Changes in water column oxygen, estimates of productivity and the development of anoxia in a major embayment of the southern Benguela eastern boundary upwelling system

2021 ◽  
pp. 103694
Author(s):  
G.C. Pitcher ◽  
T.A. Probyn ◽  
A. du Randt
Keyword(s):  
Water Column ◽  
Upwelling System ◽  
Eastern Boundary

scholarly journals Variations in mid-latitude North Atlantic surface water properties during the mid-Brunhes (MIS 9–14) and their implications for the thermohaline circulation

2010 ◽  
Vol 6 (4) ◽  
pp. 531-552 ◽  
Author(s):  
A. H. L. Voelker ◽  
T. Rodrigues ◽  
K. Billups ◽  
D. Oppo ◽  
J. McManus ◽  
...  
Keyword(s):  
Surface Water ◽  
North Atlantic ◽  
Current System ◽  
Middle Pleistocene ◽  
Boundary Current ◽  
Upwelling System ◽  
The North ◽  
Eastern Boundary ◽  
Stable Conditions ◽  
Hydrographic Conditions

Abstract. Stable isotope and ice-rafted debris records from three core sites in the mid-latitude North Atlantic (IODP Site U1313, MD01-2446, MD03-2699) are combined with records of ODP Sites 1056/1058 and 980 to reconstruct hydrographic conditions during the middle Pleistocene spanning Marine Isotope Stages (MIS) 9–14 (300–540 ka). Core MD03-2699 is the first high-resolution mid-Brunhes record from the North Atlantic's eastern boundary upwelling system covering the complete MIS 11c interval and MIS 13. The array of sites reflect western and eastern basin boundary current as well as north to south transect sampling of subpolar and transitional water masses and allow the reconstruction of transport pathways in the upper limb of the North Atlantic's circulation. Hydrographic conditions in the surface and deep ocean during peak interglacial MIS 9 and 11 were similar among all the sites with relative stable conditions and confirm prolonged warmth during MIS 11c also for the mid-latitudes. Sea surface temperature (SST) reconstructions further reveal that in the mid-latitude North Atlantic MIS 11c is associated with two plateaus, the younger one of which is slightly warmer. Enhanced subsurface northward heat transport in the eastern boundary current system, especially during early MIS 11c, is denoted by the presence of tropical planktic foraminifer species and raises the question how strongly it impacted the Portuguese upwelling system. Deep water ventilation at the onset of MIS 11c significantly preceded surface water ventilation. Although MIS 13 was generally colder and more variable than the younger interglacials the surface water circulation scheme was the same. The greatest differences between the sites existed during the glacial inceptions and glacials. Then a north – south trending hydrographic front separated the nearshore and offshore waters off Portugal. While offshore waters originated from the North Atlantic Current as indicated by the similarities between the records of IODP Site U1313, ODP Site 980 and MD01-2446, nearshore waters as recorded in core MD03-2699 derived from the Azores Current and thus the subtropical gyre. Except for MIS 12, Azores Current influence seems to be related to eastern boundary system dynamics and not to changes in the Atlantic overturning circulation.

scholarly journals Diatoms as a paleoproductivity proxy in the NW Iberian coastal upwelling system (NE Atlantic)

2017 ◽  
Vol 14 (5) ◽  
pp. 1165-1179 ◽  
Author(s):  
Diana Zúñiga ◽  
Celia Santos ◽  
María Froján ◽  
Emilia Salgueiro ◽  
Marta M. Rufino ◽  
...  
Keyword(s):  
Water Column ◽  
Surface Sediment ◽  
Sediment Trap ◽  
Coastal Upwelling ◽  
Late Summer ◽  
Early Summer ◽  
Marine Diatom ◽  
Photic Zone ◽  
Upwelling System ◽  
Water Column Stratification

