Pelagic key species and mechanisms driving energy flows in the northern Benguela upwelling ecosystem and their feedback into biogeochemical cycles

Werner Ekau,Simon Geist,Bettina Martin,Thorsten Werner,Hans M Verheye,Karolina Bohata,Rolf Koppelmann,Fritz Buchholz,Anna Schukat,Wilhelm Hagen,Holger Auel,Norbert Wasmund

doi:10.1016/j.jmarsys.2018.03.001

Abstract

Abstract The northern Benguela Upwelling System (nBUS) has been facing increasing temperatures and decreasing dissolved oxygen (DO) levels over the last decades. This has implications for key processes and trophic interactions within the ecosystem including shifts in community composition, distribution ranges, and trophic levels, changes in energy flows and migration patterns with feedbacks to biogeochemical processes. Here we summarise the results gained from the GENUS project (Geochemistry and Ecology of the Namibian Upwelling System) focussing on the geochemical and ecological structures and processes dominating the pelagic component of the nBUS. Spatial and temporal distribution patterns of key species of zooplankton and fish larvae yielded biomass estimates (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 the Oxygen Minimum Zone (OMZ). The shoaling of the 2.5 mL O2 L−1-oxycline (0.24 m y−1) 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 average respiration rate of 54.6 mL O2 d−1 (g Dry Mass)−1 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 modelling.

Highlights

Eastern Boundary Upwelling Systems (EBUS) comprise < 2% of the ocean's surface, but support 7% of global marine primary production, and provide ~20% of global fish catches (Pauly and Christensen, 1995; Sydeman et al, 2014)
We briefly present an overview of the research cruises carried out during GENUS and of those bilateral cruises realised before that are related to the topic, highlight certain methodological advances developed within the framework of GENUS, and focus on the methods used for the synthesis and integration of data for the present paper
Changes in temperature and oxygen in the northern Benguela Upwelling System have been documented over the last decades

Summary

Introduction

Eastern Boundary Upwelling Systems (EBUS) comprise < 2% of the ocean's surface, but support 7% of global marine primary production, and provide ~20% of global fish catches (Pauly and Christensen, 1995; Sydeman et al, 2014). All major coastal upwelling systems have shown some evidence of response to past climate change as demonstrated by empirical and modelling approaches (Emeis et al, 2009; Finney et al, 2010; Leduc et al, 2010; Bakun et al, 2015). These shifts in ecosystem structure were not exclusively caused by anthropogenic impacts, but may have been expressions of global or regional shifts in physical drivers (Overland et al, 2010; Rykaczewski and Checkley, 2008) This view is supported by evidence of radical regime shifts that occurred in the geological past (Finney et al, 2010). Various physical EBUS models differ significantly in their projections of the response of the systems to climate change (Wang et al, 2010)

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