Long‐Term Trends in Phytoplankton Chlorophyll <i>a</i> and Size Structure in the Benguela Upwelling System

Lamont, Tarron; Barlow, Raymond G; Brewin, Robert J. W.

doi:10.1029/2018jc014334

Cited by 31 publications

(25 citation statements)

References 121 publications

(274 reference statements)

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“…A tendency toward increasing upwelling in the SBUS was noted by several studies 22 – 24 in recent years. Conversely, Lamont et al 25 reported a decreasing trend in chlorophyll in the southern Benguela shelf region by season over the last 20 years.…”

Section: Introductionmentioning

confidence: 93%

Oceanographic anomalies coinciding with humpback whale super-group occurrences in the Southern Benguela

Dey

Vichi

Fearon

et al. 2021

Sci Rep

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Seasonal feeding behaviour of humpback whales (Megaptera novaeangliae) has been observed in the coastal waters of the Southern Benguela where the species has been observed forming super-groups during the austral spring in recent years since 2011. Super-groups are unprecedented densely-packed aggregations of between 20 and 200 individuals in low-latitude waters and their occurrences indicate possible changes in feeding behaviour of the species. We accessed published data on super-groups occurrence in the study area in 2011, 2014 and 2015, and investigated oceanographic drivers that support prey availability in this region. We found that enhanced primary production is a necessary but not sufficient condition for super-groups to occur. Positive chlorophyll anomalies occurring one month prior to the super-group occurrences were identified, but only a concurrent significantly reduced water volume export from the region throughout October were conducive to the aggregations in the specific years. Hydrodynamic model results attributed the anomalous decreased volume export to the strength and orientation of the Goodhope Jet and associated eddy activity. The combination of random enhanced primary production typical of the region and emerging anomalous conditions of reduced water export in October since 2011 resulted in favourable food availability leading to the unique humpback whale aggregations. The novelty of this grouping behaviour is indicative of the lack of such oceanographic conditions in the past. Given the recency of the events, it is difficult to attribute this reduction in ocean transport to climatic regime shifts, and the origin should be likely investigated in the distant water mass interaction with the greater Agulhas system rather than in local intensifications of the upwelling conditions. A positive trend in the humpback whale population abundance points to the need to monitor the exposure of the species to the changing climate conditions.

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Section: Introductionmentioning

confidence: 93%

Oceanographic anomalies coinciding with humpback whale super-group occurrences in the Southern Benguela

Dey

Vichi

Fearon

et al. 2021

Sci Rep

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“…Yet, its strength could be weakened with the predicted decrease in upwelling intensity in the NBUS 47,48 . Whether the predicted enhancement in upwelling of the southern system 36,49 could however compensate for the lowered biological carbon pump efficiency of the NBUS, remains open for debate. Thus, in addition to the Southern Ocean, also a better understanding of the BUSs response to climate change is essential to predict the CO2 uptake efficiency of the biological carbon pump and the future evolution of CO2 in the atmosphere.…”

Section: Air-sea Co2 Flux Estimatesmentioning

confidence: 99%

“…This is also reflected in primary productivity which in the SBUS is twice as large in the summer than winter 61 . Furthermore, studies have shown a decrease and intensification of upwelling in the NBUS and SBUS, respectively, over the past decade in response to global warming 36,49 and a southward shift of the SAA 47 . The associated increase in sea water temperatures in the NBUS 62 could potentially be related to the prominent decrease in zooplankton size spectra in the BUS 63 .…”