Abstract. The objective of the current work is to improve our understanding of how water column diatom's abundance and assemblage composition is seasonally transferred from the photic zone to seafloor sediments. To address this, we used a dataset derived from water column, sediment trap and surface sediment samples recovered in the NW Iberian coastal upwelling system. Diatom fluxes (2.2 (±5.6) 106 valves m−2 d−1) represented the majority of the siliceous microorganisms sinking out from the photic zone during all studied years and showed seasonal variability. Contrasting results between water column and sediment trap diatom abundances were found during downwelling periods, as shown by the unexpectedly high diatom export signals when diatom-derived primary production achieved their minimum levels. They were principally related to surface sediment remobilization and intense Minho and Douro river discharge that constitute an additional source of particulate matter to the inner continental shelf. In fact, contributions of allochthonous particles to the sinking material were confirmed by the significant increase of both benthic and freshwater diatoms in the sediment trap assemblage. In contrast, we found that most of the living diatom species blooming during highly productive upwelling periods were dissolved during sinking, and only those resistant to dissolution and the Chaetoceros and Leptocylindrus spp. resting spores were susceptible to being exported and buried. Furthermore, Chaetoceros spp. dominate during spring–early summer, when persistent northerly winds lead to the upwelling of nutrient-rich waters on the shelf, while Leptocylindrus spp. appear associated with late-summer upwelling relaxation, characterized by water column stratification and nutrient depletion. These findings evidence that the contributions of these diatom genera to the sediment's total marine diatom assemblage should allow for the reconstruction of different past upwelling regimes.

scholarly journals The Benguela Upwelling System: Quantifying the Sensitivity to Resolution and Coastal Wind Representation in a Global Climate Model*

2015 ◽  
Vol 28 (23) ◽  
pp. 9409-9432 ◽  
Author(s):  
R. Justin Small ◽  
Enrique Curchitser ◽  
Katherine Hedstrom ◽  
Brian Kauffman ◽  
William G. Large
Keyword(s):  
Wind Stress ◽  
Climate Model ◽  
Coastal Upwelling ◽  
Ocean Model ◽  
Wind Stress Curl ◽  
Upwelling System ◽  
Wind Structure ◽  
Eastern Boundary ◽  
Benguela Upwelling ◽  
Atmosphere Model

Abstract Of all the major coastal upwelling systems in the world’s oceans, the Benguela, located off southwest Africa, is the one that climate models find hardest to simulate well. This paper investigates the sensitivity of upwelling processes, and of sea surface temperature (SST), in this region to resolution of the climate model and to the offshore wind structure. The Community Climate System Model (version 4) is used here, together with the Regional Ocean Modeling System. The main result is that a realistic wind stress curl at the eastern boundary, and a high-resolution ocean model, are required to well simulate the Benguela upwelling system. When the wind stress curl is too broad (as with a 1° atmosphere model or coarser), a Sverdrup balance prevails at the eastern boundary, implying southward ocean transport extending as far as 30°S and warm advection. Higher atmosphere resolution, up to 0.5°, does bring the atmospheric jet closer to the coast, but there can be too strong a wind stress curl. The most realistic representation of the upwelling system is found by adjusting the 0.5° atmosphere model wind structure near the coast toward observations, while using an eddy-resolving ocean model. A similar adjustment applied to a 1° ocean model did not show such improvement. Finally, the remote equatorial Atlantic response to restoring SST in a broad region offshore of Benguela is substantial; however, there is not a large response to correcting SST in the narrow coastal upwelling zone alone.

scholarly journals Spatial and temporal variability in coccolithophore abundance and distribution in the NW Iberian coastal upwelling system

2018 ◽  
Vol 15 (1) ◽  
pp. 245-262 ◽  
Author(s):  
Blanca Ausín ◽  
Diana Zúñiga ◽  
Jose A. Flores ◽  
Catarina Cavaleiro ◽  
María Froján ◽  
...  
Keyword(s):  
Water Column ◽  
Environmental Conditions ◽  
Temporal Variability ◽  
Coastal Upwelling ◽  
Spatial And Temporal Variability ◽  
Upwelling System ◽  
Subsurface Waters ◽  
Shelf Station ◽  
Abundance And Distribution ◽  
Upwelling Event

