| 英文摘要: | Accelerated oxygen loss in both coastal and open oceans is generating complex biological responses; future understanding and management will require holistic integration of currently fragmented oxygen observation and research programmes. Deoxygenation of the ocean is one of the major manifestations of global change. It accompanies ocean warming and ocean acidification as one of three primary ocean consequences of rising atmospheric CO2. For the past half century, the study of oxygen stress (hypoxia) — its occurrence, causes and implications for life in the ocean — has been an active area of research. But there have been two separate schools of study, one that addresses eutrophication-induced hypoxia in coastal ecosystems and another that examines naturally occurring oceanic hypoxic zones (including oxygen minimum and limiting zones, and their shoaling into coastal habitats). Each has developed with somewhat different emphases and tools, and largely in isolation of the other. Even within oceanic or coastal realms, geographically based management and funding sources have led to more geographically segregated interactions than might be ideal to stimulate advances in understanding, management and adaptation. Declines in oxygen have accelerated in recent decades in both realms, as highlighted by Fifth Assessment Report of the IPCC in 20131. The number of eutrophication-induced hypoxic sites reported in the coastal zone has increased by an order of magnitude since the 1960s2. At the same time, open-ocean deoxygenation is resulting from a warming ocean, increased stratification and changing circulation3. Time-series data reveal an extensive oxygen decline in the northeast Pacific (for example, ref. 4), and a significant expansion of oxygen minimum zones in the tropical and subtropical ocean over the past half century5. Coastal and open-ocean hypoxia are largely treated as distinct — spatially and in causality. Adaptation and management discussions generally occur separately. But it is now clear that these phenomena are not distinct and in fact are highly interconnected. Carbon dioxide-induced climate change is increasing the extent and severity of both forms of hypoxia. And we are learning that nutrient enrichment, typically associated with coastal hypoxia, can also worsen oceanic hypoxia by increasing surface-layer production that ultimately fuels microbial respiration at depth. Intensified wind-driven upwelling, related to atmospheric warming and its effect on the depth of waters with low oxygen and low pH, is bathing continental shelves in hypoxic, carbonate-undersaturated waters along the US west coast and in other regions6, while other areas such as the coasts of Mexico and countries bordering the Bay of Bengal are becoming increasingly vulnerable7. Added nutrients and reduced oxygen in upwelling source waters create seasonal dead zones on the inner Oregon Shelf8. Excess nutrients from land can stimulate further biogeochemical activity and tip even open-ocean systems into anoxia. At the same time, warmer estuarine and ocean waters carry increasing numbers of eutrophic sites towards hypoxic tipping points and worsen the severity and spatial extent of oxygen depletion in systems historically experiencing hypoxia9, 10. As a result, continental margins, shelves and estuaries around the world that were previously well-oxygenated now experience hypoxia either seasonally or episodically10, 11. Declining oxygen content affects virtually all biogeochemical and biological processes within the oceans, either through direct effects on aerobic organisms or indirectly through altered ecological interactions dependent on affected taxa. At the organismal level, insufficient dissolved oxygen can affect growth, reproduction and survival. At higher levels of ecological organization, low dissolved oxygen can affect functional attributes of communities such as productivity, biodiversity, resilience and food-web structure.
Affiliations
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Lisa A. Levin is at the Center for Marine Biodiversity and Conservation, Scripps Institution of Oceanography, UC San Diego, La Jolla, California 92093-0218, USA
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Denise L. Breitburg is at the Smithsonian Environmental Research Center, PO Box 28, Edgewater, Maryland 21037, USA
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