Spatial Characteristics Of Sediment Trace Metals In An Eastern

Estuarine, Coastal and Shelf Science 65 (2005) 123e134
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Spatial characteristics of sediment trace metals in an eastern
boundary upwelling retention area (St. Helena Bay,
South Africa): A hydrodynamicebiological pump hypothesis
Pedro M.S. Monteiro a,b, Alakendra N. Roychoudhury c,*
a

Department of Oceanography, University of Cape Town, Rondebosch 7700, SouthAfrica
b
Coast Programme, CSIR, P.O. Box 320, Stellenbosch 7599, South Africa
c
Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa
Received 28 September 2004; accepted 19 May 2005
Available online 15 July 2005

Abstract
St. Helena Bay, a retention zone located in the southern Benguela upwelling system, is an important fish nursery. However, it
suffersfrom seasonal bottom water hypoxia causing major economic losses. Anoxic conditions are linked to sulfide fluxes from
bottom sediments defined by a high sedimentation rate of organic matter. It is proposed that trace metals may play an important
role in alleviating part of the ecological stress by forming sulfide complexes in such systems. A spatially intensive data set of sediment
biogeochemicalcharacteristics showed that POC and trace metals (Cr, Cu, Zn, Ni, etc.) accumulated in the central zone of the Bay.
Furthermore, trace metal concentrations were strongly correlated with both POC and Al. To explain the observed biogeochemical
relationships in St. Helena Bay, we propose a hypothesis that links the upwelling retention hydrodynamics, primary productivity
and sediment trace metaldistribution. Trace metals are incorporated into phytoplankton cells in the euphotic zone but rapidly
sediment along with particulate organics, on their senescence. Both, the biological pump and the dispersion of particulates are
primarily controlled by the hydrodynamics prevalent within St. Helena Bay, which also govern the retention zone in the shadow of
one of the major upwelling cells. Thedynamics of entrainmentestratification drives the productivity, while a residual cyclonic gyre
concentrates the surface productivity within the bay. Bed-shear stresses spatially constrain the accumulation of biogenic organic
matter, which governs the trace metal biogeochemistry of the sediments, along a narrow terrigenous mud belt.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Benguelaupwelling ecosystem; dispersion; hydrodynamics; primary production; St. Helena Bay; trace metals

1. Introduction
Retention zones in the shadow of upwelling centers in
eastern boundary upwelling systems are now known to
play key ecosystem regulation roles. These include,
productive nurseries to early life stages of fish (Bakun,
1998), areas of formation and incidence of harmful algal
blooms(Pitcher et al., 1992, in press; Probyn, 1992) and
* Corresponding author.
E-mail address: aroy@geology.uct.ac.za (A.N. Roychoudhury).
0272-7714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ecss.2005.05.013

coastal hypoxia (Bailey, 1991; Chapman and Bailey,
1991; Monteiro and van der Plas, in press). While the
ecological importance of retention areas ingeneral and
St. Helena Bay in particular (Hutchings, 1992; Roy,
1998) has been recognized, their role in the development
and maintenance of organic rich sediments and coupled
elemental cycling is less well understood.
Underlying the ecosystem roles of retention zones are
the complex hydrodynamic processes that govern the
recirculation and stratification, and upwelling and sedimentation ere-suspension characteristics (Jones et al.,

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P.M.S. Monteiro, A.N. Roychoudhury / Estuarine, Coastal and Shelf Science 65 (2005) 123e134

1998; Monteiro and Largier, 1999; Penven et al., 2000).
St. Helena Bay in the southern Benguela upwelling
system has the hydrodynamic characteristics typical of
retention areas lying ‘‘downwind’’ or equatorward from
an upwelling Cape in eastern...