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Pelagic Key Species and Mechanisms Driving Energy Flows in the Northern Journal of Marine Systems 188 (2018) 49–62 Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys Pelagic key species and mechanisms driving energy flows in the northern Benguela upwelling ecosystem and their feedback into biogeochemical T cycles ⁎ Werner Ekaua, , Holger Auelb, Wilhelm Hagenb, Rolf Koppelmannc, Norbert Wasmundf, Karolina Bohatac, Fritz Buchholzd, Simon Geista,e, Bettina Martinc, Anna Schukatb, Hans M. Verheyeg,h, Thorsten Wernerd a Leibniz Centre for Tropical Marine Research, Department of Ecology, Bremen, Germany b BreMarE - Bremen Marine Ecology, Marine Zoology, University of Bremen, Bremen, Germany c Institute of Hydrobiology and Fisheries Science, University of Hamburg, Germany d Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany e Texas A&M University Corpus Christi, Department of Life Sciences, Corpus Christi, USA f Leibniz Institute for Baltic Sea Research, Warnemünde, Germany g Department of Environmental Affairs, Cape Town, South Africa h University of Cape Town, Marine Research Institute, Rondebosch, South Africa ARTICLE INFO ABSTRACT Keywords: The northern Benguela Upwelling System (nBUS) has been facing increasing temperatures and decreasing dis- Oxygen Minimum Zone solved oxygen (DO) levels over the last decades. This has implications for key processes and trophic interactions Benguela upwelling within the ecosystem including shifts in community composition, distribution ranges, and trophic levels, changes Zooplankton biomass in energy flows and migration patterns with feedbacks to biogeochemical processes. Here we summarise the Pelagic key taxa results gained from the GENUS project (Geochemistry and Ecology of the Namibian Upwelling System) focussing Oxygen consumption on the geochemical and ecological structures and processes dominating the pelagic component of the nBUS. Trophic interactions Spatial and temporal distribution patterns of key species of zooplankton and fish larvae yielded biomass esti- − − mates (5 to 81 g Wet Mass m 2 (10 to 90% quantile) with a median of 19.5 g Wet Mass m 2 for the upper 200 m) and potential impacts on the vertical carbon flux. Vertical distribution ranges of key taxa were determined reflecting their specific abilities to tolerate hypoxia and, hence, their different adaptive mechanisms to cope with −1 −1 the Oxygen Minimum Zone (OMZ). The shoaling of the 2.5 mL O2 L -oxycline (0.24 m y ) constrains sensitive species and hampers daily and seasonal vertical migrations. It may also affect the ability of organisms to maintain themselves within nearshore habitats by hindering vertical migration into deeper onshore currents. Respiration rates of key species were determined with one standard method (optode respirometry), showing an −1 −1 average respiration rate of 54.6 mL O2 d (g Dry Mass) for the bulk fraction of mesozooplankton, allowing also the estimate of DO consumption by mesozooplankton at different depth layers. Stable isotopic ratios (N, C) revealed trophic interactions and positions of zooplankton and fish. Our results reveal many players within a small range of trophic levels and a dominance of zooplankton taxa (copepods, euphausiids) in terms of biomass over small pelagic fish (sardine, anchovy), essential to consider for future higher-resolution ecosystem model- ling. 1. Introduction highest primary production rates among the four major EBUS, very similar to those of the Canary Current EBUS off Northwest Africa, but is Eastern Boundary Upwelling Systems (EBUS) comprise < 2% of the among the lowest concerning fish production, about tenfold less than ocean's surface, but support 7% of global marine primary production, the Humboldt Current EBUS (Chavez et al., 2008). Bound between two and provide ~20% of global fish catches (Pauly and Christensen, 1995; warm water currents, the Angola Current in the north and the Agulhas Sydeman et al., 2014). The Benguela upwelling ecosystem shows Current in the south, the Benguela EBUS is divided into two very ⁎ Corresponding author at: Leibniz Centre for Tropical Marine Research, Department of Ecology, Bremen, Germany. E-mail address: [email protected] (W. Ekau). https://doi.org/10.1016/j.jmarsys.2018.03.001 Received 6 January 2017; Received in revised form 1 March 2018; Accepted 13 March 2018 Available online 27 March 2018 0924-7963/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/). W. Ekau et al. Journal of Marine Systems 188 (2018) 49–62 distinct subsystems by the strong Lüderitz upwelling cell: a northern 2013; Werner et al., 2015) and ichthyoplankton (Grote et al., 2012; part (northern Benguela Upwelling System - nBUS) influenced by South Geist et al., 2013, 2015). Changes in