Abstract. A systematic investigation of the spatial and temporal variability in coccolithophore abundance and distribution through the water column of the NW Iberian coastal upwelling system was performed. From July 2011 to June 2012, monthly sampling at various water depths was conducted at two parallel stations located at 42∘ N. Total coccosphere abundance was higher at the outer-shelf station, where warmer, nutrient-depleted waters favoured coccolithophore rather than phytoplanktonic diatom blooms, which are known to dominate the inner-shelf location. In seasonal terms, higher coccosphere and coccolith abundances were registered at both stations during upwelling seasons, coinciding with high irradiance levels. This was typically in conjunction with stratified, nutrient-poor conditions (i.e. relaxing upwelling conditions). However, it also occurred during some upwelling events of colder, nutrient-rich subsurface waters onto the continental shelf. Minimum abundances were generally found during downwelling periods, with unexpectedly high coccolith abundance registered in subsurface waters at the inner-shelf station. This finding can only be explained if strong storms during these downwelling periods favoured resuspension processes, thus remobilizing deposited coccoliths from surface sediments, and hence hampering the identification of autochthonous coccolithophore community structure. At both locations, the major coccolithophore assemblages were dominated by Emiliania huxleyi, small Gephyrocapsa group, Gephyrocapsa oceanica, Florisphaera profunda, Syracosphaera spp., Coronosphaera mediterranea, and Calcidiscus leptoporus. Ecological preferences of the different taxa were assessed by exploring the relationships between environmental conditions and temporal and vertical variability in coccosphere abundance. These findings provide relevant information for the use of fossil coccolith assemblages in marine sediment records, in order to infer past environmental conditions, of particular importance for Paleoceanography. Both E. huxleyi and the small Gephyrocapsa group are proposed as proxies for the upwelling regime with a distinct affinity for different stages of the upwelling event: E. huxleyi was associated with warmer, nutrient-poor and more stable water column (i.e. upwelling relaxation stage) while the small Gephyrocapsa group was linked to colder waters and higher nutrient availability (i.e. early stages of the upwelling event), similarly to G. oceanica. Conversely, F. profunda is suggested as a proxy for the downwelling regime and low-productivity conditions. The assemblage composed by Syracosphaera pulchra, Coronosphaera mediterranea, and Rhabdosphaera clavigera may be a useful indicator of the presence of subtropical waters conveyed northward by the Iberian Poleward Current. Finally, C. leptoporus is proposed as an indicator of warmer, saltier, and oligotrophic waters during the downwelling/winter regime.

scholarly journals Bacteriohopanetetrol-<i>x</i>: constraining its application as a lipid biomarker for marine anammox using the water column oxygen gradient of the Benguela upwelling system

2022 ◽  
Vol 19 (1) ◽  
pp. 201-221
Author(s):  
Zoë R. van Kemenade ◽  
Laura Villanueva ◽  
Ellen C. Hopmans ◽  
Peter Kraal ◽  
Harry J. Witte ◽  
...  
Keyword(s):  
16S Rrna ◽  
Water Column ◽  
16S Rrna Genes ◽  
Rrna Genes ◽  
Ammonium Oxidation ◽  
Oxygen Gradient ◽  
Upwelling System ◽  
Benguela Upwelling ◽  
Ladderane Lipids

Abstract. Interpreting lipid biomarkers in the sediment archive requires a good understanding of their application and limitations in modern systems. Recently it was discovered that marine bacteria performing anaerobic ammonium oxidation (anammox), belonging to the genus Ca. Scalindua, uniquely synthesize a stereoisomer of bacteriohopanetetrol (“BHT-x”). The ratio of BHT-x over total bacteriohopanetetrol (BHT, ubiquitously synthesized by diverse bacteria) has been suggested as a proxy for water column anoxia. As BHT has been found in sediments over 50 Myr old, BHT-x has the potential to complement and extend the sedimentary biomarker record of marine anammox, conventionally constructed using ladderane lipids. Yet, little is known about the distribution of BHT-x in relation to the distribution of ladderanes and to the genetic evidence of Ca. Scalindua in modern marine systems. Here, we investigate the distribution of BHT-x and the application of the BHT-x ratio in relation to distributions of ladderane intact polar lipids (IPLs), ladderane fatty acids (FAs) and Ca. Scalindua 16S rRNA genes in suspended particulate matter (SPM) from the water column of the Benguela upwelling system (BUS), sampled across a large oxygen gradient. In BUS SPM, high BHT-x abundances were restricted to the oxygen-deficient zone on the continental shelf (at [O2] < 45 µmol L−1, in all but one case). High BHT-x abundances co-occurred with high abundances of the Ca. Scalindua 16S rRNA gene (relative to the total number of bacterial 16S rRNA genes) and ladderane IPLs. At shelf stations with [O2] > 50 µmol L−1, the BHT-x ratio was < 0.04 (in all but one case). In apparent contradiction, ladderane FAs and low abundances of BHT and BHT-x (resulting in BHT-x ratios > 0.04) were also detected in oxygenated offshore waters ([O2] up to 180 µmol L−1), whereas ladderane IPLs were undetected. The index of ladderane lipids with five cyclobutane rings (NL5) correlates with in situ temperature. NL5-derived temperatures suggested that ladderane FAs in the offshore waters were not synthesized in situ but were transported down-slope from warmer shelf waters. Thus, in sedimentary archives of systems with known lateral organic matter transport, such as the BUS, relative BHT and BHT-x abundances should be carefully considered. In such systems, a higher BHT-x ratio may act as a safer threshold for deoxygenation and/or Ca. Scalindua presence: our results and previous studies indicate that a BHT-x ratio of ≥ 0.2 is a robust threshold for oxygen-depleted waters ([O2] < 50 µmol kg−1). In our data, ratios of ≥ 0.2 coincided with Ca. Scalindua 16S rRNA genes in all samples (n=62), except one. Lastly, when investigating in situ anammox, we highlight the importance of using ladderane IPLs over BHT-x and/or ladderane FAs; these latter compounds are more recalcitrant and may derive from transported fossil anammox bacteria remnants.

