Original Research Article
Schmidtko, S., Stramma, L., Visbeck, M. (2017). Decline in global oceanic oxygen content during the past five decades. Nature 542, 335-339. Doi: 10.1038/nature21399
The development of various organisms and their requirement for oxygen consumption is an age-old relationship. Be it on land or under water, oxygen is a vital resource for a multitude of species on Earth. For some of us, much of climate research might only seem to be about the increasing levels of the dreaded greenhouse gas CO2 and its deleterious effects on ecosystems worldwide. However, there is another gaseous problem lurking. In 2016, researchers from Princeton compiled thirty years of climate research to extrapolate 800,000 years of climate data from ice cores in Greenland and Antarctica, constructing a record of atmospheric oxygen concentration. Their findings revealed that oxygen levels in the atmosphere have declined by 0.7% over the past 800,000 years, with 0.1% of the decrease having occurred over the past 100 years alone. This study illustrates an important trend in declining global oxygen levels in the atmosphere. But one wonders, what about oceanic oxygen levels?
The way oxygen “behaves” in oceans is fundamentally different from that in the atmosphere. In the atmosphere, (contrary to popular belief), the percentage of oxygen is the same at high altitudes as it is at sea level (approximately 21%). The reason mountaineers feel out of breath as they climb summits is due to decline in air pressure. The higher you climb, the lower the atmospheric pressure, which in turn lowers the density of air. So even though the percentage of oxygen does not change, the number of oxygen molecules available to breathe in reduces. This lowering in air pressure also affects transport of oxygen in your body, causing symptoms such as altitude sickness and loss of consciousness.
How do oceans have oxygen?
Ocean waters are dynamic; they undergo daily and seasonal changes with weather and tides, and experience constant cycles of warming and cooling. Most water bodies get oxygen using two processes. The first is by photosynthesis, which occurs only near the top, since that is where sunlight penetrates. The second is by air and water mixing at the surface due to winds and waves (Fig 1). The processes mentioned above allows oxygen to become incorporated in water, therefore becoming dissolved oxygen. Studies looking at changes in dissolved oxygen usually do so by measuring its concentration, which is the ratio of the amount of oxygen dissolved in water divided by the total volume of the mixture.
Where doth the oxygen lie
Ocean layers can be broadly categorized into photic and aphotic zones. The word photic (derived from the Greek word phot, meaning “well lit”), is the upper oceanic layer that receives the greatest intensity of sunlight. This zone consists of many plants and other photosynthetic organisms, including tiny, single-celled organisms called phytoplankton, which are the dominant producers of oxygen. Below the photic zone is a region where various animals consume these phytoplankton, and use dissolved oxygen for respiration. These feeding grounds therefore turn into areas with very low oxygen concentration, usually called “oxygen-minimum zones”. Underneath the oxygen-lacking layers lie the the aphotic zones, or deep ocean waters. These cold waters (with some exceptions) have adequate dissolved oxygen, for two main reasons. The first is because oxygen concentrations in the ocean are deeply impacted by temperature; with colder water able to hold more dissolved oxygen than warmer waters. Second, there are less organisms available to consume in this zone, leading to less respiration.
Oceans act as Earth’s thermal blanket
A layer called the thermocline separates the upper, warm waters from the deep, cold and relatively calm waters below (Fig 2). With the rise in global temperatures, oceans have been the unsung heroes, playing the role of a thermal buffer by absorbing more than 90% of Earth’s excess heat production. Due to this enormous trapping of thermal energy, ocean heat levels are rising, and in turn, reducing dissolved oxygen capacity (Fig 3). Also, this temperature increase in the upper ocean is strengthening the thermocline and reducing the ability for oxygen rich, surface water to mix with deep, stagnant water that makes up the bulk of the ocean.
Despite the knowledge that oceanic oxygen is depleting, there has been little work to specifically chart the trend of these changes over decades. Dr. Schmidtko and colleagues from the Helmhotz Centre for Ocean Research performed such a historical analysis, compiling data from publicly available databases on global oceanic properties from 1960-2010. Their study is unique because unlike other studies, where calculations on global oxygen trends have been limited to the upper ocean levels (100-1000 meters), this study measured oxygen estimates for the entire ocean water column.
What’s changing and where?
After ploughing through decades of data, the authors observed that there was a statistically significant decline in oxygen concentrations for Earth’s oceans (about 2%). Out of the ten ocean basins studied, they found that the Arctic, Equatorial Atlantic, South Atlantic, North Pacific and Southern Ocean basins had the greatest decline in oxygen content. Of particular interest is that the Arctic Ocean, although accounting for only approximately 1.2% of the global ocean volume, accounts for 7.6% of the global oxygen decline! This could be a stark indication that the Arctic is changing more rapidly than other oceans due to effects of climate change.
(Dis)solving the oxygen problem
Over decades, climate change has influenced the world’s oceans by resulting in warmer sea surface temperatures. Since warmer waters have poor oxygen solubility, the researchers questioned whether the observed oxygen loss was purely due to a decrease in oxygen solubility in ocean waters. The answer seems to be- it depends where you look. If you survey the first 1000 m of the global oceans, 50% of the oxygen loss can be attributed to solubility changes. However, when analyzing the full ocean depth, that value drops to 15%. Other complex biochemical and physical processes that influence oceanic oxygen concentrations will most likely explain the remaining 85% of the puzzle.
In hot waters
While 2% loss of oceanic oxygen might not seem like a huge percentage, it exceeds estimates made under the presumption that Earth isn’t warming. Models that take anthropogenic (man-made) warming into account, on the other hand, accurately predict these patterns. A vast amount of marine organisms require oxygen to survive, and in many regions that are already low in dissolved oxygen, survival of various organisms might be dire. The continuous warming-induced ocean deoxygenation can therefore cause substantial changes in ocean ecosystems. Other far-reaching consequences will be impacts on fisheries, tourism and further increase in greenhouse gases from biological activity that thrives in low oxygen environments. However, when the cumulative effects of oceanic oxygen loss, warming waters, and increased carbon-dioxide emissions are taken into account, the consequences for marine life can be devastating. A bleak picture indeed.
I’m a fourth year PhD candidate in the Department of Psychology at Northeastern University. My research focuses on the impact of early life stress in the form of maternal separation on neurological and behavioral abnormalities that appear later in life. Being a biologist at heart, marine sciences have always fascinated me. Check out my twitter @prabarna for more science-related fun!