Section: Study Regionmentioning

confidence: 99%

Biological carbon pump affected by CO2 uptake in the Benguela Upwelling System

Siddiqui¹,

Rixen²,

Lahajnar³

et al. 2021

Preprint

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Eastern Boundary Upwelling Systems (EBUS) are well-known for their high productivity and fishery yields. However, being scarcely sampled and poorly represented in global models, their role as CO2 sources and sinks to the atmosphere remains elusive. Here, we present a compilation of shipboard measurements over the past two decades, showing how the Benguela Upwelling System (BUS) in the southeast Atlantic Ocean acts as a CO2 source in the north and CO2 sink in the south. Surface warming of upwelled waters increases the partial pressure of CO2 (pCO2) and outgassing in both regions, but in the south, the biologically-mediated drawdown of CO2 exceeds this warming effect. Here, the biological carbon pump owes its stronger impact on pCO2 to higher shares of upwelling source waters carrying preformed nutrients supplied from the Southern Ocean. Their formation increases pCO2 in surface waters and counteracts human-induced invasion of CO2 in the Southern Ocean. However, their utilization in the BUS compensates for over 20% of the CO2 loss occurring in the Atlantic sector of the Southern Ocean. This emphasizes the role of the BUS as key to improve our understanding of the ocean’s response to climate change and the future evolution of CO2 in the atmosphere.

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“…During the last two decades satellite data indicate no significant trends in productivity (Demarcq, 2009;Verheye et al, 2016;Lamont et al, 2019) but there is a tendency toward an intensification of upwelling in the SBUS (Sydeman et al, 2014;Tim et al, 2015;Wang et al, 2015;Lamont et al, 2018Lamont et al, , 2019. Associated with this trend are shifts in the ecosystem structure such as an overall decrease in zooplankton abundance and a shift from large to smaller zoo-and phytoplankton species (Hutchings et al, 2012;Jarre et al, 2015;Verheye et al, 2016;Lamont et al, 2019).…”

Section: Study Areamentioning

confidence: 99%

Oxygen and Nutrient Trapping in the Southern Benguela Upwelling System

et al. 2021

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The Benguela Upwelling System in the southeast Atlantic Ocean is of crucial socio-economic importance due to its high productivity. However, predicting its response to global change and understanding past changes are still great challenges. Here, we compile data obtained from a research cruise and an oceanographic mooring to demonstrate that a topographically steered nutrient trapping zone develops in a narrow belt along the coast during the main upwelling season in austral spring and summer in the southern Benguela Upwelling System. High nutrient concentrations within this zone increase the impact of upwelling on the productivity of the southern Benguela Upwelling System, but the efficient nutrient trapping operates at the expense of decreasing oxygen concentrations. This enhances the probability of anoxic events emerging toward the end of the upwelling season. However, at the end of the upwelling season, the front that separates the coastally trapped waters from open shelf waters weakens or even collapses due to upwelling cessation and the reversing current regime. This, in addition to a stronger vertical mixing caused by winter cooling, fosters the ventilation of the nutrient trapping zone, which reestablishes during the following upwelling season. The postulated intensification of upwelling and changes in the ecosystem structure in response to global warming seem to reduce the nutrient trapping efficiency by increasing offshore advection of surface waters and plankton blooms. The intensified upwelling and resulting lower biological oxygen consumption appears to mask the expected impacts of global warming on the oxygen minimum zone (OMZ) in the southern Benguela Upwelling System. In contrast to other OMZs, including those in northern Benguela Upwelling Systems, the OMZ in the southern Benguela Upwelling System reveals so far no detectable long-term decrease in oxygen. Thus, the nutrient trapping efficiency seems to be a critical feature mitigating global change impacts on the southern Benguela Upwelling System. Since it is topographically steered, regional impacts on the nutrient trapping efficiency appear also to explain varying responses of upwelling systems to global change as the comparison between southern and northern Benguela Upwelling System shows. This emphasizes the need for further and more comparable studies in order to better understand the response of Eastern Boundary Upwelling Systems and their ecosystem services to global change.

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Long‐Term Trends in Phytoplankton Chlorophyll a and Size Structure in the Benguela Upwelling System

Cited by 31 publications

References 121 publications

Oceanographic anomalies coinciding with humpback whale super-group occurrences in the Southern Benguela

Oceanographic anomalies coinciding with humpback whale super-group occurrences in the Southern Benguela

Biological carbon pump affected by CO2 uptake in the Benguela Upwelling System

Oxygen and Nutrient Trapping in the Southern Benguela Upwelling System

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