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 crucial for the prediction of future changes of primary productivity and carbon sequestration in the Canary Current EBUE as well as in similar environments. A multiyear sediment trap experiment was conducted at the mooring site CBmeso (“Cape Blanc mesotrophic”, ca. 20
As part of the latitudinally extended eastern boundary upwelling ecosystem (EBUE) of the Canary Current (CC) in the subtropical northeastern Atlantic, the Mauritanian upwelling is characterized by intense offshore Ekman transport and strong mesoscale heterogeneity. This physical setting facilitates the vigorous exchange between the neritic and pelagic realms off Mauritania (Chavez and Messié, 2009; Freón et al., 2009; Cropper et al., 2014). The nutrient trapping efficiency of upwelling cells (Arístegui et al., 2009), the input of wind-carried dust particles from the Sahara and the Sahel (Romero et al., 1999b, 2003; Friese et al., 2017; Fischer et al., 2016, 2019), and/or the wide shelf (Hagen, 2001; Cropper et al., 2014) additionally impact the intensity of the primary production in surface waters and the subsequent export of microorganism remains into the meso- and bathypelagic off Mauritania. This set of conditions varies strongly on different temporal patterns (from seasonal through decadal; Mittelstaedt, 1983, 1991; Hagen, 2001; Nykjær and Van Camp, 1994; Barton et al., 2013; Varela et al., 2015). Whether the strong interannual and decadal variability of physical conditions off Mauritania is related to low-frequency, global-scale climatic variations or to an intrinsic level of basin-wide atmospheric and/or oceanic variability is still a matter of debate (Cropper et al., 2014; Varela et al., 2015; Fischer et al., 2016, 2019).
EBUEs may prove more resilient to ongoing climate change than other ocean
ecosystems because of their ability to function under extremely variable
conditions (Barton et al., 2013; Varela et al., 2015). On the other hand, it
is predicted that current global warming will impact atmospheric pressure
gradients and hence the strength of coastal winds that cause upwelling (Bakun, 1990; Bakun et al., 2010). Although productivity variations in EBUEs are sensitive to the amplitude and timing of upwelling-favorable winds (Varela et al., 2015), the impact of ongoing ocean warming on the dynamics of
upwelling-favorable winds is still contentious (Bakun, 1990; Bakun et al.,
2010; Varela et al., 2015). Long-term trends in variations in upwelling
intensity and related productivity changes seem highly dependent on the length of the data series, the selected study area, the season evaluated, and the methods applied (Varela et al., 2015). The description of multiyear to
interdecadal trends of upwelling intensity in the CC EBUE has been mostly
based on variations in velocity and direction of winds and calculated
upwelling intensities. Cropper et al. (2014) found a non-significant increase
in upwelling-favorable winds along the CC EBUE between 11 and 35
A different approach for the characterization of multiyear to interdecadal trends in EBUEs is assessing fluxes of particulates and microorganisms as captured by continuous sediment trap experiments. This study builds on earlier investigations of multiyear variability of the diatom flux captured with sediment traps deployed at the mesotrophic mooring site CBmeso (Cape Blanc mesotrophic, formerly known as CB, Fig. 1; Fischer et al., 1996). Several earlier studies addressed either the variations in marine diatom fluxes between March 1988 and November 1991 (Romero et al., 1999a, 2002; Romero and Armand, 2010; Lange et al., 1998) or the land-derived signal of siliceous remains (Romero et al., 1999b, 2003). After a gap of 2.5 years (December 1991 through May 1994), the CBmeso trap experiment re-started in June 1994 (Table 1). Here, we extend the diatom record collected from early June 1994 until middle June 2009. The main goal of this study is the description of the multiyear dynamics of the total diatom flux and the shifts in the species-specific composition of the assemblage at the site CBmeso during almost 20 years (1988–2009). Our study presents the longest sediment-trap-based time series on the temporal dynamics of diatom fluxes in the world ocean. We discuss the new results in view of the high frequency of climate indices, which are proxies for atmospheric and hydrographic dynamics along the CC EBUE, and low-frequency climate variability in the North Atlantic, and we compare our new data set at the site CBmeso with previous diatom (Romero and Fischer, 2017; Lange et al., 1998; Romero et al., 1999a, b, 2002, 2020) and bulk flux results off Mauritania (Helmke et al., 2005; Fischer et al., 2016, 2019). We also discuss our new results with recent results from the nearby coastal site CBeu (Cape Blanc eutrophic) (Romero and Fischer, 2017; Romero et al., 2020).
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
Data deployment at the site CBmeso (Cape Blanc mesotrophic): trap name, coordinates (latitude and longitude), ocean bottom depth, trap depth, sampling interval, and sample amount.