scholarly journals Diatoms as paleoproductivity proxy in the NW Iberian coastal upwelling system (NE Atlantic)

2016 ◽  
Author(s):  
Diana Zúñiga ◽  
Celia Santos ◽  
María Froján ◽  
Emilia Salgueiro ◽  
Marta M. Rufino ◽  
...  
Keyword(s):  
Water Column ◽  
Sediment Trap ◽  
Coastal Upwelling ◽  
High Irradiance ◽  
Photic Zone ◽  
Upwelling System ◽  
Water Column Stratification ◽  
Resting Spores ◽  
Seafloor Sediments ◽  
Export Signal

Abstract. The objective of the current work is to better understand how diatoms species determine primary production signal in exported and buried particles. We evaluated how the diatom’s abundance and assemblage composition is transferred from the photic zone into the seafloor sediments. A combined analysis of water column, sediment trap and surface sediment samples recovered in the NW Iberian coastal upwelling system was used. Diatom fluxes (2.2 ± 5.6 106 # valves m−2 d−1) represented the majority of the siliceous microorganisms sinking out from the photic zone and showed strong seasonal variability. During downwelling seasons, diatoms export signal was strongly affected by resuspension of bottom sediments and intense Minho and Douro riverine inputs, with benthic and freshwater diatoms (17–24 %) becoming relevant in the sediment trap assemblage. Nevertheless, during upwelling productive seasons, the diatoms exported out from surface layer reflected water column diatom assemblage. They were principally represented by Chaetoceros spp. (mean 46 ± 25 %) and Leptocylindrus spp. (mean 20 ± 22 %) resting spores, demonstrating that both groups are a good sedimentary imprint during highly productive periods. Moreover, our data showed that the sink of Chaetoceros spp. resting spores dominated under persistent upwelling winds, high irradiance levels and cold and nutrient-rich waters. Otherwise, Leptocylindrus spp. spore fluxes were favoured when northerly winds relax, and surface waters warming promotes water column stratification. Further, this finding will provide a proxy of persistent vs. intermittent upwelling conditions, which is of particular relevance in palaeoceanography.

The Canary Intermediate Poleward Undercurrent: not another Poleward Undercurrent in an Eastern Boundary Upwelling System

2021 ◽  
Author(s):  
P. Vélez-Belchí ◽  
V. Caínzos ◽  
E. Romero ◽  
M. Casanova-Masjoan ◽  
C. Arumí-Planas ◽  
...  
Keyword(s):  
Pressure Gradient ◽  
Ocean Circulation ◽  
Seasonal Variability ◽  
Meridional Overturning Circulation ◽  
Driving Mechanism ◽  
Ekman Pumping ◽  
Model Simulations ◽  
Upwelling System ◽  
The North ◽  
Eastern Boundary

AbstractPoleward undercurrents are well-known features in Eastern Boundary upwelling systems. In the California Current upwelling system, the California poleward undercurrent has been widely studied, and it has been demonstrated that it transports nutrients from the equatorial waters to the northern limit of the subtropical gyre. However, in the Canary Current upwelling system, the Canary intermediate poleward undercurrent (CiPU) has not been properly characterized, despite recent studies arguing that the dynamics of the eastern Atlantic play an important role in the Atlantic meridional overturning circulation, specifically on its seasonal cycle. Here, we use trajectories of Argo floats and model simulations to characterize the CiPU, including its seasonal variability and its driving mechanism. The Argo observations show that the CiPU flows from 26°N, near Cape Bojador, to approximately 45°N, near Cape Finisterre, and flows deeper than any poleward undercurrent in other eastern boundaries, with a core at a mean depth of around 1000 dbar. Model simulations manifest that the CiPU is driven by the meridional alongshore pressure gradient due to general ocean circulation and, contrary to what is observed in the other eastern boundaries, is still present at 1000 dbar due the pressure gradient between the Antarctic Intermediate Waters in the south and Mediterranean Outflow waters in the north. The high seasonal variability of the CiPU, with its maximum strength in fall, and the minimum in spring, is due to the poleward extension of AAIW, forced by Ekman pumping in the tropics.