plankton communities have been Atlantic Central Water (SACW), accompanied by an extensive Oxygen observed, indicating horizontal and vertical shifts in distribution pat- Minimum Zone (OMZ), and a southern part (sBUS) dominated by terns. The results from physiological investigations provided informa- Eastern South Atlantic Central Water (ESACW). In the northern part, tion on respiration rates of key species and energy turnover. smaller upwelling cells are found off the Cunene River mouth (17°S), In this paper, we aim at integrating and synthesising the results around Cape Frio (19°S), and off Walvis Bay (23°S). The dynamics of the obtained from the biological components of the GENUS project with Benguela Current (BC) EBUS have been described extensively existing knowledge on the nBUS by addressing three major questions: (Shannon, 1985; Lutjeharms and Valentine, 1987; Fennel, 1999; Monteiro et al., 2008; Mohrholz et al., 2014). 1. What are the distribution patterns of key zooplankton taxa in the All major coastal upwelling systems have shown some evidence of nBUS and the likely effects of global change? response to past climate change as demonstrated by empirical and 2. How does the expanding OMZ affect zooplankton and fish commu- modelling approaches (Emeis et al., 2009; Finney et al., 2010; Leduc nities, and what is the feedback of changing pelagic communities on et al., 2010; Bakun et al., 2015). They are expected to respond to future the development of the OMZ? climate change, since they are uniquely sensitive to global, regional, 3. What are the contribution of vertical migration and feeding beha- and local changes in atmospheric circulation patterns and driving forces viour of zooplankton and fishes to the vertical carbon flux? (Bakun, 1990; Bakun et al., 2010; Demarcq, 2009; Sydeman et al., 2014; García-Reyes et al., 2015; Wang et al., 2015). Several EBUS have 2. Methods experienced dramatic shifts in ecosystem structure and fisheries yield (so-called ecosystem regime shifts) in the past (Alheit and Bakun, 2010; Results of the biological component of the GENUS project have thus Cury and Shannon, 2004; Finney et al., 2010; Kirkman et al., 2015). far been presented in a large number of peer-reviewed publications According to current thinking, these shifts in ecosystem structure were listed on the GENUS website and in the final report (GENUS, 2018). For not exclusively caused by anthropogenic impacts, but may have also the present paper, results from many GENUS publications were syn- been expressions of global or regional shifts in physical drivers thesised, and additional non-published data from former cruises in- (Overland et al., 2010; Rykaczewski and Checkley, 2008). This view is tegrated. Detailed descriptions of methods applied are given in the re- supported by evidence of similarly radical regime shifts that occurred in spective publications. Here, we briefly present an overview of the the geological past (Finney et al., 2010). However, various physical research cruises carried out during GENUS and of those bilateral cruises ff fi EBUS models di er signi cantly in their projections of the response of realised before that are related to the topic, highlight certain metho- the systems to climate change (Wang et al., 2010). dological advances developed within the framework of GENUS, and South Atlantic Central Water is a prominent water source in the focus on the methods used for the synthesis and integration of data for northern Benguela from July to December when upwelling is strongest. the present paper. Low oxygenated water originating from the Angola Dome region is upwelled at Cape Frio (18°S) during January to June when it usually dominates the central Namibian coast (Monteiro et al., 2006; Monteiro 2.1. Research cruises and van der Plas, 2006; Mohrholz et al., 2008). The Benguela EBUS, especially the northern part, has been facing increasing temperatures Cooperation started in 1999 with empirical investigations on the and decreasing dissolved oxygen (DO) concentrations over the last composition and abundance of plankton in the nBUS and was carried decades (Stramma et al., 2008). Vertical distributions of zoo- and ich- out during a number of cruises onboard German, South African, thyoplankton (Ekau and Verheye, 2005; Auel and Ekau, 2009) are Namibian, Norwegian, and British research vessels. Cruises relevant for constrained by DO concentration, and migration patterns seem to be the biological/ecological work are listed in Table 1. GENUS started with fi impacted by physical and biogeochemical factors. Hence, changes in pilot studies in 2008 and ended eld work in 2014. These cruises community composition are related to temperature and DO content. As provided insight
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