We used deep-moored (
We compare our data with those previously published at the mooring location
CBeu (ca. 20
The
The resulting counts yielded abundance of individual diatom taxa as well as
daily fluxes of
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
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
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
North of Cape Blanc (ca. 21
The chlorophyll filament extends offshore up to 400
The SACW occurs in layers between 100 and 400
The AMO is the average of sea surface temperatures (SSTs) of the North Atlantic Ocean
(from 0 to 60
Despite the indirect role of the atmosphere, the physical connection between the Atlantic Meridional Overturning Circulation (AMOC) and the AMO is typically described in terms of oceanic processes alone: since the AMOC transports heat northward over the entire Atlantic, an increase in North Atlantic Deep Water (NADW) formation should increase the strength of the AMOC, thus increasing oceanic meridional heat transport convergence in the North Atlantic, resulting in a basin-scale warming of SSTs (Knight et al., 2005). AMOC variability itself is often attributed to changes in NADW formation due to anomalous Arctic freshwater fluxes (Jungclaus et al., 2005) and/or atmospheric modes such as the North Atlantic Oscillation (NAO; e.g., 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.
ENSO is an irregularly periodic variation in winds and SST over the tropical eastern Pacific Ocean that affects the climate of much of the tropics and subtropics of other ocean basins via teleconnections. The warming phase is known as El Niño and the cooling phase as La 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 illustrated by the negative correlation of sea level pressure with eastern tropical Pacific SST.
The relationship between ENSO and other low-frequency forcings is still uncertain. It has been hypothesized that AMO could influence ENSO on multidecadal timescales (Dong et al., 2006); however, due to the comparatively low record of observations, the relationship between ENSO and other modes of multidecadal variability could just be random (e.g., Wittenberg, 2009; Stevenson et al., 2012). Levine et al. (2017) observed that AMO modifies the thermocline in the tropical Pacific, which, in turn, affects ENSO variance. Zhang et al. (2019) found that the negative ENSO–NAO correlation in late boreal winter is significant only when ENSO and AMO are in phase, while no significant ENSO-driven atmospheric anomalies are observed over the North Atlantic when ENSO and the AMO are out of phase. ENSO exhibits a considerable degree of diversity in its pattern of SST anomalies, which also complicates its connection with NAO. All these factors may increase the uncertainty of the ENSO–NAO relationship (Zhang et al. (2019) and references therein).
The NAO characterizes the difference of atmospheric sea level pressure between the Icelandic Low and the Azores High (Hurrell, 1995). These fluctuations control the strength and direction of westerly winds and location of storm tracks across the North Atlantic. A positive phase of the NAO is associated with anomalous high pressure in the Azores High region and stronger northeasterly winds along the NW African coast. Especially from November through April, the NAO is responsible for much of the weather variability 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.
As for ENSO, links between NAO and other low-frequency forcings remain debatable. Yamamoto and Palter (2016) show that some relation exists between NAO and AMO, with northerly winds associated with a positive state of AMO and zonal winds to a negative state of AMO. Winter correlations show that NAO and ENSO may have opposite effects on wind fields in the CC EBUEs and consequently on upwelling, with potential implications for the magnitude of deep-ocean mass fluxes (Fischer et al., 2016).
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
Total diatom flux (valves per square meter per day) and relative contribution of diatom groups (relative contribution, %) for the interval from March 1998 to June 2009 at the CBmeso site. Groups of diatoms are benthic (light green), coastal planktonic (black), coastal upwelling (dark green), and open ocean (orange). For the species-specific composition of each group, see Sect. 4.2. and Table 2. For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.
Estimates of annual total diatom fluxes. Values were calculated for
those calendar years with at least 300
Maxima of total diatom flux are defined here as those values that are higher
than the total diatom flux average
Estimates of annual diatom fluxes were calculated for calendar years with at
least 250
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 The benthic group is dominated by The coastal upwelling group is composed of several species of
Coastal planktonic species mostly thrive in neritic to hemipelagic and
oligo- to mesotrophic waters with moderate levels of dissolved silica (DSi).
These species become more abundant during intervals of decreased mixing, when
upwelling weakens (Romero and Armand, 2010; Romero and Fischer, 2017; Romero
et al., 2009a, b, 2020; Crosta et al., 2012). The well-silicified species Open-ocean taxa thrive in pelagic, oligotrophic, and warm to temperate waters
with low siliceous productivity due to low DSi availability and weak mixing in
surface waters (Romero and Fischer, 2017; Nave et al., 2001; Romero et al.,
2005; Crosta et al., 2012). The term “low DSi” availability (
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
Species composition of the assemblage of diatoms at the site CBmeso between March 1988 and June 2009.
Continued.
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
Comparison of clusters extracted from multivariate analysis according to total diatom flux, AMO, and Shannon diversity measured.