Do toxic Pseudo-nitzschia species pose a threat to aquaculture in the southern Benguela eastern boundary upwelling system?

Harmful Algae ◽  
2020 ◽  
Vol 99 ◽  
pp. 101919
Author(s):  
GC Pitcher ◽  
AD Cembella ◽  
B Krock ◽  
BM Macey ◽  
L Mansfield
Keyword(s):  
Upwelling System ◽  
Eastern Boundary

scholarly journals Bacteriohopanetetrol-x: constraining its application as a lipid biomarker for marine anammox using the water column oxygen gradient of the Benguela upwelling system

2021 ◽  
Author(s):  
Zoë Rebecca van Kemenade ◽  
Laura Villanueva ◽  
Ellen C. Hopmans ◽  
Peter Kraal ◽  
Harry J. Witte ◽  
...  
Keyword(s):  
16S Rrna ◽  
Water Column ◽  
16S Rrna Genes ◽  
Rrna Genes ◽  
Ammonium Oxidation ◽  
Oxygen Gradient ◽  
Upwelling System ◽  
Benguela Upwelling ◽  
Ladderane Lipids

Abstract. Interpreting lipid biomarkers in the sediment archive requires a good understanding of their application and limitations in modern systems. Recently it was discovered that marine bacteria performing anaerobic ammonium oxidation (anammox), belonging to the genus Ca. Scalindua, uniquely synthesize a stereoisomer of bacteriohopanetetrol (‘BHT-x’). The ratio of BHT-x over total bacteriohopanetetrol (BHT; ubiquitously synthesized by diverse bacteria) has been suggested as a proxy for water column anoxia. As BHT has been found in sediments over 50 Myr old, BHT-x has the potential to complement and extend the sedimentary biomarker record of marine anammox, conventionally constructed using ladderane lipids. Yet, little is known about the distribution of BHT-x in relation to the distribution of ladderanes and to the genetic evidence of Ca. Scalindua in modern marine systems. Here, we investigate the distribution of BHT-x and the application of the BHT-x ratio in relation to distributions of intact polar (IPL) ladderane lipids, ladderane fatty acids (FAs) and Ca. Scalindua 16S rRNA genes in suspended particulate matter (SPM) from the water column, sampled across a large oxygen gradient in the Benguela upwelling system (BUS). In BUS SPM, high BHT-x abundances were constrained to the oxygen deficient zone on the continental shelf (at [O2] < 45 µmol L−1, in all but one case). High BHT-x abundances co-occurred with high abundances of the Ca. Scalindua 16S rRNA gene (relative to the total number of bacterial 16S rRNA genes) and ladderane IPLs. At shelf stations with [O2] > 50 µmol L−1, the BHT-x ratio was < 0.04 (in all but one case). In apparent contradiction, ladderane FAs and low abundances of BHT and BHT-x (resulting in BHT-x ratio’s > 0.04) were also detected in oxygenated offshore waters ([O2] up to 180 µmol L−1), whereas ladderane IPLs were undetected. NL5-derived temperatures suggested that ladderane FAs in the offshore waters were not synthesized in situ but derived from warmer shelf waters. Thus, in sedimentary archives of systems with known lateral organic matter transport, such as the BUS, relative BHT and BHT-x abundances should be carefully considered. In such systems, a higher BHT-x ratio may act as a safer threshold for deoxygenation and/or Ca. Scalindua presence: in the BUS, at [O2] > 50 µmol L−1, the BHT-x ratio was < 0.18 at both off -and onshore sites (in all but one case) and a ratio > 0.18 corresponded in all cases (except one) with the presence of Ca. Scalindua 16S rRNA genes. Lastly, when investigating in situ anammox, we highlight the importance of using ladderane IPLs over BHT-x and/or ladderane FAs; these latter compounds are more recalcitrant and may derive from transported fossil anammox bacteria remnants.

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