Australia: The Land Where Time Began

A biography of the Australian continent 

 Hypoxia by degrees - Establishing Definitions for Oceans that are Changing

Significant scientific and public attention has been drawn to a marked increase in the incidence of low oxygen events that have been occurring on continental shelves, coupled with expanding low oxygen regions that have been observed in the ocean. A need for the establishment of definition of terms that are widely used such as “hypoxia” and “dead zones“ has resulted from the increasing levels and distribution of regions that have low oxygen content. Units such as μMol O₂/kg for reporting as these terms are independent of temperature, salinity and pressure, and are required for mass balances, as well as for numerical models of transport by the oceans. For historical reasons much of the dead zone occurrence reporting is in units of volumetric concentration of ml O₂/l or mg O₂/l. Reporting of the partial pressure of oxygen (pO₂) is required for direct measurements of the physiological state of marine animals. The incorporation of temperature and salinity terms is necessary and therefore accommodates changes brought about by climate warming and the influence of the very large temperature range around the world where there have been reports of an oxygen-limiting temperature range. In this paper the authors¹ examine the various definitions that have been used and the boundaries that have been set, then setting them in a common framework. They examine the large scale ocean pO₂ fields that are required for the pairing with CO₂ data for examination of the combined impacts of global warming and the acidification of the ocean. Shallow, coastal regions with low oxygen concentrations are usually termed “dead zones”, usually caused by either coastal eutrophication or the decomposition of organic matter, or by the upwelling of water that is low in oxygen content. The authors¹ point out that bathyal water, which has low oxygen levels, represents a vast global pool of low oxygen water, and that deep water species are well-adapted to these oxygen-poor waters, and that upwelling waters can readily entrain these low oxygen waters which then contribute to coastal hypoxia throughout the world, and may be characterised identically. In this study the authors’¹ examined the potential for the expansion of those low oxygen waters onto continental shelves around the world which may push the coastal waters past the limits for low oxygen content to which the coastal species are adapted and beyond the point which many coastal shallow water species can tolerate.

Sources & Further reading

1. Hofmann, A. F., E. T. Peltzer, P. M. Walz, and P. G. Brewer. "Hypoxia by Degrees: Establishing Definitions for a Changing Ocean." Deep Sea Research Part I: Oceanographic Research Papers 58, no. 12 (12// 2011): 1212-26.