Climate – Biogeochemical Interactions in the Tropical Ocean

The animation illustrates the schematic formation of an oxygen minimum zone (here shown in magenta) at the eastern continental boundaries in the tropics. In the upwelling areas of the oceans, an interplay of physics and biology causes oxygen deficiency. Here, nutrient-rich deep water reaches the surface. The abundance of nutrients and plankton at the surface leads to high oxygen consumption in deeper zones. Only weak currents occur there, which transport relatively oxygen-poor water. This has resulted in large oxygen minimum zones.

It is obvious to everyone that humans cannot breathe under water without aids. However, it is less well known that breathing can also become a problem for marine life: In some regions, oxygen is naturally present in very small concentrations in the sea, and this has a significant impact on living conditions in these regions. These are not small, hidden corners of the oceans, but rather considerable areas of the world ocean: the so-called oxygen minimum zones extend over large areas of the tropical seas.


Collaborative Research Center 754  /  Sonderforschungsbereich 754

"Climate-Biogeochemistry Interactions in the Tropical Ocean"

The distribution of oxygen in the ocean interior is controlled by an intimate interplay of physics and biology. Circulation and mixing transport oxygen into the ocean interior from the near-surface where it is produced by photosynthesis and exchanged with the atmosphere. Oxygen consumption occurs throughout the ocean and is essentially driven by bacterial respiration of organic matter. Both the supply and consumption of oxygen are sensitive to climate change in ways that are not fully understood. Recent observations, including those gathered as part of the SFB 754, suggest that the oxygen content of the ocean is declining and oxygen minimum zones are expanding. However, the regional and temporal patterns of this deoxygenation as well as the underlying mechanisms are not yet well understood and part of further SFB 754 research.

When oceanic oxygen concentrations decrease below certain threshold levels, major changes to remineralisation processes and associated marine sources and sinks of important nutrient elements such as nitrogen, phosphorus and iron can occur in the water column and underlying sediments. Paleo-records give evidence for periods of dramatically reduced oceanic oxygen that had major consequences for marine ecosystems. Low oxygen levels can therefore be viewed as a "switch" for nutrient cycling.

The tropical Oxygen Minimum Zones (OMZs) are the key regions of low oxygen in today's ocean. The effects of oxygen-dependent nutrient cycling processes occurring in these relatively small regions are carried into the rest of the ocean by the circulation. Hence "small" OMZs can impact nutrient budgets, biological productivity and CO2-fixation of the global ocean.

Recent modelling results suggest that oxygen levels will decrease significantly over the next decades in response to climate change, and altered ocean circulation. Hence the future ocean may experience major shifts in nutrient cycling triggered by expansion and intensification of tropical OMZs. There are numerous feedbacks between oxygen, nutrient cycling and biological productivity; however existing knowledge is insufficient to understand past interactions or to adequately assess the potential for future change.

Addressing the SFB 754 goals require multi-disciplinary study. Therefor, the SFB 754 builds upon wide-ranging expertise in Kiel, including biological, chemical and physical oceanography, sediment biogeochemistry, marine ecology, molecular microbiology, paleoceanography, geology, as well as climate and biogeochemical modelling. The SFB 754 is organised within 18 scientific interdisciplinary subprojects to support the research for answering the key questions of the SFB 754. In addition an outreach project (Ö) within the SFB 754 complements the routine outreach efforts with a dedicated programme.