Theory of the Benguela Upwelling System

Fennel, Wolfgang

doi:10.1175/1520-0485(1999)029<0177:totbus>2.0.co;2

Cited by 68 publications

(43 citation statements)

References 15 publications

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“…Shannon, 1985;Nelson and Hutchings, 1987;Lutjeharms and Valentine, 1987;Fennel, 1999;Mohrholz et al, 2001;Monteiro et al, 2008). There are several upwelling cells along the coast, the Lüderitz cell (at about 27 • S) is believed to be the most intense upwelling cell found in any of the world's upwelling systems, and effectively divides the system into a northern and a southern part (Duncombe Rae, 2005).…”

Section: Northern Benguela Upwelling Systemmentioning

confidence: 99%

Impacts of hypoxia on the structure and processes in pelagic communities (zooplankton, macro-invertebrates and fish)

et al. 2010

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Abstract. Dissolved oxygen (DO) concentration in the water column is an environmental parameter that is crucial for the successful development of many pelagic organisms. Hypoxia tolerance and threshold values are species-and stagespecific and can vary enormously. While some fish species may suffer from oxygen values of less than 3 mL O 2 L −1 through impacted growth, development and behaviour, other organisms such as euphausiids may survive DO levels as low as 0.1 mL O 2 L −1 . A change in the average or the range of DO may have significant impacts on the survival of certain species and hence on the species composition in the ecosystem with consequent changes in trophic pathways and productivity.Evidence for the deleterious effects of oxygen depletion on pelagic species is scarce, particularly in terms of the effect of low oxygen on development, recruitment and patterns of migration and distribution. While planktonic organisms have to cope with variable DOs and exploit adaptive mechanisms, nektonic species may avoid areas of unfavourable DO and develop adapted migration strategies. Planktonic organisms may only be able to escape vertically, above or beneath the Oxygen Minimum Zone (OMZ). In shallow areas only the surface layer can serve as a refuge, but in deep waters many organisms have developed vertical migration strategies to use, pass through and cope with the OMZ.Correspondence to: W. Ekau (wekau@zmt-bremen.de) This paper elucidates the role of DO for different taxa in the pelagic realm and the consequences of low oxygen for foodweb structure and system productivity. We describe processes in two contrasting systems, the semi-enclosed Baltic Sea and the coastal upwelling system of the Benguela Current to demonstrate the consequences of increasing hypoxia on ecosystem functioning and services.

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Section: Northern Benguela Upwelling Systemmentioning

confidence: 99%

Impacts of hypoxia on the structure and processes in pelagic communities (zooplankton, macro-invertebrates and fish)

et al. 2010

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“…Due to a positive stability of the water column, additional energy is needed to uplift the respective water mass. For this reason, it is not possible to define a simple balance as for the Benguela upwelling system and the upwelling system Off Oregon, where Ekman suction induced by wind stress curl was the major contributor to the total upward velocity (Halpern 1976;Fennel 1999). Here, we use a schematic plot to describe our conceptual model to roughly estimate the vertical water fluxes (Fig.…”

Section: Estimation the Vertical Transportmentioning

confidence: 99%

The influence of meteorological variation on the upwelling system off eastern Hainan during summer 2007–2008

