Estuarine, Coastal and Shelf Science 65 (2005) 123e134 www.elsevier.com/locate/ECSS 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, South Africa 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 …ver más…

Although St. Helena Bay is nitrogen limited (Chapman and Bailey, 1991), the retention zone is characterized by chlorophyll-a concentrations that are higher than what is expected from new production alone (O15 mg Chl mÿ3) and peaks at
68e139 mg Chl mÿ3 (Bailey, 1987; Pitcher et al., 1989,
1992). This contributes to the high sediment flux
(31.5 g mÿ2 dÿ1) observed in the bay area (Bailey,
1987). Sediment sulfide fluxes are coupled to the sedimentation of ageing phytoplankton blooms, which then enhances the hypoxic character of sub-thermocline waters. Off the west coast of South Africa between St.
Helena Bay and Elands Bay, regular episodes of oxygen depletion within the coastal ocean waters have been witnessed in the late summer (Chapman and Shannon,
1985; Monteiro and van der Plas, in press). These hypoxic conditions are maintained throughout the St.
Helena Bay area as a result of the combined effects of stratification, retention and remineralization of carbon export fluxes (Monteiro and van der Plas, in press).
Hypoxic conditions coupled with elevated near bottom sulfide concentrations have, in the past, resulted in major mortality events for organisms such as rock lobsters, fish and mussels, thereby causing significant economic losses (Cockroft et al.,