Correlogram representing Spearman's correlation rank between CA
axes (i.e., Dim.1, Dim.2, Dim.3), environmental variables, and climatic and
diversity indexes. Color scale and circle size indicate the strengths of the
correlation. Squares without an “X” indicate significant relationships
(
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;
Time series of ratio benthic : all groups (olive bars,
The long-term diatom record at the 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 the low-frequency climate variability in the North Atlantic. In Sect. 5.1, we discuss the impact of climate forcing on the long-term trends of the diatom community and the total diatom flux and the two-step shift in the species-specific composition of diatom populations. In the second subsection (Sect. 5.2), we compare the CBmeso data with those previously published at the eutrophic site CBeu (Romero and Fischer, 2017; Romero et al., 2020) and discuss (i) the effect of the giant chlorophyll filament and (ii) the impact of lateral advection from the shallow coastal area off Mauritania upon the hemi- and pelagic realms along the NE Atlantic Ocean.
Based on outstanding shifts in the species-specific composition of the diatom assemblage that occurred throughout the study (Figs. 2b and 3), we propose three main intervals in the multiyear evolution of populations and discuss them in view of major environmental forcings: (i) early 1988–late 1996 (gradually decreasing trend of coastal upwelling diatoms), (ii) 1997–1999 (highest contribution of diatoms typical of low- to moderate-productivity waters), and (iii) 2002–the middle of 2009 (major shift in the species-specific composition: extraordinary increase in and dominance of benthic diatoms).
Based on the long-term trends of our data and their statistical analysis
(Figs. 2–5), we propose that the three intervals reflect the response of the
diatom populations to the impact of low-frequency environmental forcing. As
described above in Sect. 4.2, the benthic diatom community appears positively
correlated with AMO (Fig. 6). Among low-frequency forcings affecting the
subtropical North Atlantic (see above Sect. 3.2), the AMO plays a key role in
determining decadal variations in SST and meridional circulation (e.g., Wang
and Zhang, 2013; Knight et al., 2005; McCarthy et al., 2015). It is widely
accepted that AMO is largely induced by AMOC variations and the associated
fluctuations of heat transport (Knight et al., 2005; Medhaug and Furevik,
2011; Wang and Zhang, 2013; McCarthy et al., 2015; details in Sect. 3.2.1). Using
observational data and model experiments, Wang and Zhang (2013) concluded that
the cooling of the subtropical North Atlantic (where the CBmeso is deployed)
is largely due to the meridional advection by the anomalous northward
current. The anomalous cooling appears below 100
An additional effect of the AMO impact is the significant long-term weakening
(strengthening) of the gyre during warm (cold) phase of AMO. This weakening
contributes to the anomalous northward MC in subsurface waters
(ca. 100–200
In addition to AMO forcing, the possible impact of NAO on the seasonal
dynamics of the biogenic silica (opal) fluxes and eolian input off
Mauritania has been previously discussed in Fischer et al. (2016). They
observed that winter biogenic silica fluxes had an increasing trend with an
increasing NAO index (Fischer et al., 2016). However, our statistical approach
(clustering and the Kruskal–Wallis tests) does not show any clear
relationship between each individual diatom group and the NAO index.
Nevertheless, the correlogram (Fig. 5) shows that the samples' scores of first
CA axis (Dim. 1, which discriminates the benthic diatoms from the other diatom
groups) seem to be impacted by the NAO, but with a low percentage of variance
explained (low
An extraordinary feature of the multiyear dynamics of diatom populations at
the CBmeso site is the sharp shift in the species contribution between May and June 2002 (Fig. 2b). The species shift leading to larger contribution of
benthic diatoms follows a two-step increase pattern: the first abrupt increase is observed in late May–early June 2002. The second increase occurs in winter 2006, with values mostly above 50
The intensification of the transport of AMOC intermediate waters during the
warm phase of the AMO (Wang and Zhang, 2013) might have also contributed to
the strengthening of lateral transport from subsurface shelf waters in the
Mauritanian offshore region. Earlier time-series studies at the CBmeso site
(Fischer et al., 2009, 2016) and observation-based model experiments conducted
along the Mauritanian upwelling (Helmke et al., 2005; Karakaş et al.,
2006; Nowald et al., 2015) already discussed the role of intermediate and deep
nepheloid layers in the lateral transport of particles and microorganisms
remains upon the pelagic realm. Based on the vigorous mixing in the uppermost
water column due to the confluence of northward and southward water masses and
strong, predominantly westward winds off Mauritania (Fig. 1; see Sect. 3.1),
the offshore transport from shallow into deeper waters is most intense between
20.5 and 23.5
CB8 and CB9, the traps temporally corresponding to the 1997–1999 ENSO and La Niña, were deployed at different depths (Table 1; see also above Sect. 2.1). Although this depth difference might have impacted on the total diatom flux (stronger dissolution in deeper waters, Romero and Armand, 2010; larger catchment area of lower traps, Fischer et al., 2016), the total diatom flux is low in both traps and hardly shows any dramatic increase or decrease with depth (CB9, Fig. 2a). Additionally, the species-specific composition of the diatom community shows a significant match between traps CB8 and CB9 (Fig. 2b). The dominance of open-ocean and coastal planktonic diatoms – common in waters of moderate- to low-nutrient conditions – matches well with the occurrence of low total diatom flux. This evidences that no significant difference in the record of diatom fluxes between the upper and lower traps occurred despite different depth deployments.
