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Biological oceanography

Lightheaded: Why some plankton may soon be gasping for breath

F. Wishner, B. A. Seibel, C. Roman, C. Deutsch, D. Outram, C. T. Shaw, M. A. Birk, K. A. S. Mislan, T. J. Adams, D. Moore, S. Riley. Ocean deoxygenation and zooplankton: Very small oxygen differences matterScience Advances, 2018; 4 (12): eaau5180 DOI: 10.1126/sciadv.aau5180

Earth has been called the goldilocks planet; we are close enough to the sun to have a relatively warm climate and water that stays liquid, and yet far enough that we don’t burn and the water doesn’t boil. But even within this happy medium of conditions, animal species have specific requirements, needing the proper amount of light, food, air to breath, and temperature. To animals living in the extremes of the planet, there is little wiggle room, especially when it comes to something as essential as getting enough oxygen. A new study, led by Dr. Karen Wishner from the University of Rhode Island, sheds light on the specific needs of some of the most important animals in the ocean and warns us that with climate change, their particular set of “just right” might be thrown into turmoil.

The Oxygen Minimum Zone

Figure 1: A copepod, one of the varieties of plankton that are found in the Oxygen Minimum Zone. Note: this is not the species which Dr. Wishner and her colleagues noticed in particular oxygen poor areas. Photo from Wikimedia Commons.

Animals in the ocean need oxygen just like those on land, but throughout the ocean the amount of oxygen varies dramatically. This can be due to several factors, including temperature (cold water holds more oxygen), the amount of phytoplankton in the water (phytoplankton make oxygen through photosynthesis), and even how many creatures are in that water (more animals, zooplankton, and bacteria mean more organisms are using up the oxygen). When you put all the factors together and then look at how oxygen is spread over the depth of the ocean, there is a particular region with the lowest concentration of oxygen, known as the Oxygen Minimum Zone (OMZ). As climate change continues to shape the planet around us, the OMZ is expected to grow in size and intensity.

While a region with low oxygen may seem inhospitable, there are a number of animals that interact with the OMZ on a daily basis. Some of them simply pass through as they travel from the depths of the ocean to the surface in order to gather food, but others live in this zone more permanently. Most notably, zooplankton, or small, floating animals, are particularly impacted by the size and intensity of the OMZ. These zooplankton are essential food for many species, meaning that their populations are inextricably linked to fisheries and the larger, more charismatic animals you might see in aquariums. If something happens to zooplankton, the ocean will change dramatically.

The Research

Figure 2: Oxygen sections from Wire Flyer transects, showing oxygen concentration (color) with distance. Diagonal black lines are the Wire Flyer path as it oscillated through the indicated depth zone. The light horizontal line is the horizontal MOCNESS tow path that targeted the edges of specific oxygen features seen in the earlier Wire Flyer tow (table S1). (A) Wire Flyer tow #9 and MOCNESS tow #724 (upper oxycline). (B) Wire Flyer tow #10 and MOCNESS tow #726 (upper oxycline). (C) Wire Flyer tow #12 and MOCNESS tow #728 (lower oxycline). From Wishner et al. 2018.

In order to get a better understanding of what is going on in the OMZ, the researchers visited the eastern tropical North Pacific and towed a water profiler called a Wire Flyer over about 50km. The Wire Flyer showed the scientists how much oxygen was in the water, so they were able to identify where the OMZ was and also where oxygen levels changed over depth. In particular, they noticed small changes in oxygen and temperature, with oxygen shifts of about 3-5µM and temperature changes less than 1°C (check out some of their water profiles in Figure 2). Once they identified these shifts, they used a MOCNESS (Multiple Opening-Closing Net and Environmental Sensing System) to collect plankton (see a MOCNESS in action here). By looking at oxygen levels as well as the oxygen demands of the plankton they collected, they were able to draw a compelling image of what life in the OMZ is like.

In general, what they found was that the more oxygen there was, the more zooplankton there were. This makes sense since animals would want to go where oxygen is more plentiful. However, there were two notable exceptions to this rule: the copepod Lucicutia hulsemannae and larvae of the fish Cyclothone spp. These particular zooplankton preferred shallower, warmer water closer to the center of the OMZ, which had less oxygen due to its higher temperature. They also noticed that many of the zooplankton lived closer to well oxygenated surface waters during the evening, but traveled deeper into the OMZ during the day.

Next the scientists wanted to determine if these animals were getting enough oxygen and how expected changes to the OMZ might influence them in the future. They did this by calculating the Metablic Index (MI), which is the amount of oxygen available divided by what each species needs to survive. What they found was that a few particular species, most notably L. hulsemannae and the krill N. flexipes, tended to live very close to their limits, with an MI less than 2. That means that with the animals just existing, not moving, hunting, nor feeding, there was less than twice what they needed. Active animals would need even more oxygen. In short, these animals have little wiggle room.

Figure 3: A larval fish that just hatched from its egg. Many fish species have a larval stage in which the young are transported by waves until they are old enough to effectively swim. They are considered plankton during this period. Photo from Wikimedia Commons.

No Safety Net

The researchers found that even tiny changes in the amount of oxygen in the waters of the eastern tropical Pacific (<10µM) made a big difference for the fish and zooplankton living there. In addition, they found that even the zooplankton with the highest tolerance for low oxygen conditions don’t have enough oxygen for strenuous activity; if oxygen levels were to decrease, the OMZ would become inhospitable for these hardy animals. While the logical explanation may seem that the animals might just change depth to go where there is more oxygen, this won’t always be possible. There may be more oxygen somewhere, but that doesn’t guarantee that the animals could tolerate the temperature or find food there.

There is expected to be a 10 to 15µM decrease in ocean oxygen every decade due to climate change. Considering less than 10µM is enough to affect the animals living in a region entirely, this could have dramatic impacts on the ecosystem. We don’t know how this will reshape the animals living in the OMZ, but what we do know is that zooplankton and fish are tightly linked. If zooplankton don’t get enough oxygen, what will happen to the fish? What will happen to us?


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