Su¹,

Wang

Pohlmann³

et al. 2011

Ocean Dynamics

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The influence of meteorological variation, i.e., typhoon and precipitation events, on the coastal upwelling off the eastern Hainan Island was studied based on observations taken during two upwelling seasons. The observations were made in August 2007 and July 2008, respectively. We found that, in principle, similar structure of sea surface temperature and bottom temperature prevailed in both observational periods, providing evidence that upwelling events occur frequently during the summer monsoon along the eastern Hainan shelf. Based on a simple momentum balance theory, we studied the balances between momentum fluxes, wind stress, and bottom stress. The results showed that the Burger number is S ≈ 1, indicating that the crossshelf momentum flux divergence was balanced by the wind stress and the onshore return flow occurred in the interior of the water column. Hence, a conceptual model of the upwelling structure was built for further understanding of upwelling events. In addition, it was also observed that variations in the strength of upwelling are controlled by storm events, i.e., strong northerly winds change the structure of the thermocline on the shelf significantly. The strong mixing caused by wind reduces the strength of the thermocline, in particular in coastal seas. Based on our conceptual model, a frontal zone between mixed coastal water and offshore water develops which destabilizing the water column and hence decreases the upwelling strength. Freshwaters from the two main rivers in the Wenchang Bay are confined to the coastal area less than 20-30 m deep, as confirmed by our water mass analysis. Freshwater discharge stabilized the water column, inhibiting the upwelling as shown by the potential energy calculation. Consequently, estuarine water only inhibits the upwelling in the near coastal area. Therefore, it can be concluded that estuarine water does not have a significant impact on upwelling strength on the shelf.

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“…The cross-shelf compensation current of ESACW reacts instantaneously to changing tradewind intensity and is immediately interrupted when the trade winds stop. In other words, the most important oxygen source for waters overlying the shelf is directly coupled to fluctuations in the coast-parallel component of the wind field on the time-scale of hours and/ or few days (Fennel 1999). Only the poleward undercurrent, which has a longer response time of weeks to months, remains to counteract oxygen consumption in the water column and the sediment.…”

Section: Hydrography and Oxygen Balancementioning

confidence: 99%

Variability in upwelling intensity and nutrient regime in the coastal upwelling system offshore Namibia: results from sediment archives

Emeis

Struck

Leipe

et al. 2007

Int J Earth Sci (Geol Rundsch)

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The coastal upwelling system offshore Namibia is ideally suited to address a focal question of the Integrated Marine Biogeochemistry and Ecosystem Research Programme: what are the mechanisms that drive long-term changes in ecosystems? Considerable interannual variability in climatic forcing is indicated by long time series of meteorological and remote sensing observation; these accompany considerable interannual to interdecadal changes in the upwelling intensity over the last 100 years, as well as a centennial trend. On longer time scales, the only archives available are sediment records spanning the late Holocene. To decipher the sediment record, we mapped surface-sediment patterns of proxies for physical (sea surface temperature/SST from alkenone unsaturation indexes) and nutrient (d 15 N on bulk sedimentary N) variables. Their present-day surface-sediment patterns outline the coastal upwelling cells and filaments and associated high productivity area. Analysed in an array of dated sediment cores, the spatial patterns of SST suggest long-term ([100 years) variability in the location and intensity of individual upwelling cells. The patterns of d 15 N outline an area of intense denitrification near the coast, and advection of water with low-oxygen concentrations in the undercurrent from the North. d 15 N exhibits considerable downcore variability, in particular over the last 50 years. The variability appears to be governed by differences in extent of denitrification and thus of the shelf oxygen balance, which appears to have deteriorated in the last 50 years. Together, the data suggest that SST and denitrification conditions have remained in the narrow bounds outlined by today's patterns in surface sediments, but that spatially small variability in upwelling intensity and make-up of upwelling feed waters induced considerable changes in the lower trophic levels of the coastal upwelling ecosystem over the last 6,000 years. Attempts to correlate proxy records from sediments with observational time series and regional climate reconstructions were not successful, possibly because annual to interannual environmental signals are erased in the process of sediment formation.

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Theory of the Benguela Upwelling System

Cited by 68 publications

References 15 publications

Impacts of hypoxia on the structure and processes in pelagic communities (zooplankton, macro-invertebrates and fish)

Impacts of hypoxia on the structure and processes in pelagic communities (zooplankton, macro-invertebrates and fish)

The influence of meteorological variation on the upwelling system off eastern Hainan during summer 2007–2008

Variability in upwelling intensity and nutrient regime in the coastal upwelling system offshore Namibia: results from sediment archives

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