The long-term trends mainly determined by the low-frequency AMO (see
Sect. 5.1.1) were altered in the second half of the 1990s by the impact of the
strong 1997 ENSO (McPhaden, 1999). We postulate that both low coastal
upwelling diatom values (
A positive ENSO goes along with the weakening of E–NE winds off Mauritania
(Pradhan et al., 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,
thus hindering the mixing of the water column and causing upwelling intensity
off Mauritania to decrease until early 1998 (Pradhan et al.,
2006). Additionally, the size of the Mauritanian chlorophyll filament
decreased between winter 1997 and spring 1998, while becoming unusually large
from autumn 1998 to spring 1999 (Fischer et al., 2009). Complementary support
of this ENSO-mediated impact on surface water productivity off Mauritania is
provided by variations in bulk biogenic fluxes at the CBmeso site. The almost
2.5 times higher organic carbon flux during 1998–1999 than in 1997 (Helmke
et al., 2005) led to the proposal that, after weakening of wind intensity due to the
impact of ENSO on the physical setting, upwelling intensified immediately
afterward during La Niña (Fischer et al., 2016). Similarly, the seasonal
cycle of surface chl
ENSO has a significant global impact on the dynamics of primary producers via teleconnections (McPhaden, 1999; Levine et al., 2017). Aperiodic, pronounced decreases in the total diatom flux matching the occurrence of strong ENSOs in other ocean basins have been previously associated with limiting nutrient levels due to ENSO-derived perturbations. The diatom production in hemipelagic waters in the Chilean EBUE decreased extraordinarily during the strong 1997 ENSO compared to earlier years (Romero et al., 2001). Similar negative impacts linked to ENSO teleconnections have been proposed for other ocean areas, including the southern Californian EBUE (Lange et al., 2000), the Cariaco Basin (Romero et al., 2009b), the western Mediterranean Sea (Bárcena et al., 2004; Rigual-Hernández et al., 2013), and the subarctic Pacific Ocean (Takahashi, 1987).
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 (
Comparison of seasonal values of
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 (
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
This multiyear study of diatom populations' dynamics offers an overall picture of the long-term evolution of diatom-based productivity and fluxes and the response of the community to the interaction of high- and low-frequency hydrographic and atmospheric forcing in the mid-latitude northeastern Atlantic Ocean. A unique, persistent trend in the long-term evolution of the total diatom flux, either decreasing or increasing, is not recognized in our ca. 20-year record.
The statistical analysis supports the proposed scenario of AMO as an important driver of diatom populations' dynamics off Mauritania. The occurrence of cold (1988–1996) and warm AMO phases (2001–2009) is reflected by a major shift in species-specific composition. This overall trend is interrupted by the impact of the strong 1997 ENSO. Changes in the physical setting following the 1997 ENSO (weakening of E–NE trade winds, thermocline deepening, weakened water column mixing) negatively affected diatom production off Mauritania. Less evident is a possible impact of NAO.
Our CBmeso trap results allow corroboration that the abrupt shift in the assemblage composition occurred earlier off Mauritania (starting May 2002) than previously demonstrated (Romero and Fischer, 2017; Romero et al., 2020) and followed two steps. The two-step increase in benthic diatoms' contribution at the CBmeso site suggests that the intensification of the slope and shelf poleward undercurrents into the hemipelagic environment appears linked to the warm phase of AMO and the associated AMOC changes.
Diatom remains sink not only vertically off Mauritania, but they are also laterally advected from the shelf to the deeper waters via the nepheloid-layer-mediated transport. Transported valves (siliceous remains) from shallow coastal into deeper waters beyond the slope should be considered for the calculation and model experiments of nutrient budgets (especially Si) and the paleoenvironmental signal preserved in downcore sediments.
Understanding the degree of interannual to decadal variability in the Mauritania upwelling system is 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 (e.g., diatoms) and is especially important for establishing the scientific basis for forecasting and modeling future states of this ecosystem and its decadal changes.
Data are available at
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.
The authors declare that they have no conflict of interest.
We are greatly indebted to the captains and crews of the RVs
The article processing charges for this open-access publication were covered by the University of Bremen.
This paper was edited by Ny Riavo G. Voarintsoa and reviewed by Andres Rigual-Hernandez and one anonymous referee.