What is ocean deoxygenation?.

Gregoire, Marilaure, Gilbert, Denis, Oschlies, Andreas and Rose, Kenneth (2019) What is ocean deoxygenation?. Open Access In: Ocean deoxygenation: everyone’s problem. Causes, impacts, consequences and solutions. , ed. by Laffoley, D. and Baxter, J. M.. IUCN, Gland, Switzerland, pp. 1-21. ISBN 978-2-8317-2013-5

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Abstract

Summary
• The equilibrium state of the ocean-atmosphere system has been perturbed these last few decades with the ocean becoming a source of oxygen for the atmosphere even though its oxygen inventory is only ~0.6% of that of the atmosphere. Different analyses conclude that the global ocean oxygen content has decreased by 1-2%
since the middle of the 20th century. Global warming is expected to have contributed to this decrease, directly because the solubility of oxygen in warmer waters decreases, and indirectly through changes in the physical
and biogeochemical dynamics.
• Since the middle of the 20th century, the increased river export of nitrogen and phosphorus has resulted in eutrophication in coastal areas world-wide. Eutrophication implies huge oxygen consumption, and when combined with a low ventilation, often due to vertical stratification, this leads to the occurrence of oxygen deficiencies near the sea bed. The number of reported sites affected by low oxygen conditions (>500) has dramatically increased in the last few decades. Climate warming is expected to exacerbate the decrease of oxygen by reducing the ventilation and extending the stratification period.
• The volume of anoxic zones has expanded since 1960 altering biogeochemical pathways by allowing processes that consume fixed nitrogen and release phosphate and iron, and possibly nitrous oxide (N2O). The relatively small inventory of essential elements, like nitrogen and phosphorus, makes such alterations capable
of perturbing the chemical composition equilibrium of the ocean. Positive feedback loops (e.g. remobilization of phosphorus and iron from sediment particles) may speed up the run away from this equilibrium in ways that
we hardly know or understand.
• Deoxygenation affects many aspects of the ecosystem services provided by the ocean and coastal waters.
For example, deoxygenation effects on fisheries include low oxygen affecting populations through reduced recruitment and population abundance, and also through altered spatial distributions of the harvested species causing changes in the dynamics of the fishing vessels. This can lead to changes in the profitability of the fisheries and can affect the interpretation of the monitoring data leading to misinformed management advice.
• Model simulations for the end of this century project a decrease of oxygen in the high and low emission scenarios, while the projections of river exports to the coastal ocean indicate that eutrophication will likely continue in many regions of the world. Warming is expected to further amplify the deoxygenation issue in
coastal areas influenced by eutrophication by strengthening and extending the stratification.

Document Type: Book chapter
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-BM Biogeochemical Modeling
Date Deposited: 11 Dec 2019 10:41
Last Modified: 11 Dec 2019 10:41
URI: http://oceanrep.geomar.de/id/eprint/48395

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