Doctoral Programme on Marine Ecosystem Health and Conservation
 The MARES Researchers and their Research
Zooplankton in the Future Ocean: The Effects of Hypoxia on Zooplankton Vertical Distribution, Ecology and Ecophysiology. (old topic : MARES_12)
PhD Code: MARES_11_06:
  • Host institute 1: P2 - Universität Bremen
  • Host institute 2: P18 - AZTI-Tecnalia
Research fields:
  • T1 - Future Oceans: temperature changes - hypoxia - acidifation
  • T2 - Understanding biodiversity effects on the functioning of marine ecosystems
  • Holger Auel
  • Xabier Irigoien
Contact Person and email: Holger Auel -

Subject description
Background: The expansion of hypoxic conditions in the ocean is a burning issue of growing global concern. At least partly caused by anthropogenic impacts such as global warming and eutrophication, oxygen depletion is expected to affect large areas of coastal seas (e.g. black spots in the Wadden Sea, deep basins of the Baltic Sea) as well as low-latitude open oceans in the coming decades and poses an eminent threat to marine biodiversity, global fisheries resources and marine ecosystems (Ekau et al. 2010, Stramma et al. 2010). Increasing suboxic conditions will affect nutrient cycles in the ocean, alter trophic pathways and ultimately change the structure of marine food webs and communities (Bograd et al. 2008, Stramma et al. 2010, Ekau et al. 2010).
Oxygen minimum zones (OMZ) are most pronounced in the highly productive eastern boundary upwelling systems such as the Benguela Current off SW Africa. Since the 1960s the tropical Atlantic has been suffering from the most severe decline in oxygen concentration. From 1960 to 2006, regions with oxygen concentrations of less than 90 μmol O2 kg-1 have increased both in vertical extent from 370 to 690 m and in spatial extent by 4.5 million km2 (Stramma et al. 2008). In the Benguela upwelling system, there is an almost permanent hypoxic layer above the seafloor with occasional outbreaks of toxic hydrogen sulphide in the near-bottom water (Weeks et al. 2002). In addition, very low oxygen concentrations of <1 ml O2 l-1 are found within the OMZ at 40 to 400 m water depth (Ekau et al. 2010).
Zooplankton is especially sensitive to environmental change and because of non-linear responses may cause major regime shifts in marine ecosystems (Hays et al. 2005). In the Benguela system, it shows a bimodal vertical distribution with high abundance and biomass in the upper 30 m layer or below 400 m depth, apparently avoiding the OMZ in between (Verheye et al. 2005, Auel & Verheye 2007). Many key zooplankton species including copepods and krill (Euphausiacea) perform vertical migrations, either on diurnal or ontogenetic scales. Therefore, it is likely that extended OMZs will interfere with or even block zooplankton vertical migrations and prevent key species from completing their life-cycles. For instance, the dominant copepod Calanoides carinatus was able to survive surprisingly low oxygen concentrations of less than 1 ml O2 l-1, but could not tolerate
hypoxic conditions of less than 0.6 ml O2 l-1 prevailing in the centre of the OMZ (Auel & Verheye 2007). In contrast, certain species specifically adapted to hypoxic conditions, may find a refuge from predation and competition by less tolerant opponents in the centre of the OMZ. Effects on lower trophic levels propagate along the food chain and affect fish stocks and top predators. Actually, it has been proposed that the lack in recovery of small pelagic fish (sardine and anchovy) in the northern Benguela region is related to the expansion of hypoxic zones.
Objectives and work programme: The proposed MARES Ph.D. project, for which Bremen University and AZTI will join their complementary expertises and infrastructure, will focus on the effects of hypoxia on zooplankton vertical distribution, ecology and ecophysiology. It will consist of three work packages. In the first phase at AZTI, the candidate will analyse stratified zooplankton samples collected in the northern Benguela upwelling region by means of computer-based image analysis (ZooImage), for which AZTI has a lot of expertise. The objective here is to quantify zooplankton abundance and biomass in relation to the oxygen concentration above, within and below the OMZ. In addition, the candidate will be trained in zooplankton species identification. The second work package will include field work during a GENUS research cruise to the northern Benguela upwelling region. Respiration measurements and hypoxia tolerance experiments will be conducted on board
by optode respirometry to establish the physiological limits of zooplankton species living within vs. outside the OMZ. During the third phase at Bremen University, the candidate will analyse metabolic enzyme activity (LDH, etc.) and ETS (Electron Transport System) activity to study physiological adaptations of key zooplankton species to hypoxic conditions. Finally, the ecological implications of physiological limits regarding hypoxia tolerance will be assessed. This will allow mapping of potential future distribution ranges based on species-specific hypoxia tolerance limits and oceanographic field data/modelling results on present and future oxygen concentrations in the sea.

Expected outcomes
The candidate is expected to publish the results of the project together with the supervisors in peer-reviewed scientific journals. He/she will present the outcomes at two international conferences. Close cooperation and exchange with other international research projects, e.g. GENUS (Geochemistry and Ecology of the Namibian Upwelling System) and BCC (Benguela Current Commission), will ensure that the outcomes of the Ph.D. project will be recognised by the scientific community and the stakeholders in the region. Since the expansion of hypoxic zones in the oceans is a burning issue of global concern, it is expected that the outcomes of this project will be of high relevance for a wider audience and also gain interest of the general public.

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