Seasonality in the oxygen minimum layers at the extremities of the Benguela system

Chapman, Piers; Shannon, L. V.

doi:10.2989/025776187784522162

Cited by 57 publications

(54 citation statements)

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“…[15] Wind has been known to be an important controlling factor for dissolved oxygen concentration for open upwelling systems [Chapman and Shannon, 1987;Grantham et al, 2004;Sobarzo et al, 2007]. The timing for the upwelling wind start and the intensity of the upwelling wind stress control the strength of the on-shore advection of oxygenpoor and nutrient-rich deep water, and thereby the bottom oxygen concentration on the continental shelf [Barth et al, 2007;Chan et al, 2008].…”

Section: Discussionmentioning

confidence: 99%

Relative role of wind forcing and riverine nutrient input on the extent of hypoxia in the northern Gulf of Mexico

Feng¹,

DiMarco

Jackson

2012

Geophysical Research Letters

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[1] Seasonal hypoxia of the northern Gulf of Mexico has been observed for more than 25 years. It is generally accepted that the variation in the areal extent of hypoxia is determined by changes in nutrient addition from the Mississippi River. In this study, we investigate the statistical relation between the hypoxic area and a new variable, the duration of west wind, using the available measurements for the period 1985-2010. Special consideration was paid to the 1993-2010 period, a time when a large shift in the seasonal hypoxia pattern has been reported. When excluding the years in which hurricanes directly impacted the hypoxic area observation, we find that the duration of west wind is correlated with the hypoxic area at r 2 = 0.32 for the 1985-2010 period, and r 2 = 0.52 for the 1993-2010 period. Multilinear regressions using both wind duration and May-June nitrate loading improve the statistical relationships for both periods to r 2 = 0.69 and 0.74 for the long and short time periods, respectively. Mechanistically, the statistical relationships reflect the movement and changes in horizontal river plume position associated with the wind and the influence of stratification on the hypoxic area. Citation: Feng, Y., S. F. DiMarco, and G. A.Jackson (2012), Relative role of wind forcing and riverine nutrient input on the extent of hypoxia in the northern Gulf of Mexico, Geophys. Res. Lett., 39, L09601,

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Section: Discussionmentioning

confidence: 99%

Relative role of wind forcing and riverine nutrient input on the extent of hypoxia in the northern Gulf of Mexico

Feng¹,

DiMarco

Jackson

2012

Geophysical Research Letters

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“…This poleward undercurrent of South Atlantic Central Water (SACW) originates in the Gulf of Guinea and entrains 'old' and oxygen-depleted waters of the Angola Dome (Chapman and Shannon 1987). The mixed water mass arrives at the northernmost Namibian offshore zones with oxygen concentrations well below 90 lmol/L and hugs the shelf break as a focused jet.…”

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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“…As regards the maximum southward reach of the undercurrent, 27 S is considered a good estimate (Mohrholz et al 2008). Owing to its low oxygen content, SACW is held partly responsible for the sub-to anoxic condition above the shelf (Chapman & Shannon 1987;Shannon & Nelson 1996).…”

Section: Coastal Upwellingmentioning

confidence: 99%

Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ15N-signals of suspended matter and surface sediments

2011

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The study deals with the modern situation of the northern Benguela Upwelling, directing particular attention to the shelf region off central Namibia (21 to 24° S). At the centre of the investigation is the comparison of δ15N‐records in surface sediments (δ15Nsediment) with suspended particulate matter (δ15NSPM) from the surface ocean. In addition to that, water column profiles (including hydrographic data) provide an insight into changes of δ15NSPM with depth and elucidate potential offsets between δ15NSPM and δ15Nsediment. The parallel spatial trend of δ15Nsediment and surface ocean δ15NSPM shows that secondary processes are not so pronounced as to obliterate the signal generated in the surface waters. Highest δ15N‐signatures are found right off the coast where water temperatures are lowest. Concomitantly high productivity rates and low bottom oxygen suggest the upwelling of denitrified source waters. With increasing distance offshore, δ15N declines unexpectedly, reaching a minimum above the shelf break. Beyond that, the trend reverses to “normal” with δ15N‐signals continuously increasing towards the mesopelagic ocean. The decrease in δ15Nsediment and surface ocean δ15NSPM with increasing distance to the coast disagrees with the concept of Rayleigh fractionation kinetics, viz. the progressive 15N‐enrichment of the nitrate pool as it is gradually used up by phytoplankton growth. On the basis of the available evidence, the downward trend of δ15N results from decreased relative nitrate consumption, resting on a combination of reduced primary production and the existence of an ulterior source of nutrients. Nutrient replenishment seems to occur via an additional upwelling front at the edge of the shelf as well as tapping of subsurface nitrate through sufficiently deep penetration of wind‐ and wave‐induced mixing over large areas of the shelf. Both mechanisms are considered capable of working against the expected nutrient drawdown (i.e. 15N‐enrichment) as surface waters travel offshore. It is important to keep these caveats in mind when interpreting δ15N‐variations in sediment cores retrieved from this area. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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Seasonality in the oxygen minimum layers at the extremities of the Benguela system

Cited by 57 publications

References 1 publication

Relative role of wind forcing and riverine nutrient input on the extent of hypoxia in the northern Gulf of Mexico

Relative role of wind forcing and riverine nutrient input on the extent of hypoxia in the northern Gulf of Mexico

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

Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ15N-signals of suspended matter and surface sediments

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