Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys Connecting pigment composition and dissolved trace elements to phytoplankton population in the southern Benguela Upwelling zone (St. Helena Bay) Supriyo Kumar Das a,⁎ , Joyanto Routh b , Alakendra N. Roychoudhury c , Marcel J.W. Veldhuis d , Hassan E. Ismail e a Department of Geology, Presidency University, 86/1 College Street, Kolkata 700073 West Bengal, India b Department of Thematic Studies, Environmental Change, Linköping University, SE-58183 Linköping, Sweden c Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa d Marine Eco-Analytics, Sluiskolkkade 2, 1779 GP Den Oever, The Netherlands e Department of Environmental Affairs, Oceans and Coasts, 2 East Pier, V & A Waterfront, Cape Town 8012, South Africa ARTICLE INFO Keywords: Benguela Upwelling Nutrients Trace elements Phosphate Nitrate Pigments ABSTRACT Rich in upwelled nutrients, the Southern Benguela is one of the most productive ecosystems in the world ocean. However, despite its ecological significance the role of trace elements influencing phytoplankton population in the Southern Benguela Upwelling System (SBUS) has not been thoroughly investigated. Here, we report pigment composition, macronutrients (nitrate, phosphate and silicate) and concentrations of dissolved Cd, Co, Fe and Zn during late austral summer and winter seasons in 2004 to understand the relationship between the selected trace elements and phytoplankton biomass in St. Helena Bay (SHB), which falls within the southern boundary of the SBUS. Chlorophyll a concentrations indicate higher phytoplankton biomass associated with high primary pro- duction during late summer in SHB where high diatom population is inferred from the presence of fucoxanthin. Diminished phytoplankton biomass and a shift from diatoms to dinoflagellates as the dominant phytoplankton taxa are indicated by diagnostic pigments during late winter. Dissolved trace elements (Cd, Co and Zn) and macronutrients play a significant role in phytoplankton biomass, and their distribution is affected by biological uptake and export of trace elements. Continuous uptake of Zn by diatoms may cause an onset of Zn depletion leading to a period of extended diatom proliferation during late summer. Furthermore, the transition from diatom to dinoflagellate dominated phytoplankton population is most likely facilitated by depletion of trace elements (Cd and Co) in the water column. 1. Introduction The Benguela Current is one of the four major eastern boundary currents in the oceans, and displays substantial seasonal, interannual and decadal variability (Shannon, 1985; Shannon and Nelson, 1996). In particular, the Southern Benguela Upwelling System (SBUS) along the western coast of South Africa is of considerable biogeochemical interest due to intense primary production, high phytoplankton biomass and associated carbon export into the open ocean (Compton et al., 2009). The ocean current in the SBUS causes upwelling of nutrient-rich bottom waters that fuel primary production in the near-coastal waters (Shannon, 1990). The ten-month long upwelling season lasts from August to May, and supports commercially valuable pelagic, demersal and rock lobster fisheries (Hutchings et al., 2009). However, the fisheries industry often suffers by mass mortalities during low oxygen events caused by inshore accumulation and decay of massive dino- flagellate blooms, known as ‘Harmful Algal Blooms’ or previously as ‘red tides’ (Pitcher, 1998; Andrews and Hutchings, 1980). The role of coastal upwelling and associated physical and environmental condi- tions in the variation of phytoplankton species in the SBUS has been examined extensively (Monteiro and Largier, 1999; Pitcher et al., 2010; Pitcher et al., 1991; Tyrrell and Lucal, 2002), but little is known of the influence of trace elements (i.e., micronutrients) supplied by the up- welled current (Moore et al., 2009; Noble et al., 2012) on driving these variations (Bailey and Rogers, 1997; Monteiro and Roychoudhury, 2005). Trace elements have significant influence on the growth, spatial distribution and taxonomic variations of phytoplankton in seawater http://dx.doi.org/10.1016/j.jmarsys.2017.07.009 Received 26 April 2016; Received in revised form 25 July 2017; Accepted 26 July 2017 ⁎ Corresponding author. E-mail address: sdas.geol@presiuniv.ac.in (S.K. Das). Journal of Marine Systems 176 (2017) 13–23 Available online 31 July 2017 0924-7963/ © 2017 Elsevier B.V. All rights reserved. MARK