Oxygen – the renaissance of a hydrographic parameterOxygen distribution in the ocean depends on both biological processes, like the respiration of organisms, and on physical processes such as current flow. Changes in either of these processes should therefore lead to changes in the oxygen distribution. In fact, dissolved oxygen can be viewed as a kind of sensitive early warning system for global (climate) change in the ocean. Scientific studies show that this early warning system can detect the expected decrease in oxygen transport from the atmosphere into the ocean that is driven by global current and mixing processes, as well as possible changes in the marine biotic communities. In recent years, this knowledge has led to a renaissance of oxygen in the field of global marine research.
- In oceanography, dissolved oxygen has been an important measurement parameter for over a hundred years. A method for determining dissolved oxygen was developed as early as the end of the 19th century, and it is still applied in an only slightly modified form today as a precise method. This allowed for the development of an early fundamental understanding of the oxygen distribution in the world ocean, with the help of the famous German Atlantic Expedition of the “Meteor” in the 1920s. Research efforts in recent years have recorded decreasing oxygen concentrations for almost all the ocean basins. These trends are, in part, fairly weak and mainly limited to water masses in the upper 2000 metres of the ocean. Therefore, no fully consistent picture can yet be drawn from the individual studies. Most of the studies do, however, show a trend of decreasing oxygen concentrations. This trend agrees well with an already verified expansion and intensification of the natural oxygen minimum zones, those areas that are deadly for higher organisms. If the oxygen falls below certain (low) threshold values, the water becomes unsuitable for higher organisms. Sessile, attached organisms die. Furthermore, the oxygen deficiency leads to major changes in biogeochemical reactions and elemental cycles in the ocean – for instance, of the plant nutrients nitrate and phosphate.
- Oxygen levels affect geochemical processes in the sediment but also, above all, bacterial metabolism processes, which, under altered oxygen conditions, can be changed dramatically. It is not fully possible today to predict what consequences these changes will ultimately have. In some cases it is not even possible to say with certainty whether climate change will cause continued warming, or perhaps even local cooling. But it is probable that the resulting noticeable effects will continue over a long time period of hundreds or thousands of years
- Even today, however, climate change is starting to cause alterations in the oxygen content of the ocean that can have negative effects. For the first time in recent years, an extreme low-oxygen situation developed off the coast of Oregon in the United States that led to mass mortality in crabs and fish. This new death zone off Oregon originated in the open ocean and presumably can be attributed to changes in climate. The prevailing winds off the west coast of the USA apparently changed direction and intensity and, as a result, probably altered the ocean currents. Researchers believe that the change caused oxygen-poor water from greater depths to flow to surface waters above the shelf. The death zone off Oregon is therefore different than the more than 400 near-coastal death zones known worldwide, which are mainly attributed to eutrophication, the excessive input of plant nutrients. Eutrophication normally occurs in coastal waters near densely populated regions with intensive agricultural activity. (Chapter 4)
The Atlantic Expedition For the first time, during the German Atlantic Expedition (1925 to 1927) with the research vessel “Meteor”, an entire ocean was systematically sampled, both in the atmosphere and in the water column. Using an echosounder system that was highly modern for its time, depth profiles were taken across 13 transits of the entire ocean basin.
Oxygen – challenge to marine researchThe fact that model calculations examining the effects of climate change almost all predict an oxygen decline in major parts of the ocean, which agrees with the available observations of decreasing oxygen, gives the subject additional weight. Even though the final verdict is not yet in, there are already indications that the gradual loss of oxygen in the world ocean is an issue of great relevance which possibly also has socio-economic repercussions, and which ocean research must urgently address. Intensified research can provide more robust conclusions about the magnitude of the oxygen decrease. In addition it will contribute significantly to a better understanding of the effects of global climate change on the ocean. In recent years marine research has addressed this topic with increased vigour, and has already established appropriate research programmes and projects. It is difficult, however, to completely measure the temporally and spatially highly variable oceans in their totality. In order to draw reliable conclusions, therefore, the classic instruments of marine research like ships and taking water samples will not suffice. Researchers must begin to apply new observational concepts. “Deep drifters” are an especially promising tool: these are submersible measuring robots that drift completely autonomously in the ocean for 3 to 4 years, and typically measure the upper 2000 metres of the water column every 10 days. After surfacing, the data are transferred to a data centre by satellite. There are presently around 3200 of these measuring robots deployed for the international research programme ARGO, named after a ship from Greek mythology. Together they form a worldwide autonomous observatory that is operated by almost 30 countries. So far this observatory is only used on a small scale for oxygen measurements. But there has been developed a new sensor technology for oxygen measurements in the recent past that can be deployed on these drifters. This new technology would give fresh impetus to the collection of data on the variability of the oceanic oxygen distribution.