Journal article Open Access

Environmental factors influencing benthic communities in the oxygen minimum zones on the Angolan and Namibian margins

Hanz, Ulrike; Weinberg, Claudia; Hebbeln, Dierk; Duineveld, Gerard; Lavaleye, Marc; Juva, Katriina; Dullo, Wolf-Christian; Friewald, André; Tamborrino, Leonardo; Reichart, Gert-Jan; Flögel, Sascha; Mienis, Furu

ABSTRACT.

Thriving benthic communities were observed in the oxygen minimum zones along the southwestern African margin. On the Namibian margin, fossil cold-water coral mounds were overgrown by sponges and bryozoans, while the Angolan margin was characterized by cold-water coral mounds covered by a living coral reef. To explore why benthic communities differ in both areas, present-day environmental conditions were assessed, using conductivity–temperature–depth (CTD) transects and bottom landers to investigate spatial and temporal variations of environmental properties. Near-bottom measurements recorded low dissolved oxygen concentrations on the Namibian margin of 0–0.15 mL L−1 (0 %–9 % saturation) and on the Angolan margin of 0.5–1.5 mL L−1 (7 %–18 % saturation), which were associated with relatively high temperatures (11.8–13.2 C and 6.4–12.6 C, respectively). Semidiurnal barotropic tides were found to interact with the margin topography producing internal waves. These tidal movements deliver water with more suitable characteristics to the benthic communities from below and above the zone of low oxygen. Concurrently, the delivery of a high quantity and quality of organic matter was observed, being an important food source for the benthic fauna. On the Namibian margin, organic matter originated directly from the surface productive zone, whereas on the Angolan margin the geochemical signature of organic matter suggested an additional mechanism of food supply. A nepheloid layer observed above the cold-water corals may constitute a reservoir of organic matter, facilitating a constant supply of food particles by tidal mixing. Our data suggest that the benthic fauna on the Namibian margin, as well as the cold-water coral communities on the Angolan margin, may compensate for unfavorable conditions of low oxygen levels and high temperatures with enhanced availability of food, while anoxic conditions on the Namibian margin are at present a limiting factor for cold-water coral growth. This study provides an example of how benthic ecosystems cope with such extreme environmental conditions since it is expected that oxygen minimum zones will expand in the future due to anthropogenic activities.

ACKNOWLEDGMENTS. The authors thank the captain of the RV Meteor cruise M122, Rainer Hammacher, his officers and crew, who contributed to the success of this cruise. We also would like to thank the scientific and technical staff for their assistance during the cruise and their work in the laboratory. Greatly acknowledged are the efforts from the German Diplomatic Corps in the German embassies in Windhoek and Luanda and in the Foreign Office in Berlin. We thank the German Science Foundation (DFG) for providing ship time on RV Meteor and for funding the ROV Squid operations to investigate the cold-water coral ecosystems off Angola and Namibia. FINANCIAL SUPPORT. This research has been supported by the DFG Research Center/Cluster of Excellence "MARUM – The Ocean in the Earth System". Ulrike Hanz is funded by the SponGES project, which received funding from the European Union's Horizon 2020 research and innovation program (grant no. 679849). Furu Mienis is supported by the Innovational Research Incentives Scheme of the Netherlands Organisation for Scientific Research (NWO-VIDI grant no. 016.161.360). Gert-Jan Reichart is supported by the Netherlands Earth System Science Centre (NESSC), financially supported by the Ministry of Education, Culture and Science (OCW). Katriina Juva is funded through the FATE project (Fate of cold-water coral reefs – identifying drivers of ecosystem change), supported by the Norwegian Research Council (NRC). AUTHOR CONTRIBUTIONS. UH analyzed the physical and chemical data, wrote the paper and prepared the figures with contributions from all authors. FM, GD and ML designed the lander research. DH and CW led the cruise and wrote the initial cruise plan. FM and ML collected the data during the research cruise. WCD was responsible for water column measurements with the CTD. AF and ML provided habitat characteristics, including species identification of both CWC areas. KJ performed the tidal analysis and provided together with SF data from the SML lander. All authors contributed to the data interpretation and discussion of the paper. DATA AVAILABILITY. CTD data presented in this work are available at https://doi.pangaea.de/10.1594/PANGAEA.904176 (Hebbeln, 2019). Lander time series data were submitted to Pangaea and are also available at the NIOZ data archive system (https://doi.org/10.25850/nioz/7b.b.q, Hanz, 2019). COPYRIGHT. © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License.
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