustering and the values of low-frequency forcings (Fig. 4). AMO, the Shannon diversity index, and total diatom flux show significant differences between groups (Kruskal–Wallis test; p value < 0.05), whereas no statistical differences have been observed for the NAO, ENSO, and Pacific Decadal Oscillation indices. Only benthic diatoms (group 4) show higher AMO values compared to the three other groups (pairwise Wilcoxon rank sum test; p value < 0.05). In addition, a gradient in the Shannon diversity index of the diatom populations (Fig. 4c) is observed with predominantly low values (1.7–2.5) corresponding to benthic (group 4), intermediate values (2.7–3) for coastal planktonic (group 3), and high values (3.1–3.45) in samples dominated by coastal upwelling and open-ocean populations (groups 2 and 1) (pairwise Wilcoxon rank sum test; p value < 0.05). The statistical analysis also shows that, during intervals dominated by coastal upwelling populations, the total diatom flux was higher compared to values when other diatom groups dominated the community. The correlogram performed between CA axes and the low-frequency climate indices also confirms these trends (Fig. 5). A significant positive and negative correlation was found between the first CA axis sample scores and AMO and Shannon diversity index (Fig. 3). Given that the first CA is positively driven by the benthic group, this confirms the outstanding dominance of the benthic diatom D. surirella after May 2002, which also appears to be linked to the strengthening of AMO. In the same way, the fact that the second CA axis is positively correlated with total diatom flux confirms that coastal upwelling diatoms deliver a large number of diatom valves.

In this subsection, we compare the total diatom flux and the assemblage composition at the site CBmeso 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 coastline and the outer CBmeso site (Fig. 1). These two trap locations are under different nutrient availability and upwelling intensity (Romero and Fischer, 2017; Fischer et al., 2016, 2019).

The less favorable conditions for diatom productivity in waters overlying the site CBmeso (Fig. 1b–d) are evidenced by lower total diatom flux than at the site CBeu. In the seasonal pattern, the total diatom flux at the site CBmeso is always 2 orders of magnitude lower than values obtained at the site CBeu (Fig. 7a). This also happens during fall, when the highest average seasonal flux is recorded at CBmeso (5.6×105 valves per square meter per day vs. 3.3×106 valves per square meter per day). We hypothesize that these flux differences reflect (i) the more intense upwelling in waters overlying the Mauritanian slope (Mittelstaedt, 1983, 1991; Cropper et al., 2014), (ii) the weakening of the offshore transport via the chlorophyll filament (Fig. 1b), (iii) the seaward decreasing concentration of nutrients within the filament (Lathuilière et al., 2008; Meunier et al., 2012), and (iv) the offshore weakening of the lateral transport (Karakaş et al., 2006; Nowald et al., 2015). According to satellite imagery (Van Camp et al., 1991; Gabric et al., 1993; Fischer et al., 2016; Fig. 1b–d), the CBmeso mooring locates only occasionally beneath the giant chlorophyll filament. In general, the larger DSi availability (approximately 10 vs. 5 µM) and the higher Si : N ratios of the source waters (SACW vs. NACW = 0.6 vs. 0.3; Arístegui et al., 2009) in coastal water bathing site CBeu are reflected in ca. 3-fold-higher biogenic silica fluxes at the coastal CBeu – whose particle downward transport is additionally affected by strong ballasting due to higher lithogenic input from the nearby western African continent – compared to the offshore CBmeso site (Fischer et al., 2019).

Complementary support of the scenario of lower (higher) productivity levels at CBmeso (CBeu) is provided by the species-specific composition of the assemblage: relative contribution of groups related with oligo-mesotrophic waters is higher at CBmeso than at CBeu (coastal planktonic and open ocean, Fig. 7d and e), while the opposite is true for diatoms typical of eutrophic waters (Fig. 7c). Despite the difference in the relative contribution, the species-specific composition of diatom groups is remarkably similar at both sites. All the main taxa of diatom groups at the site CBmeso (Table 3; see also Sect. 4.2) are also found in CBeu samples (see Table 2 in Romero and Fischer, 2017). Both trap sites are linked via lateral advection through near-surface, intermediate, and deeper nepheloid layers (Fischer et al., 2016).

In their earlier study, Romero and Fischer (2017) observed that the shift in the species composition at the site CBeu toward a benthic-dominated assemblage occurred in early winter 2006. Since benthic diatoms in the deeper CBmeso traps are transported via nepheloid layers from shallow coastal waters (see Sect. 5.1.1), the high percentage of benthic species at the CBmeso site (Fig. 7b) evidences the impact of particulates derived from the Mauritanian inner shallow shelf (Romero and Fischer, 2017; Fischer et al., 2009, 2016, 2019; Romero et al., 2020). The simultaneous occurrence of the second increase in benthic diatoms at CBmeso and the increase at the neritic site CBeu (Fig. 7) is a striking feature of the population shift over a large part of the Mauritanian upwelling system. 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 transport of particulates and microorganism remains from their source in shallow coastal waters into the hemipelagic realm probably occurs within weeks (Karakaş et al., 2006, 2009). The MC might have helped in detaching benthic diatoms from their substrata (Romero and Fischer, 2017) and in transporting them northwestward into the hemipelagic realm (where the CBmeso traps of this study were deployed). These observations offer additional evidence of the impact of AMO via the strengthening of the meridional advection, the major nutrient input via the MC, and the nepheloid-layer-mediated transport into the deeper Mauritanian waters.